ID-139
Developed by: University of Kentucky Multidisciplinary Extension Team
Written by: Morris Bitzer, Agronomy, Co-Editor; James Herbek, Agronomy, Co-Editor; Ric Bessin, Entomology; J.D. Green, Agronomy; Greg Ibendahl, Agricultural Economics; James Martin, Agronomy; Sam McNeill, Biosystems and Agricultural Engineering; Michael Montross, Biosystems and Agricultural Engineering; Lloyd Murdock, Agronomy; Paul Vincelli, Plant Pathology; Ken Wells, Agronomy
Partial financial support for some of the research reported in this manual as well as full financial support for the printing provided by the Kentucky Corn Growers Association.
Mention or display of a trademark, proprietary product, or firm in text or figures does not constitute an endorsement and does not imply approval to the exclusion of other suitable products or firms.
Morris Bitzer and James Herbek
The corn (Zea mays L.) grown in Kentucky is used mainly for livestock feed (60 percent) and as a cash crop. As a cash crop sold from the farm, corn ranks third behind tobacco and soybeans but is the number one row crop in terms of acreage. However, in total crop value, as reported by the Kentucky Agricultural Statistics Service, corn ranks third after tobacco and hay. Corn is grown in every county in Kentucky, with a major portion of the acreage in Western Kentucky. Corn acreage in Kentucky dropped from a high of 3.6 million acres in 1911 to a low of 1.13 million acres in 1972. Acreage increased slightly in the 1980s to an average of 1.5 million acres but then declined to an average of 1.34 million acres in the 1990s (Figure 1).
Figure 1. Acreage, yield, and production in Kentucky.
Corn yields have risen dramatically over the last few decades. The average state yield in the 1970s was 85.5 bushels per acre; in the 1980s, 94.1 bushels per acre; and in the 1990s, 112.0 bushels per acre. Since 1990, the highest state average ever was 132 bushels per acre in 1992, and the lowest average during this period was 100 bushels per acre in 1991.
Because the cost of producing an acre of corn is high and the value per bushel has declined in recent years, producers must manage and market their corn crop more carefully for adequate profits. The goal of this publication is to serve as a guide for corn production strategies that focus on efficient use of resources and provide the principles and practices for obtaining maximum, profitable corn yields.
With the introduction of biotechnology in the marketplace, producers now have to make a new decision when selecting corn hybrids. Biotech-derived crops have been altered and improved to include resistance or tolerance to pesticides and improved food and feed qualities. A more thorough discussion of the impact of biotechnology on corn production is presented later in this publication.
Corn may be classified by kernel characteristics such as dent, flint, flour, sweet, pop, and pod corn. Except for pod corn, these types are based on the endosperm composition of the kernel. The quantity or volume of endosperm determines the size of the kernel (e.g., the difference between dent and flint corns or flint corn and popcorn) is polygenic (controlled by many genes). The pod corn trait is monogenic and more of an ornamental type.
This publication deals mostly with the dent corns that originated from the hybridization of the southern dent or late-flowering maize race called Gourdseed and the early-flowering northern flints. Dent corn is characterized by the presence of corneous, horny endosperm at the sides and back of the kernels. The central core is a soft, floury endosperm extending to the crown of the endosperm where, upon drying, it collapses to produce a distinct indentation.
Dent corn is used primarily as animal food but also serves as a raw material for industry and as a staple food. There are two types of dent corn, yellow and white. Except for some sweet corn and popcorn, dent corn is the main commercial type of corn grown in Kentucky. The majority of dent corn in Kentucky has yellow kernels; however, Kentucky is one of the leading states in the production of white corn, which is grown mainly for the food industry and is about 10 percent of the total corn acreage. In 1995, Kentucky had 116,000 acres of white corn, and this acreage remains fairly constant from year to year. Very little flint or flour corn is grown in the United States. Pod corn is mainly a curiosity and is not grown commercially.
Some corn hybrids have been altered genetically to produce changes in starch, protein, oil, or other properties of the kernels. Some of these special-purpose corns grown in Kentucky are waxy, high-amylose, high-lysine, high-oil, and low-phytate varieties. A very limited acreage of waxy and high-amylose corn is being grown, and only a few swine producers are raising high-lysine corn, but several thousand acres of high-oil corn are contracted each year in Kentucky.
Waxy corn is used as the raw material for the production of waxy cornstarch by wet-corn millers for industry and food uses. Waxy cornstarch contains more than 99 percent amylopectin, whereas regular corn contains 72 to 76 percent amylopectin and 24 to 28 percent amylose. High-amylose corn has an amylose content greater than 50 percent. It is grown exclusively for wet milling for the textile industry, gum candies, biodegradable packaging materials, and as an adhesive in the manufacture of corrugated cardboard. High-lysine corn contains the single recessive gene, opaque-2, that reduces the zein in the endosperm and increases the concentration of lysine, thus improving the nutritional quality of the grain. Its primary use in the United States is feed for nonruminants.
The most recent improvement in special-purpose corn has been the development of hybrids with higher concentrations of oil. The high-oil seeds are produced by a topcross procedure in which the planted seed is a mixture of 9 percent of a very high-oil inbred pollinator seed and 91 percent seed of a male-sterile, high-yielding, single-cross hybrid. The seed produced contains upwards of 8 percent oil compared to a normal hybrid, which contains only 3.5 to 4 percent oil. The added oil makes a high energy feed. Most high-oil corn is contracted and sold at a premium price. The average yield of these high-oil hybrids has usually been about 10 percent lower than normal hybrids. It is usually recommended to plant these at a 10 percent higher seeding rate in an effort to offset some of this yield loss.
Another recent development has been the testing and release of low-phytate corn hybrids. Phosphorus in regular corn is stored as phytate, but phosphorus in kernels of low-phytate corn is digested more efficiently. This results in lowering the need for supplemental phosphorus, better use of the phosphorus by the animal, and less phosphorus excreted into the environment. Initial tests of low-phytate corn hybrids have been encouraging, but economic viability remains to be determined.
Special-purpose corns are usually grown under contract at a price premium. It is important to understand the contract requirements before the special-purpose corn is grown. There may also be certain recommended production management practices, e.g., soil type, fertility, population, planting date and harvest, drying, and handling practices to obtain the highest possible yields while maintaining grain quality. It is important that grain identity of special-purpose corns be preserved from planting through storage to avoid contamination that would eliminate premium prices and decrease marketability. Special-purpose corns also require isolation from other corn to eliminate cross-pollination.
Most, but not all, special-purpose corns have an inherently lower yield compared to normal dent-corn hybrids. However, special-purpose corns can compensate for this reduction in yield potential with adequate premiums. Before producers decide to grow a specialty corn, it is imperative that they determine potential yield reductions, production risks, contract requirements, and the premium amount needed to ensure a profitable return. Because of improved hybrid development, the yield of some specialty corns has improved as compared to normal hybrids.
Kentucky is one of the leading states in the production of white and yellow corn for food. Food grade corn is used to make corn flakes, tortilla flour, and cornmeal. The hybrids for this market are usually selected by the company offering the production contract. The regional testing of the yellow food corn hybrids has been discontinued; however, the white food corn hybrids are still being tested, and results are available from the University of Kentucky corn testing program.
Morris Bitzer and James Herbek
A cornfield is a complex and constantly changing community made up of many individual corn plants. Within the corn plant, the raw materials (water and minerals from the soil and carbon dioxide and oxygen from the air)—with sunlight providing the energy—combine to produce yield. The growth and yield of a corn plant are functions of the plant's genetic potential to interact with its environmental conditions. Although climatic conditions account for a major portion of the environmental influence on corn growth and development, a corn producer can manipulate the environment with various management practices. By understanding how a corn plant develops, a producer can use the proper production practices to obtain higher yields and profit. Following is a brief discussion of the growth and development of the corn plant.
The corn seed contains adequate stored nutrient reserves to get the seedling established. Seedling emergence usually occurs six to 10 days after planting (four to five days under warm, moist soil conditions). If the seed is placed in a cool, dry soil, it may take two weeks or longer for seedling emergence. The depth of planting also will influence how long it takes for the seedling to emerge. The depth at which the permanent root system (nodal roots) develops is not affected by planting depth and occurs approximately 1 inch below the soil surface. Three or four fully developed leaves are produced during the first three weeks after the plant emerges. A leaf is fully developed when the collar of that leaf is visible. Initiation of all the leaves, ear shoots, and tassel has occurred at the growing point by this stage, and the growing point of the plant is still approximately 1 inch below the soil surface. Damage to the seedling above the ground from frost, hail, or livestock would have little or no effect on the growing point or final yield.
After the tassel and all the leaves and ear shoots are initiated, the stalk begins a period of rapid growth. When six or seven leaves have fully emerged, the growing point has moved above the soil surface and any damage to the leaves and growing point could affect final yield. Plant height increases dramatically during this rapid growth phase, and the plant reaches its maximum height when the tassel is fully emerged from the whorl. Although the ear shoots were formed just before tassel formation (five leaves emerged), the length of the ear and potential number of ovules or kernels per row is determined between the development of 10 or 11 emerged leaves to 17 or 18 emerged leaves or about one week before silking. Moisture or nutrient stresses during this period of ear size determination may seriously reduce the number of potential seeds on an ear. Earlier maturing hybrids will advance through these stages in a shorter time, which usually results in smaller ears than later maturing hybrids. The nodal root system is developing rapidly during this stage, which allows for more rapid uptake of soil nutrients and water to meet the demands of this rapid growth rate. At tasseling, less than half of the final weight of the corn plant has been produced; however, more than 60 percent of the nitrogen, 50 percent of the phosphorus, and 80 percent of the potassium uptake have already occurred.
As vegetative growth nears completion, the ear develops very rapidly. The flowering stage, which includes pollination, is the most critical period in the development of the corn plant. The flowering stage occurs about 65 days after corn emergence in a medium maturity hybrid. Pollen shedding begins two to three days after the tassel has fully emerged from the last leaf sheath and just prior to silk emergence. Under favorable conditions, all silks will emerge within three to five days after tasseling, and the tassel will continue to shed pollen for five to eight days. The silks from near the base of the ear emerge first, and emergence progresses up the ear to the tip. When a pollen grain falls on a corn silk, it germinates and produces a pollen tube that grows the length of the silk in about 24 hours, after which fertilization occurs and a new kernel begins to develop. The silk is released by the kernel immediately upon pollination. Stress (moisture, temperature, nutrient) from one week before to one week after flowering may delay silking until after most of the pollen is shed, resulting in poor pollination, especially on the tips of the ears.
Grain production occurs between pollination and maturity. Drought or nutrient stress during this period can result in unfilled kernels, less weight per kernel, and light, chaffy ears. The grain filling period covers about 55 days for most corn hybrids. Plant physiological maturity is achieved when the kernel has reached its maximum dry weight. A black layer forms at the tip of each kernel at physiological maturity. The average moisture of the kernel at this stage is 30 to 35 percent. Grain drying is a matter of physical moisture loss and varies with climatic conditions but should average at least 0.5 percentage point per day.
