ID-125
It is important to recognize that the types of cropping practices used in Kentucky and surrounding states are unique compared with other regions where wheat is produced. No-tillage is increasing in acreage in wheat production and is already widely used in the production of corn and soybeans. Wheat is usually planted in the fall after corn and then harvested in the spring before double-crop soybeans.
The spectrum of weeds found in conventional and no-tillage plantings of wheat are similar; however, there are some species that tend to be more troublesome where no-tillage practices are used. Wild garlic is a perennial that is widely recognized for its aerial bulblets that contaminate wheat during harvesting. Larger populations of wild garlic are usually present in no-tillage programs compared with programs that use plowing and disking for seedbed preparation. Common chickweed and henbit are cool-season broadleaf weeds that can also occur in wheat. These broadleaf weeds can reduce potential wheat yield by 20 percent and are especially troublesome in no-tillage plantings if they are not controlled in the fall.
The unique rotation system used in Kentucky and neighboring states provides opportunities for weeds to compete in wheat as well as other crops. Italian ryegrass is a cool-season weedy grass that can cause almost complete yield loss of wheat and compete in spring months with no-till corn and soybeans. Common and giant ragweeds and johnsongrass, which are warm-season weeds, can emerge in late spring and develop enough green vegetation to interfere with wheat harvest as well as compete with double-crop soybeans after wheat harvest.
Another unique feature about growing wheat in a rotation with corn and double-crop soybeans is associated with the risk of crop injury caused by carryover of herbicide residues. Growers must use caution in selecting herbicides that do not persist in soil for long periods and result in injury to rotational crops.
Periodically monitoring fields helps detect problems before weedy plants become too large to effectively control. Critical periods for monitoring weeds are:
Scouting procedure for weeds in wheat involves:
Follow these steps for surveying your field:
Step 1. Randomly select survey sites so they are representative of the entire field. Do not survey within 100 feet of
a fence or roadway. As Table 6-1 indicates, the minimum number of sites varies according to field size. At each
survey site, walk forward 60 feet (approximately 20 steps) and observe for weeds occurring within 5 feet on either side
(see Figure 6-1). Each site should be approximately 600 square feet.
| Table 6-1. Minimum number of survey sites based on field size. | |
| Field size (acres) | Number of survey sites |
| 1 - 20 | 3 |
| 20 - 30 | 4 |
| 30 - 40 | 5 |
| 40 - 50 | 6 |
Step 2. Collect the following information:
Record the average size of each weed species present. This information can help you select herbicide options and determine when to treat.
Although economic thresholds for weeds in wheat are not well defined, you might find some general guidelines helpful. Table 6-2 gives treatment guidelines relative to the estimates for weed cover and wild garlic counts. These guidelines are general and vary depending on several factors, including weed species, cost of treatment, and price of wheat.
| Table 6-2. Treatment guidelines. | |||
| Infestation level a | |||
| Total weed cover | Wild garlic counts per 600 sq. ft. | Treatment guideline | |
| Light | 1%-5% | <3 plants | Probably no economic benefit to treat. |
| Moderate | 6%-30% | 4-25 plants | Treatment may or may not be justified. |
| Severe | >30% | >25 plants | Treatment may be justified if implemented in a timely manner. |
| a The infestation level is the total weed cover or wild garlic counts averaged across all survey sites. In some instances, the average infestation level may suggest that there is no need for treating, yet a few sites were heavily infested and warranted treatment. It may be feasible to spot-treat portions of the field where severe infestations are present. | |||
A weed map, the second step in the scouting procedure, can help chart special weed problems and may isolate areas of the field that need treatment. The map can also be a useful reference for planning future weed management programs. Figure 6-2 is a sample weed map of a wheat field.
Follow these procedures to create a weed map:
Step 1. Outline the general shape of the field and indicate approximate locations of fences, roads, buildings,
woods, etc.
Step 2. Mark the approximate locations of severe weed infestations or weeds not listed on the survey form, and
mark the locations where you make your counts.
Step 3. Indicate any weed problems on the map that will help in making management decisions.
Step 4. Draw this map each time you scout the field.
