Can
Aphid Control Reduce Barley Yellow Dwarf Incidence in Wheat?
(A case study - Caldwell Co., Ky 1998-99)
Calibrate
Drills to Control Seed Costs
Grain producers often express concern about herbicide carryover to cause injury to rotational crops during periods of dry weather. Parts of Kentucky have experienced below average rainfall this year, and as the wheat planting season approaches, wheat producers should consider if some fields have the potential for herbicide carryover problems.
The potential for herbicide carryover depends on the herbicide degradation rate, the amount of herbicide applied per acre, and the sensitivity of a rotational crop, such as wheat, to the herbicide applied to the previous crop. The duration of herbicide persistence is determined by the rate at which the herbicide degrades in the soil. This rate of degradation depends on the chemical structure of the herbicide, and soil characteristics such as pH, clay content and organic matter. Climatic conditions play a large role in regulating this rate of degradation. Maximum chemical and microbial degradation occurs in warm, moist soils. Extreme conditions, such as long periods without rain, slow the degrading process.
Herbicide carryover to cause rotational crop injury has not been a widespread problem in Kentucky during the past 25 years. Instances of carryover injury have occurred each year and will occur again this year. What is the likelihood of herbicide carryover problems in wheat this planting season? Unfortunately, there is not a simple answer to this question.
According to the rotational intervals on product labels, those herbicides with the greatest risk of carryover to wheat in Kentucky include those with the active ingredient atrazine (AAtrex), simazine (Princep), or clomazone (Command). The rotational intervals are based on risk of crop injury and residue tolerances for feed or food.
Injury is the issue in cases with atrazine and simazine, whereas, both crop injury and tolerances for feed and food uses of wheat are the issues dealing with clomazone. Historically, herbicide injury to wheat has occurred only in instances of where the total amount of atrazine and simazine exceeded 3 pounds active ingredient (ai). Rates of atrazine and simazine this high have not been used for many years. As the amount of atrazine and simazine has declined, so has the occurrence of wheat injury from these commonly used corn herbicides. However, this is not to say that carryover injury to wheat will not occur this year.
The following items should be considered in determining the potential for herbicide carryover injury to wheat.
Rainfall. The most important time of rainfall for herbicide degradation is the first month following herbicide application. Fields receiving normal rainfall during this time will be much less likely to have herbicide carryover.
Soil pH. If the pH is above 7.0, the likelihood of atrazine and simazine carryover will increase greatly. Conversely, carryover of clomazone will increase when the soil pH is below 6.
Herbicide rate and timing. Fields in which atrazine, at less than 2 LB ai per acre, was applied in April and had normal, or near normal rainfall, will usually not persist to cause wheat injury. The later in the season that atrazine was applied, the greater the potential for wheat injury. Many atrazine-containing products recommend rotating to only corn or sorghum when applications are made after June 10. Remember that atrazine is an ingredient in many corn herbicides (see AGR-6, Chemical Control of Weeds in Kentucky Farm Crops).
Tillage. Any herbicide remaining in the soil will be distributed throughout the zone of tillage and this results in the herbicide concentration being diluted in this tilled zone. This dilution decreases the potential for herbicide injury but does not eliminate the injury potential.
Date of wheat planting. Fields suspected of herbicide carryover should be planted as late as possible.
Herbicides of concern. The following
herbicides should be considered the most likely to cause carryover problems
in wheat.
CORN HERBICIDES:
Atrazine (especially at 2 lb ai or more)- Products containing atrazine include:
AAtrex, Atrazine, Bicep II, Bicep II Magnum,
Buctril/Atrazine, Extrazine II, FieldMaster, Fultime, Guardsman, Laddok,
Leadoff, Marksman, Surpass 100
Princep (especially at 2
lb ai or more)
SOYBEAN HERBICIDES:
Command
More specific information on herbicide persistence and carryover can be found in the following University of Ky publications:
AGR-6, Chemical Control of Weeds in Kentucky Farm Crops
AGR-139, Herbicide Persistence and Carryover in Kentucky
AGR-140, Herbicides with Potential to Carryover
and Injure Rotational Crops in Kentucky
Pioneer 2510 wheat was planted using a
no-till planter on 22 Oct 1998 following a corn crop on the University
of Kentucky Research and Education Center in Caldwell Co. KY. The 4' by
15' plots were arranged in a randomized complete block design with five
replications. Fertility was applied as 100 lbs of nitrogen on 26 Feb 99
(Feekes GS 3-4). The treatments included three different insecticide application
dates and an untreated control. Two treatments consisted of single applications
of Warrior ® (lambda-cyhalothrin) at 3.2 fl. oz./ac, made with a backpack
sprayer in 26 gal of spray per acre, on 24 Nov 98 (Feekes GS 2-3 ) or 17
Feb 99 (Feekes GS 3). The third set of plots were treated on both dates.
These were compared to an untreated control. Regular aphid counts were
not made but plots were checked for aphids just before applications were
made. Plots were rated for BYD on 5 May 99 (Feekes GS 10) by randomly selecting
50 individual plants and examining them for symptoms. Percent of plants
displaying BYD symptoms were analyzed for differences using the SAS GLM.
procedure.