Having a knowledge of the growth and development of the corn plant provides the producer with a better understanding of how different problems and stresses affect final yield. By understanding the effects that management practices have during the various stages of corn development, the producer can manage the corn plant more intelligently so that it can nearly reach its yield potential.
Lloyd Murdock and Ken Wells
Traditionally, tillage has been practiced for the purpose of mixing surface residues deeper into the soil, loosening the soil prior to seedbed establishment and to aid in weed control. The primary tillage implement for many years was the moldboard plow. The rough surface left by primary tillage was smoothed by secondary tillage implements, usually a disk harrow followed by one or more passes of another fine-toothed harrow for final smoothing of the surface in preparation for seeding. These techniques have been described as "conventional tillage." Another traditional application of secondary tillage has been the use of a myriad of cultivating tools to provide mechanical weed control and to break up surface crusts. However, the advent of widespread use of chemical weed control during the late 1950s greatly reduced the amount of secondary tillage used for weed control. The major disadvantages of these conventional tillage techniques were increased risk of soil erosion on sloping land and breakdown of soil structure.
Largely due to massive nationwide loss of topsoil from conventional tillage, additional primary tillage techniques were developed to leave varying amounts of the residues from the previous crop lying on the soil surface for the purpose of lowering the erosion potential. Several implements, mostly a variation of the chisel plow, were developed to accomplish this. When followed by a shallow harrowing, these conservation tillage techniques provided a seedbed smooth enough for successful planting of corn but still left some residue cover.
Further developments in chemical weed control and planting equipment that could successfully plant through surface residues resulted in development of no-tillage seeding techniques. The only tillage involved in no-tillage seeding is the narrow, in-row disturbance made by the coulter and furrow-opener on the planter. No-tillage results in most prior crop residues remaining on the surface, which causes a dramatic reduction in soil erosion and increased water infiltration. No-till techniques, pioneered by farmers and researchers in Kentucky, are now so widely used in Kentucky that they dominate seeding methods for corn and soybeans (Figure 1). When combined with other conservation tillage practices, greater use of no-till has resulted in only a small percentage of Kentucky's corn and soybean crop being established by conventional techniques (Table 1).
| Table 1. Tillage systems used for corn, soybean, and fall-seeded small grain in Kentucky, 1998.1 | ||||
| Crop | Total Acres | % Planted | ||
| No-Till | Conservation Till2 | Conventional Till3 | ||
| Full Season Corn | 1,345,000 | 51.8 | 34.5 | 13.7 |
| Double Crop Corn | 62,100 | 64.4 | 29.3 | 6.3 |
| Full Season Soybeans | 882,700 | 51.3 | 30.8 | 17.9 |
| Double Crop Soybeans | 474,700 | 86.7 | 12.4 | 0.9 |
| Fall-Seeded Small Grains | 603,000 | 24.6 | 62.0 | 13.4 |
| All Crops | 3,852,500 | 47.6 | 33.5 | 18.9 |
| 1 Conservation Technology Information Center data.
2 Greater than 15 percent of residues left on surface. 3 Fewer than 15 percent of residues left on surface. |
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No-tillage has a number of advantages, including less soil erosion as compared with clean-tilled systems, and fuel, machinery, and time savings are all impressive. There is also a tendency toward better crop yields on soils that are moderately well drained to well drained, due to higher water capture and conservation often associated with the mulch of crop residue maintained on the soil surface.
No-tillage is best suited to soils that are moderately well drained to well drained. The residue cover keeps soils cooler and wetter throughout much of the growing season under no-till conditions. This is particularly true with heavy residue. Surface residues that leave somewhat poorly drained soils wetter can be an advantage during dry periods, but no-till planting on such soils during cool, wet springs can cause delayed emergence and reduced stands that reduce yields.
Management practices that can improve the performance of no-till corn in cool, wet conditions are the use of in-row (pop-up) fertilizer (see fertility section) and row cleaners. The row cleaners aid in warming and drying the soil over the row, and the in-row fertilizer improves plant growth under stress early in the season. Seed treatments that protect against root shoot rots (Pythium ultimum) are quite helpful and are often routinely added by seed companies.
Conservation tillage is a better choice for poorly drained soils. The tilled surface allows these soils to warm and dry faster in the spring. However, conservation tillage practices used on sloping fields that are prone to erosion should leave at least 30 percent of the soil surface covered by residue at planting to protect the field from excessive erosion. This can be done by reducing the amount of secondary tillage that is done on the field. Secondary tillage is costly, time consuming, and frequently a major culprit in causing soil compaction. It also contributes to erosion, water pollution, and subsequent crop drought stress. Protection against loss of topsoil is of much economic importance. Recent research by the University of Kentucky indicates that each inch of topsoil on a Crider soil, up to the first 8 inches, increases annual corn yield by more than 10 bushels per acre.
Soil compaction comes in a number of forms and from several causes, but in Kentucky the most common causes are either traffic or tillage when the soil is too wet. There is a water content at which any soil is most easily compacted. In the words of one expert, "This is when it is a little too wet to work, but I am going to do it anyway."
Sidewall compaction can result from planting a crop when the soil is a little too wet. This damaging effect can be even greater on soils with a relatively high clay content at the surface. It occurs when the double disc opener leaves the side wall of the planting furrow smooth and compacted (slick as opposed to shattered) as it pushes the soil aside. The trailing press wheel then increases the compaction with its downward force. If the soil stays very moist or wet, the roots may be able to penetrate the compacted mud at the sidewall and expand further into the soil. However, if the weather turns dry after planting, the sidewalls then harden, and roots are not able to push through since there are no pores or cracks. This causes the roots to grow within the planting furrow, along the direction of the row. Although plants may look normal at emergence, they will begin to show nutrient and drought stress after the corn is several inches high. This problem may be more common in no-tillage because no-tillage soils have better structure, and it is easier to traffic them in a wetter condition. The old adage of "waiting on no-till" is a good one. Sidewall compaction can also occur with conventional tillage. If you can mold the soil into a ball in your hand and the soil ball will not easily crumble apart, it is too wet to plant.
Wheel tracks on a wet field can also contribute to a compaction problem. The trend to larger and heavier equipment means that axle weights have increased. A four-wheel drive tractor, a large combine with a full grain hopper, a loaded manure wagon, a fertilizer buggy or truck, or a loaded grain cart can all exert great pressure on the soil below the wheel. These weights, in combination with greater tire pressures, can compact the soil 12 to 18 inches deep. When the degree of compaction is sufficient to diminish pore space to the point that oxygen diffusion, water movement, and root penetration into and through the soil are restricted, crop yields are likely to be lowered.
Disc harrows are tillage tools that can cause severe compaction on wet soils. The weight of a disc transmitted to the soil at the bottom edge of each blade creates enough pressure in a wet soil to compact a zone 4 to 6 inches thick just below the disc blades. This is most common in disc-only tillage systems or where soils are excessively tilled and a disc is used when the soil is a little too wet.
A survey of 175 fields in Kentucky in 1992 and 1993 indicated that 46 percent had no compaction, 18 percent were slightly compacted, 18 percent were moderately compacted, and 18 percent were severely compacted. This survey used soil penetrometers to classify the amount of compaction. Limited research indicates that the moderate and severe categories should be considered possible yield-limiting situations. This means that about 30 to 40 percent of Kentucky's cropped fields are compacted enough to possibly limit the growth and yield of some crops. The more poorly drained fields had the most compaction, with 77 percent of the poorly drained soils being moderately or severely compacted, while only 20 percent of the well- drained soils were in this range. When the primary tillage was discing, fields were twice as likely to have moderate or severe compaction as those where a chisel or moldboard plow was used. The least likely fields to have compaction were no-till fields.
When compaction was found, it was most likely to begin at depths between 6 and 9 inches and to terminate between 12 and 15 inches. However, compaction was found at other depths and depth thicknesses.
The effect of compaction on yield varies with the crop, weather conditions, and soil type. Corn is more sensitive to soil compaction than soybean or wheat. Based on research in Kentucky and surrounding states, the estimated yield reduction for corn is 30 to 50 percent with extreme compaction such as that found under end rows and at field entrances, 10 to 20 percent for fields with severe compaction, and 5 to 10 percent for those with moderate compaction.
The best way to solve compaction is to prevent it. Some simple things can make a difference.
Sometimes soils are deep-tilled when there is no compaction. This is costly and does not improve yields. The best way to identify compaction in a field is by using a soil penetrometer (soil compaction tester), a tiling rod, or a 3-ft length of 1/2- to 3/4-inch diameter steel rod sharpened on one end with a T-handle on the other end. These tools should be marked (notched) for depth and should only be used when the soil is at field capacity after a rain (too wet to till, but not sloppy muddy). This is best done in December through March when the profile is wet throughout. Under these conditions, compacted layers can be found and the depth and thickness of the compacted zone can be identified. Each Cooperative Extension Service office in Kentucky has a soil penetrometer with instructions on how to use it and a form to record the results. The form also has a method to classify the amount and type of tillage found in the field. When readings reach 300 pounds per square inch, the compaction is considered root limiting. If one-third of the field has readings of 300 or more, a corrective action and change in tillage practices should be considered. When one-half of the field has readings of 300 or more, corrective action and changes in tillage practices definitely are needed.
After moderate to severe compaction (lesser amounts of compaction are not harmful) has been identified, there is more than one way to correct it. When tillage or subsoiling is used, be sure the compacted zone is dry enough to shatter. Fall is generally the best time because the subsoil is usually drier and will shatter better. This means that fields with identified problems will be cropped for another summer prior to compaction alleviation. Rotations to some other crops can also help alleviate compaction. Alfalfa, sweet clover, and fescue all have root systems that are helpful but are rather long-term solutions.
Compaction can be caused by traffic and some tillage operations and can cause yield reductions in some crops. The yield reduction may not be easily seen unless the compaction is extreme. A lot of money is wasted on deep tillage done in response to fear of compaction that does not exist. The key is using a total management system that prevents compaction but also monitors fields for the problem and then corrects it when and where it is found.
Figure 1. Percent of Kentucky's corn, soybean, and wheat acreage established with no-till technology.
Morris Bitzer and James Herbek
One of the most important decisions that a producer must make when planning for the next corn planting season is what hybrid or hybrids to plant. Currently, most commercial corn producers plant single-cross hybrids, and most of these hybrids are produced and marketed by private seed companies. The corn producer's challenge is to select those hybrids that are appropriate for each management situation, keeping in mind the risks associated with potential weather extremes and field limitations. Managing to get the highest possible yield starts with selecting those corn hybrids that are best adapted to your farm and farming practices. Among the agronomic characteristics to consider in choosing hybrids are yield, maturity, standability, insect and disease tolerance, seedling vigor, and stress tolerance.