Correctly identifying weeds during their early stages of development helps in properly selecting and initiating control strategies. Many weed species look similar during early stages of development. Vegetative characteristics such as shape, color, and arrangement of leaves, and location of pubescence (hairs) can aid in identification, providing these characteristics remain consistent under a wide variety of conditions. However, it is not unusual for these vegetative characteristics to vary for some weed species, so they are not always reliable for identification. See Figures 6-3 and 6-4 for descriptions and visual aids to be used in identifying weed species.
Figure 6-1. Surveying diagram.
Figure 6-2. Sample weed map.
Figure 6-3. Key identifying characteristics of a few major problem weeds that occur in wheat in Kentucky.
Plate 6-1.
Henbit
(Lamium amplexicaule)
Weed control is important in maintaining high quality and yield potential in wheat. An effective overall weed management program for Kentucky wheat involves a combination of cultural and chemical practices.
Establishing and maintaining a competitive wheat stand contributes to weed control. A seeding rate that results in 25 wheat seedlings per square foot is ideal for achieving optimum wheat yields and often limits the amount of weedy vegetation and weed seeds. Planting wheat in narrow rows increases the likelihood for achieving early-season shading and competition to weeds compared to wheat planted in wide rows. Applying nitrogen at recommended rates and times can promote tillering of wheat and limit the presence of warm-season weeds that affect harvest.
Crop rotation often reduces weed populations. For example, infestation levels of wild garlic, common chickweed, and henbit tend to be lower following corn than soybeans. A rotation of corn/wheat/double-crop soybeans is common in Kentucky and is often more favorable for managing weeds in wheat than a soybean/wheat/soybean rotation.
Disking prior to wheat planting can remove emerged cool-season weeds. The benefits of tillage are greater if weather conditions prior to tillage are favorable for germination of weed seed. Practices that use plowing and disking for seedbed preparation for wheat can reduce wild garlic infestations by 50 percent compared to no-tillage production practices.
Tillage prior to planting corn or soybeans can also be beneficial in controlling broadleaf weeds, such as common chickweed or henbit. This practice limits production of weed seed by destroying weedy plants before they mature.
Plate 6-9.
2,4-D or Banvel (dicamba) injury. Wheat treated during boot stage of growth with auxin-type herbicides may result in trapped
heads, missing florets, or twisted awns.
Plate 6-10.
Atrazine or Princep (simazine) carryover. Wheat plants emerge, then die back from leaf tips of oldest leaves.
Plate 6-11.
Command (clomazone) carryover. Wheat plants emerge and often have chlorotic or bleached appearance. Plants may recover
from early-season injury.
Herbicides play a major role in managing weeds in wheat. Herbicide recommendations for wheat production are discussed in the Cooperative Extension publication Chemical Control of Weeds in Kentucky Farm Crops (AGR-6). Always read and follow the restrictions and precautions stated on the label of the herbicide container.
You might want to consider the following issues when using herbicides:
Burndown herbicides. Fields planted to no-till wheat often require a burndown herbicide to control weeds
before wheat emerges. Spray volume and weed size are two key factors to consider when using burndown
herbicides. Gramoxone Extra, a contact herbicide, is usually applied in 20 to 40 gallons of clean water or clear liquid
fertilizers. Gramoxone Extra is normally applied at 1.5 to 2 pints per acre for 1- to 3-inch-tall weeds, 2 to 2.5 pints per acre for
3- to 6-inch-tall weeds, and 2.5 to 3 pints per acre for 6-inch-tall weeds.
Roundup Ultra, a translocated herbicide, should be applied in 10 to 20 gallons of water per acre at a rate of 1 quart per acre for weeds less than 6 inches tall and 1.5 quarts per acre for weeds taller than 6 inches. The rate of Roundup Ultra also may vary depending on weed species and carrier volume.
Fall and spring applications. Most herbicides are applied postemergence to emerged wheat and weeds in mid to late November or in early March to early April.
Wheat is tolerant to fall or spring applications of such herbicides as Buctril, Hoelon, or Harmony Extra, providing the wheat is within the recommended stage of growth. Banvel or 2,4-D is traditionally applied after wheat has overwintered and is fully tillered. The risk of wheat injury from 2,4-D or Banvel is least when applications are made after wheat has fully tillered but has not yet jointed (i.e., Feekes 4 or 5). See plate 6-9 for injury symptoms when 2,4-D or Banvel is applied during the boot stage of wheat. Avoid applying herbicides to wheat that is stressed from cold temperatures or during extreme day and night temperature fluctuations because crop injury can occur.
Some weeds, such as common chickweed, henbit, and ryegrass, are best controlled when plants are small and actively growing. Weedy grasses are especially difficult to control if they have tillered or overwintered. Also, weeds that are stressed from cold temperatures might not be effectively controlled. If conditions during the fall are favorable for weed growth, fall application of a herbicide may be necessary, especially in no-till plantings.
Wild garlic emerges during the fall and winter months. Achieving optimum control of this weed is important; therefore, growers tend to delay herbicide applications until late winter or early spring to ensure that most of the population of wild garlic plants has emerged.
Preharvest treatments. Some weed species that emerge in wheat can cause problems during wheat harvest and/or in double-crop soybeans, particularly when weather conditions or other factors cause wheat stands to be poor. Pennsylvania smartweed, horseweed (also referred to as marestail), ragweeds (common and giant), and johnsongrass can impede wheat harvest and compete for soil moisture in double-crop soybeans. Roundup Ultra and certain formulations of 2,4-D are examples of herbicides registered for preharvest weed control in wheat. The response of weeds to these herbicides is slow and does not occur as rapidly as with certain harvest-aid applications used in other crops. Drift to nearby sensitive crops is a concern when using these treatments. Preharvest treatments can injure wheat or reduce seed germination or seedling vigor and are generally not recommended for wheat grown for seed production.
Tank mixing. Tank mixing of herbicides with one another or with other chemicals, such as fertilizers or additives, can sometimes lead to problems with application or crop injury. Consult the label(s) for potential problems with physical compatibility of the mixture as well as the potential for crop injury or poor weed control. Also, be certain the application timing is within the recommended period for all tank mix components.
The following are examples of tank mixing problems associated with Harmony Extra:
Herbicide-resistant weeds. Biotypes of some weed species in other states have been identified as resistant to sulfonylurea herbicides, such as Harmony Extra. A biotype is a naturally occurring individual of a species that often looks the same but has a different genetic makeup than other individuals of the species. The difference in genetics among biotypes within a species accounts for the presence of herbicide-resistant weeds.
The potential for weed resistance to develop increases with repeated use of herbicides that have the same mode of action (i.e., similar chemistry). Therefore, monitor herbicides used in all rotational crops and use production practices that prevent or reduce the potential for the development of herbicide-resistant weedy biotypes.
Carryover. Some herbicides used in rotational crops can persist in the soil and injure wheat. Princep or atrazine used in corn or Command used in soybeans or tobacco, for example, have the potential to carry over in soil and injure wheat (plates 6-10 and 6-11).
Certain wheat herbicides also can persist long enough in soil to injure rotational crops. For example, Peak can remain active in soil for several weeks and injure double-crop soybeans. Label directions for Harmony Extra prohibit the planting of any rotational crop until 60 days following the application of this herbicide. Also, the rotational restrictions for planting soybeans after a preharvest treatment of 2,4-D vary from seven to 30 days, depending on formulation and rate of herbicide.
Many problems associated with herbicide carryover can be avoided by following restrictions indicated on the herbicide label(s).
Harvesting and grazing restrictions. Most herbicides used in wheat have label restrictions regarding use of the crop as grain or for forage purposes. The EPA has established these restrictions to prevent illegal residues in the harvested grain or forage for livestock feed. When more than one product is included in the spray tank mixture, follow the label that is most restrictive.
Cleaning of spray equipment. If spray equipment is not rinsed properly, herbicide residues can accumulate in the spraying system and dislodge in subsequent applications, causing injury to susceptible crops. Check the herbicide label for recommended procedures for cleaning equipment. The procedures may appear cumbersome but are often necessary to remove small amounts of herbicide that could injure other crops.