The bottom line for most producers, all other things being equal, is to use the highest yielding hybrids available. Under stress conditions, high yielding hybrids with superior stalk quality are most desirable. If a hybrid cannot stand under stress conditions, lodging can severely decrease yields. State yield trial reports provide the most complete and unbiased information on the relationship between yield and lodging. Most state trials are conducted at several locations under varying degrees of stress conditions and include most of the hybrids sold in the state. Each year, the University of Kentucky College of Agriculture conducts the Kentucky Hybrid Corn Performance Tests. This information is made available both on a Web site and as a progress report available from your county Cooperative Extension Service office.
The process of hybrid selection should consider the stability of performance across years and locations. Selection of more than one hybrid will reduce risk from weather and disease. Each year several new hybrids are included in the test. Selecting new hybrids that are within one least standard deviation (LSD) of the best hybrids in the test will provide more chance of stability of performance. In addition to yield, data are presented on moisture at harvest, percent stand, lodging, and test weight. Separate tables are presented on the protein, oil, and starch composition of the corn hybrids.
Other good sources of information about hybrid performance are from well-managed local corn demonstration plots sponsored by county Extension groups, FFA chapters, and seed corn companies. To be meaningful, these plots should have at least three replications of each hybrid or a check hybrid between plots of every two or three hybrids with yield adjustments made for location in the field. Many corn companies today combine data from several locations, which does improve the reliability of the data. Strip test or plots with each hybrid entered only once are of little value for yield comparisons, as field variation is usually greater than most differences among the hybrids.
Choosing the appropriate maturity or maturities for each field, situation, or farm operation is important when selecting hybrids. The Kentucky Hybrid Corn Performance Test is a good source of information on relative maturity of hybrids. The hybrids are divided by maturity: early, medium, and late. Once you have selected the desired maturity, you can choose among the hybrids within a maturity group based on their performance characteristics.
Deciding which maturity or maturities to plant depends on a number of factors that may be unique to each field or farm operation. In general, full-season hybrids (hybrids that use most of the growing period in that area) produce the highest yields. However, recent hybrid development has resulted in early and medium maturity hybrids having about the same yield potential as the full-season hybrids. Currently, the majority of the corn grown in Kentucky is of medium maturity. Early and medium maturity hybrids will have an earlier harvest and a lower moisture content than later maturing hybrids. Early maturity hybrids are useful for late plantings (after early June) because of the shorter growing season. Yield potential of early maturity hybrids is comparable to later maturity hybrids when planted at later planting dates with a lower moisture content at harvest. Early and medium maturity hybrids are also a good choice for stress situations, particularly soils with low water-holding capacity since they require less moisture to mature.
Producers should plant several hybrids differing in maturity, particularly if a large acreage of corn is planted. Hybrids that differ in maturity reduce the risk of adverse weather (heat or drought) and stress at pollination. It also spreads the harvest period so corn can be harvested at optimal grain moisture levels. The optimal proportion of different maturities differs for each farm operation and depends on acreage, soil types, and other management factors. A typical recommendation of different maturities might be 10 to 15 percent early hybrids, 60 to 70 percent medium hybrids, and 15 to 20 percent late hybrids.
Most producers consider corn maturity as the number of calendar days from planting to maturity. This system allows a farmer to compare the maturities between different hybrids but does not necessarily indicate how many days it will take for that hybrid to reach physiological maturity. The number of days that are required for a hybrid to reach maturity depends on location, date of planting, and the weather during the growing season. A hybrid that is labeled as a 115 day hybrid may take from 110 to 120 days to mature depending on the above factors. This system of measuring corn maturity does not take into account the complicated physiological processes that control growth and development of corn.
Each day that a corn plant grows from emergence to maturity does not contribute equally to the development of the plant. Development is faster during warmer days than it is during cooler days. Although factors other than temperature may enter into determining rate of growth, the corn industry adopted the Growing Degree Days (GDD) system in 1970. This system uses a heat unit approach to the prediction of maturity that is more accurate than the old days-to-maturity ratings and is based on the number of heat units necessary for corn to reach physiological maturity.
Growing degree days are calculated by subtracting the base temperature (50°F) from the average of the maximum and minimum daily temperatures. Little or no corn plant growth occurs when the temperature drops below 50°F, and when the temperature rises above 86°F development is reduced. Consequently, a GDD is calculated according to the following equation:
The maximum temperature is the highest temperature for the day (adjusted downward to 86°F, if necessary), and the minimum temperature is the lowest for the day (adjusted upward to 50°F, if necessary). For example, if the high temperature for the day is 90°F and the minimum is 60°F, the GDD = (86 + 60)/2 - 50 = 23 for that day. The University of Kentucky Agricultural Weather Center (AWC) starts recording GDDs for corn on April 1. These graphs are available at the following URL: wwwagwx.ca.uky.edu/cgi-bin/cropdd_www.pl. By knowing the GDDs required for a particular hybrid to mature, one can determine from the AWC when a particular hybrid should mature from the date that it emerged. For example, if the corn emerged on April 15 and required 2,700 GDDs to mature, corn would reach physiological maturity about August 26. This assumes fairly normal weather. The same site can also tell you on August 26 how many GDDs has accumulated by that date. This information can be used to determine if a particular hybrid will mature before the average date of the first frost in the fall.
Hybrid seed corn is available in different kernel sizes and shapes. Location on the ear influences the size and shape of the kernels. Large round seed comes from the base of the ear; small round seed, from the tip; and flat seed, from the center of the ear. The key to accurate planting is to select kernel size and shape to fit their planting equipment. For plateless-type planters that use vacuum or air pressure to hold seed to a plate or drum or finger pickup units, seed size and shape are not as important. These types of planting units can use different seed sizes and shapes.
Research has not found any relationship between kernel size or shape and emergence on yield. Thus, within a given hybrid, seed of any size or shape has the same genetic potential. Growers with plateless planters can take advantage of lower prices often associated with less popular seed sizes and shapes. Corn hybrids should be selected on the basis of their agronomic performance, not on their kernel size or shape, if the planting equipment is suitable.
The following equation can be used to determine the number of live plants that can be expected from corn seed at a given seeding rate:
It is fairly common to find that as many as 10 to 15 percent of the seeds planted do not produce a live plant under field conditions.
Ric Bessin
Agricultural biotech crops on the market today have been given genetic traits from other organisms to provide protection from pests and tolerance to pesticides or to improve food and feed quality. To transform a plant, the gene that produces the trait of interest is identified and separated from the rest of the genetic material in a donor organism. Most organisms have thousands of genes, and a single gene represents only a tiny fraction of the total genetic makeup of an organism. A donor organism may be a bacterium, fungus, or even another plant species. In the case of Bt corn, the donor organism was a naturally occurring soil bacterium, Bacillus thuringiensis, and the gene of interest produces a protein that kills Lepidoptera larvae, in particular, European corn borer. The donor gene along with a genetic promoter (which turns the gene on in the corn plant) and a genetic marker (which allows plants breeders to quickly identify transformed plants) were inserted into corn embryos. These new genes are then incorporated into commercial corn hybrids using traditional backcrossing breeding methods.
Plants produced through biotechnology are closely regulated by the USDA APHIS, the EPA, and the FDA. Producers should not select a hybrid based solely on the fact that it is biotechnology derived. Selection of a biotechnologically derived hybrid for pest-resistant traits should depend on whether the resistant traits are needed. Likewise, selection of biotechnologically derived hybrids with improved food or feed quality should depend on market value and profit potential.
Producers wanting to use ag biotech hybrids should always check with their grain buyers prior to seed purchase to be certain that these hybrids are approved and will be accepted at the market. Some biotech crops have not been approved or accepted in certain markets. The recall of foods containing traces of StarLink corn taught us an important lesson that the utmost care must be taken to prevent commingling of grain intended for different markets. Because corn is pollinated with wind-blown pollen, field isolation of up to 660 feet may be needed to prevent cross-pollination between different hybrids to ensure product identity.
Morris Bitzer and James Herbek
Planting corn early in Kentucky is not as important as it is in states farther north. Kentucky's growing season is long enough that corn may be planted from early April to mid-May in most years and still obtain high yields. Optimal planting dates usually range from April 1 to May 1 in Western Kentucky and April 15 to May 15 in Central and Eastern Kentucky. In some years, corn is planted in March, but often it must be replanted because of poor stands due to cold soil. The most critical factor in determining when to start planting corn is the soil temperature. Planting when soil temperatures are above 50°F at a 2-inch depth for three or four days appears to be an excellent guide. A soil temperature of 50°F at 7:00 a.m. or 55°F at 1:00 p.m. should assure that temperatures are suitable for germination and growth for at least several hours during the day. Because of residue cover, soils for no-tillage planting usually do not warm up as early as tilled soils. If using no-till, planting may have to be delayed by four to seven days.
Earlier planted corn has usually had fewer insect and disease problems. For maximum yields, corn should be planted before May 1 in extreme Western Kentucky, by May 10 in west-central Kentucky, and by May 15 in Eastern Kentucky. If corn planting is delayed past June 5, an earlier-maturing hybrid should be planted. Several years of research have shown that a 1 percent per day yield loss can be expected in corn planted after May 10-15.
The speed of germination and uniformity of plant emergence depend not only on soil temperature but also on planting depth. Under good conditions of temperature and moisture, a 11/2- to 2-inch depth is ideal. Some research in the Midwest has shown that 2 inches is the best depth for highest yields. For early planting, especially when the soil is cooler, plant at a slightly shallower depth of 1 to 1 1/2 inches. If the soil is dry, which is sometimes the case when planting late, you may need to plant 2 1/2 to 3 inches deep to get the seed to moisture. Soil temperatures in the upper 2 inches are greatly influenced by air temperature and solar radiation and can fluctuate as much as 10°F during a single day.
Planting too deep or too shallow can adversely affect corn performance. Early in the season, soils are colder at deeper depths and may slow germination and subject the seed to disease or insect injury. A seed treatment for insects is recommended with early planting. Planting depths greater than 2 inches may result in seedlings with less vigor, slower growth and development and lower yield. Planting corn seeds too deep can result in the coleoptile growth ceasing below the soil surface leaving the tender shoot to grow unprotected toward the soil surface. An unprotected shoot would be damaged and leaves unfurled before it emerges. Planting depths over 3 inches should not be considered under any soil conditions because of emergence problems. Conversely, planting too shallow can lead to poor nodal root development, shallow rooting depth, and poor drought tolerance. Do not plant less than 1 inch deep under any circumstances because poor nodal root development (permanent root system) may occur, which can result in plants falling over, known as suicidal corn.
Depth is particularly critical in no-tillage planting. For germination to occur rapidly and uniformly, the seed must be at a uniform depth and surrounded by soil. Some types of seed firmers may improve uniform planting depth. Careful control of planting depth improves stands and uniform emergence.
The optimum plant population depends on the yield level that a particular environment (soil, moisture) permits. Average corn plant populations have gradually increased over the years as have corn yields. These increases can be attributed to improvements in production technology as well as genetic improvement in yield potential, standability, and stress tolerance. Today's corn hybrids have higher yield potentials because of greater yield stability over a wider range of environments, superior stalk strength and standability, and better tolerate competitive stress (less barrenness) at high plant densities than previous hybrids. If a stressful environment occurs under recommended high populations with modern-day hybrids, extremely high yields will not occur, but, it is less likely that a significant yield decrease will occur unless the population has greatly exceeded the recommended optimum range.
Recent studies at the University of Kentucky have shown trends for maximum yields at higher plant populations. In the 3 year study (Table 1), corn yields increased significantly at each increased level of plant population. In the 2-year study with two hybrids (Table 2), there were no significant increases in yields with increased plant populations; however, there was a trend toward slightly higher yields at 28,000 plants per acre.
| Table 1. Effect of plant population and row width on corn yields in Kentucky (eight-location average, 1995-97). Bitzer and Herbek. | ||
| Treatment | Yield (bu/ac) | |
| Plant population | 22,000 | 164a* |
| (Plants/acre) | 26,000 | 171b |
| 30,000 | 178c | |
| Row width | 20 inch | 170b |
| 30 inch | 175a | |
| 36 inch | 169b | |
| *Means followed by different letters are significantly different at 0.05 level of significance. | ||
Corn can compensate for low populations by producing larger ears or additional ears. However, most hybrids today produce only one ear. Hybrids also respond differently to plant populations. When the population is too high, some hybrids may have barren stalks and lodging potential tends to increase. Consult seed company recommendations for desired plant populations of specific hybrids.
Using the data from Tables 1 and 2 and data collected by R. Barnhisel, University of Kentucky Agronomy Department, during the last five years of variable rate seeding studies, the recommended corn seeding rates for Kentucky are presented in Table 3. Corn planted on low yielding soils should not be seeded above 22,000 seeds per acre, and on high yielding, uniform soils, top yields are obtained with seeding rates of 28,000 to 30,000 seeds per acre. For intermediate yields (120 to 175 bushels per acre), use intermediate populations. Many times yields close to 200 bushels per acre can be achieved at 26,000 to 28,000 seeds per acre. Excessive populations can lead to more lodging, more disease pressure, and lower yields in most years. The final population should be approximately 85 to 90 percent of the seeding rate as shown in Table 4.
| Table 2. Effect of plant population and row width on corn yields in Kentucky (four-location average, 1998-99). | ||||
| Treatment | Yield (bu/ac) | |||
| 1998 | 1999 | Ave. | ||
| Plant population | 24,000 | 167 | 130 | 149* |
| (Plants/acre) | 28,000 | 174 | 129 | 152 |
| 32,000 | 172 | 126 | 149 | |
| Row width | 20 inch | 171 | 126 | 148 |
| 30 inch | 171 | 131 | 151 | |
| * There were no significant differences among means at 0.05 level of significance. | ||||
| Table 3. Recommended corn seeding rates for Kentucky. | |
| Seeding rate*
(seeds/acre) |
|
| Grain | 22,000 - 30,000 |
| Silage | 24,000 - 30,000 |
| Irrigated | 26,000 - 32,000 |
| * Range depends on potential yield of soil ranging from less than 100 bu/ac for the low range to more than 200 bu/ac for the high range. | |
| Table 4. Corn population planting guide. | |||||
| Harvest population1 | Required planting rate | Inches between kernels when planting at
various row widths |
|||
| 20" | 30" | 36" | 38" | ||
| 16,200 | 18,000 | 17.4 | 11.7 | 9.7 | 9.2 |
| 17,100 | 19,000 | 16.5 | 11.1 | 9.2 | 8.7 |
| 18,000 | 20,000 | 15.7 | 10.5 | 8.7 | 8.3 |
| 19,800 | 22,000 | 14.3 | 9.5 | 7.9 | 7.5 |
| 21,600 | 24,000 | 13.1 | 8.7 | 7.2 | 6.9 |
| 23,500 | 26,000 | 12.1 | 8.1 | 6.7 | 6.4 |
| 25,200 | 28,000 | 11.2 | 7.5 | 6.2 | 5.9 |
| 27,000 | 30,000 | 10.5 | 7.0 | 5.8 | 5.5 |
| 28,800 | 32,000 | 9.8 | 6.5 | 5.4 | 5.2 |
| 1 Allows 10 percent stand loss. | |||||
Studies in Kentucky during the 1970s and 1980s showed no advantage in yield for corn planted in rows narrower than 36 inches. However, by the early 1990s, a large percentage of the corn was grown in 30-inch rows because producers had switched to narrower rows for soybean and were using the same equipment for corn. In the early 1990s, much interest was generated for using 20-inch rows for corn. However, research from most of the states surrounding Kentucky did not show any advantage for 20-inch rows over 30-inch rows. Research was started in the mid-1990s comparing 20-inch, 30-inch, and 36-inch row width for corn in Kentucky (Table 1). These data showed an advantage for 30-inch over 36-inch row widths but that there was no advantage for 20-inch rows over 30-inch rows. Actually, 20-inch rows were no better than 36-inch rows. In Table 2, two more years of research on row width gave the same results. Consequently, the recommended row width for corn production in Kentucky is 30-inch rows.
Any consideration for a change in row spacing must take into account the economic return of that change. Most economic analysis comparisons indicate that a yield increase of at least 6 to 8 percent on large acreages (>500 acres) over a seven to 10 year period is needed to cover expenses incurred when switching row widths unless new equipment is needed to replace old equipment.
If a corn crop has been damaged or the stand is poor early enough to consider replanting, there are several factors that need to be considered. Some of these factors are seeding rate and expected plant stand, plant stand after damage or loss of stand, uniformity of plant stand being considered, replanting date and seed costs to replant, and potential pest problems with replanted corn. Whether to replant or not comes down to deciding whether the replant-crop yields would be sufficient to cover the costs of replanting and net enough to make it worth the effort. The key factor to consider is found in Table 5. This table will help you decide if replanting will yield more corn than leaving the present stand. The information in this table was obtained and adapted from the National Corn Handbook, NCH-30, "Guidelines for Making Corn Replanting Decisions." Refer to this publication for a much more detailed explanation of making a replanting decision, or contact your state corn specialist.
| Table 5. Grain yields for various planting dates and population rates, expressed as a percent of optimum planting date and population rate (uniformly spaced within row). | |||||||
| Planting date | Plants per acre at harvest | ||||||
| 12,000 | 14,000 | 16,000 | 18,000 | 20,000 | 22,500 | 25,000 | |
| (% of optimum yield) | |||||||
| April 15 | 70 | 76 | 81 | 85 | 88 | 91 | 93 |
| April 20 | 72 | 78 | 83 | 87 | 90 | 93 | 95 |
| April 25 | 75 | 81 | 86 | 90 | 93 | 96 | 98 |
| May 1 | 77 | 83 | 88 | 92 | 95 | 98 | 100 |
| May 6 | 78 | 83 | 88 | 92 | 95 | 98 | 100 |
| May 11 | 77 | 83 | 88 | 92 | 95 | 98 | 99 |
| May 16 | 75 | 81 | 86 | 90 | 93 | 96 | 98 |
| May 21 | 73 | 78 | 83 | 87 | 91 | 94 | 95 |
| May 26 | 69 | 75 | 80 | 84 | 87 | 90 | 92 |
| May 31 | 64 | 70 | 75 | 79 | 82 | 85 | 87 |
| June 5 | 59 | 64 | 69 | 73 | 77 | 80 | 81 |
| June 10 | 52 | 58 | 63 | 67 | 70 | 73 | 75 |
Table 5 contains the percentage of expected corn yield for planting date and harvest populations. Optimum population is considered to be 25,000 plants per acre with the optimum planting date to be the first week to 10 days of May. Information in this table along with consideration of the above- mentioned factors should aid in making a replanting decision. To use this table, consider this example: Suppose a field was planted on May 1 with an expected harvest population of 25,000 plants per acre. Later, the stand was reduced to 14,000 plants per acre; the yield loss penalty for the reduced population would be 17 percent (100 percent minus 83 percent). If it was decided to replant the field on May 21 to obtain a desired population of 25,000 plants per acre, a yield of 95 percent of optimum could be expected; for a net gain of 12 percent (95 percent minus 83 percent). Thus, replanting should be profitable in this case. However, if the stand was reduced to 16,000 plants per acre on May 31, a decision to replant would not be profitable, as an expected yield of only 87 percent would be realized as compared to an 88 percent yield if the stand was left standing. This is simply a guide to help you make a decision concerning replanting. Table 5 takes into account the loss of yield at later plantings but does not take into account non-uniform stands. All these factors must be weighed against expected replanting yield gains. If after considering all the factors, there is still doubt as to whether a field should be replanted, it will probably be correct more often if the field is left as is.
Morris Bitzer and James Herbek
There are many cropping sequences that can be used for growing corn in Kentucky. Economically and agronomically, it is difficult to justify growing corn in a monoculture instead of using a rotation. Data from many states have shown that a yield loss up to 10 percent occurs when corn is grown two or more years in succession. Most of that loss occurs in the second year.
There are several benefits from growing corn in rotation. With less pressure from disease, insects, and weeds, production costs are lower and profits are higher due to higher corn yields. Rotation studies in Kentucky have shown a yield increase of about 10 bushels per acre for corn grown in a rotation with soybean or soybean and wheat. Rotations also improve the use and availability of nutrients, and with the proper selection of a rotation crop, the productivity of the complete cropping system. Corn fits well into most crop rotations. The corn/soybean or corn/wheat/double-cropped soybean (three crops in two years) cropping sequences are commonly used in Kentucky.
Lloyd Murdock
The purpose of developing a fertility program is to ensure that adequate levels of nutrients are available for plant uptake in support of the yield potential for the climatic, plant genetic, and soil environmental factors impacting plant growth in any given field. A regular soil sampling program is the best way to obtain the information necessary to develop such a fertility program. An occasional tissue sampling program helps augment the soil sampling program. For corn production, nutrient application most commonly involves lime for pH, as well as nitrogen (N), phosphorus (P), and potassium (K). Zinc (Zn) or magnesium (Mg) is needed occasionally. In rare cases, boron (B) may be necessary.
When you take soil test samples, keep in mind that a few ounces of soil are being tested to determine lime and fertilizer needs for millions of pounds of soil in the field. It is absolutely necessary that the soil sample you send to the laboratory accurately represent the area sampled.
Soil samples can be collected during much of the year, although September to December or February to April are the best times. There will be a small difference in soil test results depending on the time of the year of sampling. So, once a time of the year is selected, always sample in the same season.
A soil probe, auger, garden trowel, or a spade and knife are all the tools you need to take the individual cores that will make up the field sample. You will also need a clean, dry bucket (preferably plastic) to collect and mix the sample cores. Soil sample boxes or bags and information forms for submitting samples are available at all county Cooperative Extension services offices.
The most representative sample can be obtained from a large field by sampling smaller, more uniform areas on the basis of soil type, cropping history, erosion, or past management practices. A sample should represent no more than 20 acres except when soils, past management, and cropping history are quite uniform. When troubleshooting problem areas in fields during the growing season, take a sample from the problem area and adjacent areas with good crop growth.
Collect at least 10 soil cores in small areas and up to 30 cores in larger fields. Take the soil cores randomly throughout the area to be sampled and place in the bucket.
Tilled areas--Take soil cores to the depth of the tillage operation (usually about 6 inches).
No-tilled areas--Take soil cores to a depth of 4 inches where fertilizer or lime remains on the soil surface or is incorporated only in the surface 1 to 2 inches.
Lime and fertilizer applied continuously to the surface of no-till fields results in a build-up of immobile nutrients within the top 1 to 3 inches of the field, with little effect on increasing soil test values below this depth. This stratification of P, K, Ca, and Mg has not been a problem in no-till corn production in Kentucky, but no-till fields are sampled to a 4-inch depth because of nutrient stratification. Also, if most or all of the N is applied on the soil surface, continuous no-tillage does cause increased acidity in the top 1 to 2 inches of soil. This surface acidity reduces the activity of some herbicides, particularly the triazines. This surface acidity may need occasional monitoring with a separate 2-inch soil sampling.
Certain areas should be avoided when taking soil samples. Do not include soil from the following areas:
Many farmers now sample fields to delineate soil-test variability so that they can make variable-rate applications of lime and fertilizer within the field. This is most commonly done by sampling fields on a grid. Grid sampling involves establishing some measured grid intersects within a field and then taking a composite soil sample within a small area either around the grid intersects or from the center of the grid. The question of concern is what grid size to use. A widely used method is to grid fields into 330- x 330-foot (2.5 acre) blocks and sample each block by compositing six or eight cores taken within a 60-foot radius of the center of the block. While such regimented grid sampling gives a better picture of soil-test variability within a field, it does require more intensive sampling, which increases costs. Research on grid size has shown that the smaller the grid, the more accurate the map of a field's availability. Grids on 100-foot intersects (0.23 acre per grid) are much more accurate than 330-foot intersect grids, but they require the expense of a soil test for every 0.23 acre in a field.
The expense of the large number of soil tests required by grid sampling has resulted in some farmers resorting to a procedure presently called "smart sampling." This procedure is identical to the long-standing University of Kentucky recommendation of (a) sampling fields in units no larger than 20 acres and (b) separately sampling areas known to be different within the field. Currently, "smart sampling" protocols are derived from field maps of crop yield made with yield monitors, where low-producing areas are identified and then sampled separately.
After all cores are collected and placed in the bucket, crush the soil material and mix the sample thoroughly by hand. Take about a pint volume from the bucket and allow it to air dry in an open space free from contamination. Do not dry the sample in an oven or at an abnormally high temperature.
Extractants. Soil pH is nearly always measured on a slurry of soil and distilled water or a buffer solution, but nutrient measurements are made after their extraction from the soil. Different laboratories may use different extractants or extraction procedures. The most commonly used extractants are:
The ultimate concern is that fertilizer nutrients be recommended on the basis of crop response that has been correlated with, and calibrated for, each specific extractant. For example, UK's fertilizer recommendations are correlated and calibrated for soil test values determined with the Mehlich-3 extractant. Using UK's recommendations for soil test values determined with the Mehlich-1 extractant would be totally invalid and might result in fertilizer rate recommendations that are much greater than needed.
Some laboratories report results in parts per million (ppm), while others report in pounds per acre. If there is need to convert from one to the other, use the following formulas to estimate this comparison:
It is not uncommon for a farmer to receive vastly different fertilizer recommendations after splitting a soil sample and sending half to different labs. Such differences are due to the differing philosophies used in interpreting soil test values and making fertilizer recommendations.
Several different philosophies are used in Kentucky, depending on who is making the recommendation. Farm supply dealers, agricultural consultants, and soil test laboratories use different approaches. Philosophies commonly used in making recommendations are discussed below. Each of these philosophies is based on different assumptions about crop needs and how crops respond to applied nutrition at different soil test levels and to different amounts and ratios of available nutrients. For any of these philosophies to have value in Kentucky, they must be correlated to the soil types and climatic conditions of Kentucky.
The crop response is the focus of this philosophy. The expected response of the crop at any given soil test level is what determines the fertilizer rate recommended for each nutrient. The amount of fertilizer recommended is determined from many field trials on different soils over many years. The approach is based on research data that adequately predict a crop response under normal to good conditions.
The theory behind this philosophy is that the correct nutrient balance results in maximum crop response. This approach is often adopted when wide extremes in soil type are encountered or when the research base for the soil types encountered is limited.
According to this philosophy, the nutrients removed at harvest should always be replaced. This approach is used especially on soils that test medium to high in P and K. This method is often used in combination with a recommendation made by either the nutrient balance or crop sufficiency approaches, which use a soil test as a basis for recommendation. A yield response to this extra maintenance fertilizer is usually not expected, but the fertilizer is added to maintain soil test levels over time.
This concept is based on testing the soil for secondary nutrients and micronutrients, and recommendations are made based only on this information, regardless of whether the correlation and calibration research base exists. Using a soil test in this way greatly increases the chance of adding a nutrient where it may not be needed. This is significantly different from making recommendations for these nutrients when both tissue and soil tests are used to determine deficiency or when an area or soil type is known to have a consistent secondary nutrient or micronutrient problem.
Normally recommendations are made from a combination of these philosophies. The philosophy that usually stands alone is the crop sufficiency philosophy. The maintenance philosophy frequently is used with either the sufficiency or the nutrient balance approaches. The philosophy of recommending micronutrients based only on a soil test is sometimes used with all approaches but is most commonly used with the maintenance and nutrient balance philosophies.
All of these philosophies or combinations of philosophies have been evaluated in Kentucky. All resulted in excellent crop yields when the weather conditions were good. In almost all cases, there was no real difference in yields. However, there were always fairly large differences in the amount and kinds of fertilizer recommended. This resulted in large differences in the costs, with very high fertilizer costs giving no yield advantage. Fertilizer rates based on the crop sufficiency philosophy usually cost the least and produce yields equivalent to the more costly recommendations derived from the other philosophies tested. Soil tests taken a few years following the application of the various recommendations indicated that surplus fertilizer was being stored in the soil.
Greater soil acidity is the result of naturally occurring processes, mostly the decomposition of soil organic matter and plant residues and the removal of bases from the soil. Acid-forming fertilizers accelerate the formation of acidity, and the "salt" effect from fertilizer use also increases soil acidity.
The commonly used N fertilizers are the most usual source of acid-forming fertilizers. When used at high rates for a number of years, these N fertilizers cause the soil pH to drop rapidly. Table 1 shows the amounts of lime needed to neutralize acidity from various N fertilizers.
| Table 1. Approximate pounds of ag lime needed to neutralize the acidity generated by nitrogen fertilizers. | |||
| Pure product | Lime needed for 1 lb of
actual N added |
||
| % N | 100% pure fine lime | Normal
ag lime |
|
| Ammonium Nitrate | 34 | 1.8 | 2.7 |
| Urea | 46 | 1.8 | 2.7 |
| Anhydrous Ammonia | 82.5 | 1.8 | 2.7 |
| N Solutions | 28-32 | 1.8 | 2.7 |
| Ammonium Sulfate | 21 | 5.3 | 7.9 |
| Diammonium Phosphate | 18 | 1.8 | 2.7 |
Soils that contain higher levels of active hydrogen and aluminum or both in relation to Ca and Mg are acidic. The degree of acidity is expressed in terms of pH. A pH of 7 is neutral; pH values below 7 are acidic, and those above 7 are alkaline. Each pH unit represents a 10-fold change in acidity. For example, a soil with pH 5 has 10 times more active acidity than one with pH 6. Most crops grow best at soil pH values between 6 and 7.
The pH of the soil is a measurement made on a slurry of soil and water. It is a measure of the acidity in the soil solution that is in contact with plant roots. The soil buffer pH is a measure of reserve soil acidity that is held on the surface of soil mineral and organic particles and that must also be neutralized in order to increase the soil pH. In the soil buffer test, a buffer solution is mixed with soil, and the pH of the slurry is measured. The result from the buffer test is reported as buffer pH. The buffer pH is used only to determine lime requirements. The buffer pH and the soil pH together can be used to determine the lime required to change soil pH to some desired level.
Lime neutralizes soil acidity, raises soil pH, and adds Ca and Mg to the soil. The range in soil pH for optimal nutrient availability is generally between 6 and 7, with a target pH of about 6.5. Outside this range, one or more nutrients may become deficient. Liming acid soils also improves the environment for beneficial soil microorganisms and promotes a more rapid breakdown of soil organic matter, releasing nutrients for growing plants.
Corn is somewhat less sensitive to acid soils than wheat and soybean with which it is usually rotated. Nevertheless, at very low pH, corn suffers from both manganese and aluminum toxicity. Manganese toxicity causes striped leaves and stunted growth, and many times there is a string of necrotic spots on the interveins of the leaves. Aluminum toxicity results in poor root growth that causes short thick roots with few fine roots, which results in drought injury. Both symptoms are common in soils with pH values of 4 to 5.2; yields are often greatly reduced, and many nutrients are rendered much less available for plant uptake. This is especially true for phosphorus but also the availability of calcium, magnesium, nitrogen, sulfur, potassium, and molybdenum (see Figure 1).
Between pH 5 and 5.5, no visual symptoms are likely, and plant growth may appear normal, but yield will probably be reduced by 10 percent or more. The nutrients listed above are more available than at a pH below 5 but are still reduced in availability. The efficiency of most added fertilizers, especially P, will be reduced. Fertilizer P efficiency will probably be reduced by 25 percent or more when compared to pH 6.5.
Corn grows well with little or no yield reduction between pH 5.5 and 6.0, but fertilizer efficiency is still reduced. The reduction in the availability of P will be in the 0 to 25 percent range when compared to pH 6.5.
Although corn can tolerate moderately acid soils, growers need to keep two points in mind. First, adding ammonical N fertilizer to corn greatly accelerates soil acidification. Second, in no-till corn fields where most of the N is added to the soil surface, the soil surface can become very acid (below pH 5) within three to four years. Once this happens, toxic amounts of aluminum and manganese are produced, and the triazine herbicides (atrazine and simazine) are rapidly degraded and do not provide adequate weed control.
At pH 7.0 or above, manganese and zinc may become deficient. For example, zinc deficiency of corn has been observed in Kentucky soils at these pH levels, especially when available P is also high.
The best liming program for corn involves a soil test every two years and lime applied according to soil test recommendations. On the average, one can expect the need for about one-half ton of lime per acre per year, but this is usually added at a rate of 2 to 3 tons per acre every three to six years.
Figure 1. Symptoms of corn growing in low pH soils.
The most important source of lime for agricultural use is ground limestone called agricultural lime. The quality of agricultural lime is determined by its purity and fineness of grind. The Kentucky lime law specifies that agricultural lime must be 80 percent pure (calcium carbonate equivalence) and must be ground fine enough that 90 percent will pass a 10-mesh screen and at least 35 percent will pass a 50-mesh screen. This is a minimum standard for the lime to be effective in neutralizing soil acidity.
Relative neutralizing value (RNV) estimates the percent of agricultural lime that will dissolve in a three- to four-year period. The higher the RNV, the higher the lime's quality. Lime whose RNV is 80 will require a smaller amount to reach a desired pH than one whose RNV is 60. The average RNV in Kentucky is about 67, and this is the basis for University o f Kentucky's lime rate recommendations. County Extension agents have information on the RNV levels for sources of agricultural lime being sold in Kentucky.
Other liming materials are sometimes available in an area. These are usually by-products of industry or are liquid suspensions of finely ground limestone. Use of these materials should be based on their purity (expressed as percent CaCO3) and fineness. With suspensions, the actual amount of lime in the mix determines the liming value. For example, a ton of lime suspension may contain only 1,000 pounds of lime. The rest is water and suspension agent. Specialty products like bagged, finely ground limestone, pelletized lime, hydrated lime, ground oyster shells, and others are available. These are usually more expensive but are convenient to use on small areas. Be careful in using these products so that an area is not over-limed.
When cornfields are limed, enough should be used to raise the soil pH to the mid-6 range (pH 6.2 to 6.4 is suggested by the University of Kentucky). The exact amount needed is largely due to the amount of reserve acidity that is held on the soil particle surface as measured by the buffer pH. By knowing the buffer pH together with water pH, the amount of ag lime necessary to raise the soil to pH 6.4 can be determined. The rates can be found in the Extension publication Lime and Fertilizer Recommendations (AGR-1).
The adjustment of soil pH by lime is affected by five factors:
When applying lime rates greater than 4 tons per acre, the lime should be thoroughly mixed in the plow layer by applying one-half the recommended rate before plowing and the other half after plowing, followed by disking.
Lime can be applied at any time. With adequate soil incorporation and moisture, a measurable pH change can occur within 4 weeks. However, it takes six to 12 months for a significant amount of the lime to dissolve and make the desired change in soil pH. For this reason, lime should be applied at least six months before the target crop is to be planted. Fall is a good time to apply lime so dissolution can occur during the winter. Also, fall weather is usually better for getting on the land with spreading equipment.
Nitrogen is the fertilizer element required in the largest amounts, and at the greatest cost, for corn production. Each bushel of grain harvested will contain almost a pound of N. Properly fertilized silage corn removes slightly more than 10 pounds of N for each 1,000 pounds of dry matter. Availability of N in most soils is too low to supply all the N required for optimal corn production without fertilizer N. Recommended rates take into account that only one-third to two-thirds of the fertilizer N added is recovered in the harvested corn.
Nitrogen recovery is variable and largely unpredictable. This is primarily due to the powerful effect of weather on the release of native soil N and on the fate of fertilizer N. This makes it impossible to precisely predict the quantity of N required for maximum yield or maximum economic return and is the reason that meaningful soil tests for N availability are not very useful for most situations in Kentucky.
Neither the amount of organic matter nor the amount of soil nitrate has proven to be a reliable indicator of the available N for field crops grown under Kentucky conditions. For this reason, N recommendations for field crops are based on past cropping history, soil management, and soil properties.
Young corn plants show a general chlorosis or yellowing of the entire plant when N is limiting. Under severe early growth deficiency, the bottom leaves may "fire" and desiccate. If N supply becomes limiting after stalk elongation begins, through the remainder of the growing season N is translocated from the most mature leaves at the lower stalk positions to the newer leaves or the ear. This causes the lower leaves to show a characteristic "V"-shaped yellowing extending from the leaf tip along the midrib toward the stalk, with the open end of the "V" at the leaf tip. The effect on growth and grain yield can range from stunted, chlorotic plants, which may not even form an ear, to normal-appearing plants with ears that do not have fully formed kernels toward the tips of the ears (see Figure 2).
Figure 2. Corn leaf with nitrogen deficiency symptoms (right) and a normal leaf.
The absolute amount of N needed during the first few weeks of growth is small and uptake is slow. Uptake progressively increases as the plant becomes larger with rapid uptake of N beginning about 3 weeks before tasseling. Most of the N taken up will be held in the leaves until grain formation begins. After grain formation begins, there is translocation of much N from other plant parts to the ear. About half the total N uptake occurs by the time of pollination.
Organic soil N is found in large quantities in virtually all soils. A soil that has 3 percent organic matter contains more than 3,000 pounds of organic N per acre. However, only a small part of this, 1 to 5 percent each season, is broken down to inorganic N forms that are available to plants. A greater rate of N release can be expected from fresh plant residues and from plowed-down or killed grass and legume sods. For this reason, cropping history is an important consideration when estimating fertilizer requirements. Inorganic ammonium (NH4+) is either released by organic matter decomposition or added as fertilizer. Ammonium is a relatively immobile ion, and it is not susceptible to leaching or denitrification as is nitrate (NO3-). Corn takes up NH4+ less readily than NO3-. In most Kentucky soils suitable for corn production, NH4+ is rapidly converted to NO3- in a process called nitrification. This reaction is largely completed shortly or within 30 days after fertilization.
Nitrate is a highly mobile ion because its solubility in water is essentially unlimited. It is readily available to plants but also is susceptible to leaching below the root zone, mainly in well-drained soils subjected to long-lasting or very intense rainfall. Denitrification loss of nitrate N is a microbiological transformation that can proceed very rapidly when soils become saturated with water. Therefore, it is most important in soils with impaired drainage.
Some nitrate is lost almost every year in all Kentucky soils, but such losses become serious when heavy rains or flooding occur within a month after fertilizer application. These losses result in more N fertilizer being needed on poorly drained soils. The tillage system also influences these processes. Denitrification, leaching, and immobilization can all be greater in no-till soils, so N rates should generally be slightly increased when using the no-till system.
A winter legume cover crop can also provide a substantial amount of N for corn, either with no-tillage or conventional tillage. Research conducted by the University of Kentucky indicates that some legume cover crops can provide yield advantages beyond N. Benefits from hairy vetch have been greater than from crimson clover or big flower vetch.
Most of the mixed fertilizers used in Kentucky contain some N, with the amounts varying depending on the grade. The first number in the guaranteed analysis of a fertilizer refers to the percentage of N. An 18-46-0 grade is 18 percent N and contains 18 pounds of N in each 100 pounds. Most of the N in mixed fertilizer is in the ammonium form. Diammonium phosphate and monammonium phosphate are also commonly available N-containing fertilizers in which all the N is in the ammonium form.
Fertilizers that contain only N are sometimes referred to as straight N fertilizers. They are marketed in both solid and liquid forms. Nitrogen materials commonly sold in Kentucky are discussed below.
Ammonium Nitrate (NH4NO3) is a solid N fertilizer that contains 33.5 to 34.5 percent N. One-half of the N is in the ammonium form, and one-half is in the nitrate form. Ammonium nitrate dissolves rapidly in the soil and is an excellent source of nitrogen, especially for surface-applied N on no-till corn.
Urea (CO(NH2)2)) contains 45 to 46 percent N in the solid form. When applied to the soil, the enzyme urease quickly converts urea N to ammonium. Consequently, urea N behavior in soil is essentially the same as that of ammonium except for the volatilization loss of NH3. The soil near the urea granule becomes alkaline, which favors the formation of NH3 gas from NH4+. A large fraction of the N sometimes can be volatilized as NH3 and lost to the air. Some of the factors that affect the amount of loss are temperature, tillage, vegetative cover, moisture, and soil pH.
If the urea is moved into the soil by a rain (0.25 inch is enough) or by tillage within two days after application, the volatilization loss is little or none. When the urea is applied before May 1, the loss is little to none, even without tillage or a rain within two days. However, after May 1 the volatilization N loss is about 5 percent or less if urea is applied to the surface of a tilled soil, although it can be higher if the soil pH is near 7 or above. If the urea is applied to the surface of a no-till field after May 1, the losses can range from 0 to 25 percent, but the average is about 10 percent. The higher losses come with a soil pH of 7 or above or if the soil is warm and moist but drying due to a good breeze. Surface application of urea to no-till corn after May 1 is risky.
Volatilization loss from urea can be greatly reduced or almost eliminated by the use of urease inhibitors with the fertilizer. Urease inhibitors are very effective, but their use is best justified economically with surface application of urea to no-till corn after May 1.
Nitrogen solutions contain N that range from 28 to 32 percent; 28 percent N solution is used in Kentucky because of its low salt-out potential. In N solutions most commonly used for direct soil application, one-half of the N is from ammonium nitrate, and one-half is from urea. Each gallon of 28 percent, 30 percent, and 32 percent N solution contains 2.98, 3.25, and 3.54 pounds of N, respectively. The volatilization losses of N from surface-applied N solutions are much smaller than from urea even though one-half of the fertilizer is in the urea form.
Anhydrous Ammonia (NH3) is the highest analysis N fertilizer available, containing about 82 percent N. At ordinary temperatures and pressure, it is a gas and must be kept under pressure to be stored as a liquid.
When anhydrous ammonia is released from pressure during application, the liquid immediately changes to a gas. For this reason, anhydrous ammonia must be injected 6 or more inches deep into the soil and then covered immediately to prevent loss of ammonia gas to the atmosphere. To prevent losses in no-tillage, extra sealing devices must be used. A winged or beaver-tail-shaped piece of steel on the injection knife is very helpful, but many times an additional device, such as a solid or spoked closing wheel or an inverted disc, is needed to close the knife opening. When injected into the soil, the ammonia molecule (NH3) reacts with water and becomes ammonium (NH4+). The positively charged ammonium ion is then held by soil particles until it is either converted to nitrate N by nitrification over a period of several weeks or is absorbed directly by plant roots or soil microorganisms.
The N in the injection band moves very little laterally, so the roots must grow to the vicinity of the injection band to come in contact with the N. Therefore, the plants may be N-deficient early in the growing season if root growth is slowed by cool and wet conditions or sidewall compaction. If some N is broadcast before planting or applied as in-row fertilizer, the potential for temporary N deficiency is often relieved.
Anhydrous ammonia can also be applied as a supercooled liquid. In this process, anhydrous ammonia is released and depressurized in a specially built converter that keeps 70 to 85 percent of it as a liquid during application. In this state, the anhydrous ammonia can be metered and calibrated much more accurately.
Ammonium sulfate (NH4)2SO4 contains about 21 percent N and 24 percent sulfur. All the N is in the ammonium form, which is temporarily absorbed by the clay and organic matter of the soil until it is nitrified to nitrate N or used by plants or microorganisms. Ammonium sulfate acidifies the soil much more quickly than other sources of N. It is not subject to volatilization loss.
Nitrate of Soda (NaNO3) contains 16 percent N, all of which is in the nitrate form and readily soluble in the soil solution.
The amount of N loss on wet soils depends on the source of N used, the time between N application and the onset of waterlogging, and the number of days the soil is saturated. Nitrogen can only be lost, due to excessive water, when the N is in the nitrate form and is leached or lost by denitrification. Denitrification is the more common cause of loss in Kentucky soils. The expected N loss from periods of heavy rains is found below.
These soils probably have not lost much N because it takes two to three days of saturated conditions to begin the denitrification process and these soils usually do not remain saturated between rains. There may be some exceptions here.
These soils stay saturated longer for several reasons, and the corn usually looks bad. The amount of N loss is still not as great as one might assume. A N rate of 50 pounds per acre probably would be the most a grower could justify adding to replace lost N in these situations. Replicated trials by the University of Kentucky in 1993 showed increased corn yields of 11 bushels per acre from sidedressing N under these conditions.
Since only nitrate is lost, we must first estimate the amount of applied N that was in the nitrate form at the time of flooding. Below are estimates of fertilizer in the nitrate form at 0, 3, and 6 weeks after application. It is estimated that 3 to 4 percent of the NO3-N in the soil will be lost by denitrification for each day the soil is saturated.
The NO3-N in a flooded sandy soil is leached more rapidly than other soils, and the nitrate level would be expected to be very low after the water recedes.
| N source | Week after application | ||
| 0 | 3 | 6 | |
| % Fertilizer as NO3-N | |||
| Anhydrous Ammonia (AA) | 0 | 20 | 65 |
| AA with N-Serve* | 0 | 10 | 50 |
| Urea | 0 | 50 | 75 |
| Urea with N-Serve* | 0 | 30 | 70 |
| UAN | 25 | 60 | 80 |
| Ammonium Nitrate | 50 | 80 | 90 |
| * Nitrification inhibitor that slows transformation of ammonium to nitrate. | |||
An additional tool for determining NO3-N in the soil after flooding is a NO3-N soil test. The sample should be taken down to 12 inches deep, and several samples should be taken in each field of both the low and higher ground. If the NO3-N is 0 to 10 ppm, a full rate of N for the crop potential should be added as a supplemental application. At 25 ppm, no additional N would be needed. One would extrapolate between these two figures, keeping in mind the amount of NH4 left in the soil from the first application.
There are two types of inhibitors. They are unrelated and are helpful in two totally different situations.
Nitrification Inhibitors: Nitrification inhibitors protect from loss of N due to excessively wet soils. They are most effective on N fertilizers that are mainly in the ammonium form, such as anhydrous ammonia, urea, and N solutions. When N in the ammonium form is added to soil, it is rapidly transformed to nitrate. Nitrification inhibitors slow the transformation for about 4 weeks. This keeps N as ammonium longer so that it is not likely to leach or be lost by denitrification due to excessive wetness. Economic benefits are more likely on poorly drained soils that usually remain wet during spring. The economics of the use of a nitrification inhibitor must be weighed against other methods, such as adding more N to offset the loss (about 35 pounds per acre) or sidedressing at least one-half of the nitrogen when the corn is 6 to 12 inches high.
Urease Inhibitors: Urease inhibitors protect against losses of N from urea-based N sources to the atmosphere (volatilization). The losses are greatest for surface applied urea on no-till corn. See the urea section for discussion of this.
Timing N Applications: Probably the most practical and effective method of increasing N recovery by corn is to delay or split the N application. This practice works because young corn plants (up to 4 to 6 weeks) require very little N, and in Kentucky most of that can be supplied by the soil. Also, soils are typically wettest and most prone to N losses early in the season. Delayed N is most beneficial where the potential for denitrification and leaching losses are greatest, particularly on poorly, somewhat poorly, and moderately well-drained soils. As a general guideline for these soils, if two-thirds or more of the N is applied 4 to 6 weeks after planting, the total N can be reduced by 25 to 50 pounds per acre. Fall application of N for corn is never recommended in Kentucky, and use of nitrification inhibitors with fall-applied N does not eliminate the sizeable overwinter N loss likely in Kentucky.
Placement of N Fertilizer: The application of N below the soil surface improves efficiency of N use in no-till corn but has very little benefit with tilled corn. When the N fertilizer is placed below the residue layer of no-till corn, the N is less likely to be immobilized in the residue layer as happens when fertilizer N is broadcast on the surface. Subsurface application reduces the amount of N required by 10 to 15 percent when compared to surface broadcasting under no-till conditions. More is discussed later under row fertilizers.
Recommended Rates of Nitrogen: Amounts of fertilizer N recommended in Kentucky are affected by cropping history, type of tillage, internal soil drainage, irrigation, and time of application. Recommended rates can be found in the University of Kentucky publication Lime and Fertilizer Recommendations (AGR-1).
Both phosphorus (P) and potassium (K) are required in large quantities for good corn growth and yield. A good yielding crop will take up to 50 to 70 pounds of phosphate (P2O5) and 130 to 170 pounds of potash (K2O) per acre (see Nutrient Content and Removal section). Of this total uptake, about three-fourths of the phosphate and about one-third of the potash is in the grain. The remainder is in leaves, stalk, roots, husks, and cob. So for a grain production system where all crop residues are left on the field, 40 to 50 pounds P2O5 and 40 to 50 pounds K2O per acre are removed from the soil each year. In silage production, all P2O5 and K2O taken up by the plant, except for that in the roots and stubble, are removed from the soil.
It is particularly important that adequate P2O5 and K2O be available for plant uptake during the first half of the season. By the time kernels start filling rapidly (70 to 75 days after seedling emergence and 10 to 15 days after silking), the plant will have taken up about 70 percent of its P2O5 requirements and nearly 90 percent of its K2O requirements.
Availability from Soil: Both P and K are considered immobile elements in the soil since they react with the soil in ways that minimize their movement with soil water. This is particularly true for P since, once in the soil, it forms compounds with calcium, iron, aluminum, manganese, and zinc, which are less soluble than the P compounds in the fertilizer. If soil pH is in the range of 6.0 to 6.5, much of the fertilizer P will react to form calcium phosphates, which are more soluble than the iron, aluminum, and manganese phosphates that form at lower pH levels. Therefore, greater P availability is one benefit of good liming practices. Potassium is retained on clays and organic matter by cation exchange. Except for very sandy soils, soil cation exchange capacity is great enough to hold an adequate reservoir of readily available K+. For these reasons, leaching of P and K from Kentucky soils is of little importance. By comparison, loss of P and K by erosion of topsoil is of much greater concern.
Corn grown on fields being rotated from a tilled sod may respond less to P fertilization than expected from the soil test results. This is because P will be released as organic residues from the sod as it decomposes.
Requirements: The amount of P and K fertilizer required for good corn growth is directly related to the amount of plant-available P and K already in the soil. Using a reliable soil testing laboratory that makes fertilizer recommendations based on field-tested procedures is the best way to determine levels of plant-available soil P and K. The annual amount of P and K taken up by the plant from fertilizer is not likely to exceed 15 to 20 percent of the P or 25 to 40 percent of the K applied.
Sources: Commercial fertilizer is the most widely used source of P and K for corn production. The sources of P most commonly used are triple superphosphate (0-46-0), diammonium phosphate (18-46-0), monoammonium phosphate (11-48-0), and a wide array of other ammoniated phosphates, both liquid and dry. Most commonly used sources of fertilizer P are considered equally effective for agronomic purposes when used at recommended rates and properly applied. Solid and liquid forms of P are also considered equally effective.
Almost all K fertilizer used for corn is muriate of potash (0-0-60). Other available sources are sulfate of potash (0-0-50) and sulfate of potash magnesia (0-0-18, 11 S, 18 Mg). All are considered equally effective.
Organic sources of P and K such as animal manures and sewage sludge may also be used. Since their nutrient content varies, analysis is necessary to determine appropriate rates. It is important to know the content of heavy metals (nickel, cadmium, and chromium) in municipal and industrial sludges in order to prevent toxic build-up.
Placement: Broadcasting P and K is the most convenient method of application, although at low to very low soil test levels, large amounts are required. Banded applications (2 inches to the side and 2 inches below the seed) can increase agronomic efficiency of P and K, making it possible to decrease the usual rate by one-third to one-half. A "starter" effect (improved initial growth) is likely to result from band placement. This may appear very significant during the early growing season, but in Kentucky it rarely increases yield, provided that broadcast P and K fertilizers are used at recommended rates.
Rates: Rates of phosphate and potash recommended by the University of Kentucky can be found in the AGR1 publication.
Magnesium levels in soils range from high (chiefly the loess-derived soils) to low (primarily some sandstone-derived soils). Soil test levels and recommended Mg rates can be found in AGR-1. Deficiency of Mg is rare in Kentucky and is most likely to be found on sandy soils.
Calcium deficiency of corn has never been documented in Kentucky. Despite concerns about reduced atmospheric sulfur fallout, no verified sulfur deficiency on corn has been recorded. University of Kentucky tests of sulfur application to corn during the 1990s and before did not show a yield response to its use. If deficiency occurs, it is most likely to be found on sandy soils.
Zinc deficiencies in corn are common in Kentucky in limestone soils, particularly when soil pH is above 6.5. The deficiency symptom most commonly noticed are broad whitish streaks down the leaves of young corn seedlings (see Figure 3). If corn plants are carefully removed from the soil and the stalk is carefully split all the way to the bottom tip of the plant, the presence of a purplish discoloration at the lower nodes is another distinctive indicator of zinc deficiency. If the deficiency is severe, seedlings may die. In mild cases, internode growth is limited, stunting plant height. Leaves may also show purplish edges, and ears may cup to one side and not fill completely. Where zinc deficiency of corn is suspected or has occurred previously, a zinc soil test is helpful in determining if zinc should be applied. A table found in AGR-1 lists soil test zinc levels at various soil pH ranges and soil test P levels below which a response to zinc fertilization is likely to occur. However, many other factors, including weather conditions, affect availability of soil zinc to corn, making it difficult to predict a response to added zinc for a specific growing season. Zinc fertilizer recommendations can be found in AGR-1.
Boron deficiencies in corn have been documented in Kentucky, but they are not common. Plant tissue analysis is the best way to test for this deficiency. If the ear leaf sample contains less than 5 parts per million (ppm) and the soil test value is less than one ppm, an application of 2 pounds of boron per acre might be beneficial.
Figure 3. Corn leaf with zinc deficiency (left) and a normal leaf.
Deficiencies of other nutrients such as manganese, iron, copper, molybdenum, chlorine, and cobalt are extremely unlikely for corn in Kentucky. If a problem is suspected, tissue analysis is recommended.
The use of row fertilizer and its potential benefits vary with conditions. The efficiency of fertilizer is greatly increased by banding fertilizer and is helpful on soils with a low soil test. In such cases, the rate of P and K can be reduced by one-third to one-half. For soils testing medium or high, a sufficient amount of P and K nutrients exists in the soil such that additional fertilizer applied near the row is not likely to increase yields. Regardless of soil test, banded fertilizer will usually increase the vigor and early growth of corn.
Yield increases may sometimes be achieved with starter fertilizer containing N and P, when placed beside or in the row, but they are not always economical. The consistency and amount of the yield increase response depends on soil type, tillage, planting date, and weather. Conditions that place the corn under prolonged stress early in the growing season increase the chances of a positive response and the amount of the response. The response is more consistent and larger for early planting of no-till corn on soils that are not well drained. Although not as consistent, responses to starter fertilizers are also found on early planted no-till corn on well-drained soils. Responses will be much smaller in warmer years and with later plantings. The average yield increase expected from row fertilizer is shown in Table 2.
| Table 2. Expected yield response to row fertilizer. | |||
| Tillage | Soil drainage | Consistency of response | Average yield* increase (bu/ac) |
| Tilled | All types | Occasional | 0-1 |
| No-tilled | Well-drained | Sometimes | 1-6 |
| No-tilled | Not well-drained | Most of the time | 5-7 |
| * The average yield response includes all yield responses, both positive and negative. There will be times when the yield increase is greater due to cooler and wetter years than normal, and in some unusual situations there can even be a negative response. | |||
Most of the response to starter fertilizer in Kentucky soils is response to N. The rest of the response can be achieved by adding P. Potassium has very little effect on the early growth. If the fertilizer is placed in the seed furrow, only 10 to 15 pounds per acre each of N and P2O5 are needed. Increasing the rate higher than this will not improve the starter effect and may adversely affect seed germination. If the fertilizer is banded beside the row (2" x 2"), research indicates that 20 pounds per acre each of N and P2O5 are needed to achieve an optimal effect.
To prevent germination and emergence problems, the amount of N plus K2O should be limited to no more than 15 pounds per acre (as shown from recent research) in the furrow and no more than 100 pounds per acre in a 2" x 2" placement beside the row. An N source that contains only urea adds additional risk due to high levels of ammonia generated in the placement area.
Plant analysis is the laboratory determination of nutrient elements on a sample of plant tissue. In recent years, this technique has been more frequently used to diagnose nutritional problems related to soil fertility or to monitor effectiveness of fertilizer practices on growing crops.
A plant analysis program is not a substitute for soil testing but is most effective when used in conjunction with a regular soil testing program.
The most common elements analyzed for plant analysis are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), boron (B), copper (Cu), zinc (Zn), and aluminum (Al). Others that may be measured either routinely or upon request include sulfur (S), sodium (Na), molybdenum (Mo), cobalt (Co), silicon (Si), cadmium (Cd), nickel (Ni), lead (Pb), chromium (Cr), arsenic (As), and selenium (Se). Although some of these are not essential for plant growth, the results may be used for interpretation and recommendations, or for identifying toxic levels of some elements. Considerable care must be given to collecting, preparing, and sending plant tissue to the laboratory for analysis.
Randomly sample plants throughout a uniform field or sampling area. When a nutrient deficiency is suspected or abnormal growth is present, collect one sample from the affected area and a sample from an adjacent normal area. Collect the plant tissue in a new, clean brown paper bag. Dusty or soil-covered leaves and plants should be avoided. If leaves have a slight dust cover, brush gently with a soft brush or perform a "quick rinse" with distilled water. Do not prolong the quick rinse or use a soap solution as nutrient elements will be leached out of the tissue. Do not include damaged, diseased, or dead tissue in your sample.
Good results require sampling a definite plant part. For corn less than 12 inches tall, cut 20 plants at 1 inch above the soil surface. For corn taller than 12 inches but which has not tasseled, pull the entire first mature leaf (completely unrolled) below the whorl from 20 plants. Fully developed plants should be sampled when 50 percent of the ears show silks. Sample the whole ear leaf (the leaf just below the ear) from 20 plants. Do not take samples after the silks have turned brown.
For diagnostic purposes, a good representative soil sample should also be collected. When problem areas exist in the field, take one sample from the affected area and one sample from an adjacent normal area. Take cores or subsamples adjacent to plants that are selected for tissue sampling.
Table 3 summarizes nutrient levels that would be considered sufficient. Levels below those shown might be insufficient for optimal yields.
| Table 3. Nutrient sufficiency levels for corn.1 | |||
| Nutrient | Type of sample | ||
| Whole plants
less than 12 inches tall |
Leaf below whorl, plants more than
12 inches tall |
Ear leaf at tasseling before silks turn brown | |
| N | 3.5-5.0% | 3.00-3.50% | 2.75-3.00% |
| P | 0.3-0.5% | 0.25-0.45% | 0.25-0.45% |
| K | 2.5-4.0% | 2.00-2.50% | 1.75-2.25% |
| Ca | 0.3-0.7% | 0.25-0.50% | 0.25-0.50% |
| Mg | 0.15-0.45% | 0.13-0.30% | 0.13-0.30% |
| S | 0.15-0.50% | 0.15-0.50% | 0.15-0.50% |
| Mn | 20-300 ppm | 15-300 ppm | 15-300 ppm |
| Fe | 50-250 ppm | 30-200 ppm | 30-200 ppm |
| B | 5-25 ppm | 4-25 ppm | 4-25 ppm |
| Cu | 5-20 ppm | 3-15 ppm | 3-15 ppm |
| Zn | 20-60 ppm | 15-60 ppm | 15-60 ppm |
| Mo | 0.10-10.0 ppm | 0.1-3.0 ppm | 0.1-3.0 ppm |
| 1 From Plant Analysis Handbook for Georgia. Bulletin 735. Univ. of Georgia Cooperative Extension Service, 1979. | |||
Estimated nutrient content of healthy, mature corn and the amounts of nutrients taken up are shown in Table 4. Data were provided by the University of Kentucky.
| Table 4. Nutrient content of and removal by corn plant parts. | |||||
| Plant part | Content (% by dry weight) | ||||
| N | P1 | K2 | Ca | Mg | |
| Grain | 1.30 | 0.28 | 0.50 | 0.12 | 0.16 |
| Stover | 0.70 | 0.15 | 1.20 | 0.37 | 0.16 |
| Plant part | Unit | Removal (lb/unit) | ||
| N | P2O5 | K2O | ||
| Corn Grain | Bu. | 0.7 | 0.4 | 0.35 |
| Corn Silage | Ton | 7.5 | 3.6 | 8.0 |
| Corn Stover | Ton | 15 | 7 | 30 |
|
1 P x 2.29 = P2O5 2 K x 1.2 = K2O |
||||
J. D. Green and James R. Martin
The most economically important pests that reduce corn yield each year are unwanted plants that interfere with corn growth, development, or harvest. These plants, called weeds, compete with corn for water, light, and soil nutrients to reduce crop yield. Some weeds are capable of naturally releasing substances into the soil that are allelopathic, or toxic, to the crop. Weeds can serve as hosts for corn diseases, such as the maize dwarf mosaic and maize chlorotic dwarf virus complex (MDM/MCD) on johnsongrass rhizomes, which can be vectored and transported by insects to corn plants, thus reducing crop yield. Weeds also provide shelter and serve as a food source for insects and diseases that overwinter or provide habitat for wildlife species such as prairie voles that reduce corn stands.
A number of decisions must be considered in developing a successful weed control program. To assist in weed management decisions, a corn producer must be able to properly identify the specific weed problems in each field along with other aspects and factors that might influence weed emergence and growth. It is also important to understand the life cycle of weedy plants, their growth habit, and their potential competitiveness or impact on the crop.
Weeds can be grouped into three major categories. Annuals complete their life cycle in one growing season and reproduce only by seed. Summer or warm-season annuals, such as large crabgrass (Digitaria sanguinalis) and common cocklebur (Xanthium strumarium), germinate in the spring and set seed in late summer or fall. These plants are more likely to directly compete with the corn. Winter or cool-season annuals typically germinate in the fall and complete their reproductive cycle in the spring or early summer. Therefore, cool-season annual plants, such as common chickweed (Stellaria media) and Italian ryegrass (Lolium multiflorum) are generally more of a concern at the time of planting and during the early stages of corn growth in no-till corn production.
Biennials are capable of completing their life cycle during two growing seasons. The first year normally consists of vegetative growth, whereas the second year involves both vegetative and flower development. Biennials, such as musk thistle (Carduus nutans), reproduce only by seed. Sometimes these plants may complete their life cycle within one year.
Perennial plants are capable of existing for more than two years. Reproduction can be by seed and by vegetative structures such as rhizomes, stolons, tubers, taproots, or creeping roots. For example, johnsongrass (Sorghum halepense) plants frequently encountered in corn fields emerge from seed; however, johnsongrass plants are capable of emerging from rhizomes. Warm-season perennial weeds have become of increasing concern as no-tillage practices have increased in Kentucky's crop production systems. Ten of the most common and troublesome weeds found in Kentucky corn fields are listed in Table 1.
| Table 1. Common and troublesome weeds and their life cycle in Kentucky corn fields. | ||||
| Weed species | Life cycle | Primary reproduction | Native/
Introduced |
|
| 10 Most Commonly Occurring Weeds | ||||
| smooth pigweed | SA | seed | N | |
| giant foxtail | SA | seed | I | |
| large crabgrass | SA | seed | I | |
| johnsongrass | P | seed, rhizome | I | |
| morningglory (ivyleaf & pitted) | SA | seed | I | |
| honeyvine milkweed | P | seed, creeping root | N | |
| fall panicum | SA | seed | N | |
| common cocklebur | SA | seed | N | |
| giant ragweed (horseweed) | SA | seed | I | |
| yellow nutsedge | P | tuber, rhizome, seed | N | |
| 10 Most Troublesome Weeds to Control | ||||
| honeyvine milkweed | P | seed, creeping root | N | |
| broadleaf signalgrass | SA | seed | N | |
| burcucumber | SA | seed | N | |
| trumpetcreeper | P | creeping root, seed | N | |
| giant ragweed (horseweed) | SA | seed | I | |
| johnsongrass | P | seed, rhizome | I | |
| common pokeweed | P | seed, taproot | N | |
| ivyleaf morningglory | SA | seed | I | |
| fall panicum | SA | seed | N | |
| Italian ryegrass | WA | seed | I | |
| Life cycle: SA = summer or warm-season annual; WA
= winter or cool-season annual;
P = warm-season perennial. Origin: I = introduced plant; N = native plant. |
||||
Proper weed identification is an essential component of any successful weed management program. It is even more critical in no-tillage systems because herbicides are the primary method of weed control. Training and a skilled eye are often needed to properly identify weeds during early vegetative growth stages. In fact, an effective postemergence control strategy for weeds often depends on proper identification when weeds are less than 4 inches tall. Thus, field scouting should begin within 2 weeks of corn planting and continue at weekly intervals for 8 to 10 weeks into the growing season. Scouting methods recommended for weeds in corn can be found in Kentucky Integrated Crop Management Manual for Field CropsCorn (IPM2) available at your county Extension office.
A history of previously known weed problems in a field greatly aids in preparing an overall weed control strategy at the beginning of the growing season. Knowing the previous field history can also provide insight on their identity when weeds emerge. A good method for developing a field history of weed problems is by mapping weeds from previous and current field scouting reports and from observations made at harvest. A detailed weed map for each field will provide information on the location of weed infestations and help monitor changes in these infestations from year to year.
An economic threshold exists when a weed population reaches a level whereby it becomes economically justified to control because of