Significant differences in percentages
of plants displaying BYD symptoms, as related to insecticide treatments,
were detected (F (3,12 df) = 3.83, Pr>F =0.039) (Table 1). Although very
few aphids were seen before the final insecticide application; they were
widespread and numerous during the spring.
Table 1. Mean percentages
(± s.e.) of wheat plants showing BYD symptoms in plots treated with
Warrior insecticide on selected dates to control aphid vectors of barley
yellow dwarf virus.
| Time of Application | % of Plants Showing BYD Symptoms ± SE1 |
| No Insecticide | 13.2 ± 5.0 a |
| 24 Nov 98 | 5.6 ± 1.0 ab |
| 24 Nov 98 and
17 Feb 99 |
1.6 ± 0.4 b |
| 17 Feb 99 | 3.2 ± 1.2 b |
Variations in plant stands among plots due to establishment problems prevented valid yield comparisons. The variation due to stand difficulties would not have allowed a fair comparison of the yield effects.
The November treatment, often made as an 'insecticide only' application, costs about $11.00 per acre. The February insecticide application is often made in conjunction with other inputs, so the application cost may be saved. Therefore, in this location and in this year, the fall, winter, and combination treatments would have cost $11.00, $6.00 and $17.00 respectively.
Assuming the entire difference in percentage
of plants showing BYD symptoms was a result of insecticide timing, and
that a damaged plant would have about a 20% yield loss, we can compare
the relative merits of treating -vs- not treating.
No Insecticide Treatment
Using an estimate of 13.2% damaged plants
with a 20% yield reduction for each damaged plant, the effective yield
loss was calculated to be 2.64%. If this were 100 bu/acre wheat, the resulting
loss would be 2.6 bushels. At a price of $2.50 / bushel, the untreated
acre of wheat would bring about (97.4 bu at $2.50/bu) $243.50 or a loss
of $6.60 per acre due to this aphid-vectored disease.
24 Nov & 17 Feb Insecticide Treatment
The best insecticide treatment (two applications)
contained an average of 1.65% damaged plants. This indicates that about
88% of the loss to BYD was prevented by the two treatments. As calculated
above, this is a 0.3% yield loss per acre. For 100 bu/acre wheat, this
loss would be 0.3 bushel, leaving a per acre yield of 99.7 bushels. At
$2.50/bu the resulting loss would be $0.75, bringing a per acre return
of (99.7 bu at $2.50/bu) $249.25. However, this level of protection was
obtained by making two insecticide applications, at a cost of about $17.00
per acre. Reducing the per acre return by this cost leaves a net return
of ($249.25 - $17.00) $232.25.
24 Nov. Only Insecticide Treatment
The 24 Nov. treatment had 5.6% damaged
plants. Assuming the standard plant yield loss, this is the equivalent
of a 1.1% yield loss per acre. For 100 bu/acre wheat, this loss would be
1.1 bushels, leaving a per acre yield of 98.9 bushels. At $2.50 /bu the
resulting loss would be $2.75, bringing a per acre return of (98.9 bu at
$2.50 /bu) $247.25. However, this level of protection was obtained by making
an insecticide applications which would cost about $11.00 per acre. Reducing
the per acre return by this cost leaves a net return of ($247.25 - $11.00)
$236.25.
17 Feb. Only Insecticide Treatment
The incidence of damaged plants in the
17 Feb. treatment was 3.2%. For 100 bu/acre wheat, this loss would be 0.6
bushels, leaving a per acre yield of 99.4 bushels. At $2.50/bu the resulting
loss would be $1.50 bringing a per acre return of (99.4 bu at $2.50/bu)
$248.50. However, this level of protection was obtained by making an insecticide
applications which would cost about $6.00 per acre. Reducing the per acre
return by this cost leaves a net return of ($248.50 - $6.00) $242.50.
Summary
Under these test conditions, the insecticide
applications did cause statistically significant differences in BYDV symptom
expression. However, it is clear that the assumed associated protection
of yields resulting from this level of symptom reduction was not cost effective.
If all other things are equal, the cost of the insecticide applications
was greater than the reduction in damage (Table 2).
Table 2. Net return ($/ac)
from plots treated at selected times with an insecticide application to
control aphid vectors of BYDV in Caldwell County, KY, 1999
| Treat. | No-Insect. | 24 Nov
& 17 Feb |
24 Nov | 17 Feb |
| Net Ret/ac | $243.50 | $232.25 | $236.25 | $242.50 |
The circumstances and yield potential on
your farm will alter these figures. As prices and yields decline and treatment
costs increase, the insecticide treatments will look even less appealing.
However, a rise in prices and yields coupled with a lower treatment costs
will make the returns from insecticide applications look much more favorable.
Choosing a 100 bushel per acre yield as
a basis for comparison may be misleading. 'Intensive Wheat Management'
has used 100 bushels as a benchmark; however, many fields will not support
this level of production. When yields change so do the level of expenses
that can be supported. Using the percent damage estimates, and assumed
costs of control from the previous examples we have calculated the necessary
value of a bushel of wheat needed to support the three treatments at various
yield levels, using the BYD intensity seen in the 1998 experiment (Table
3).
Table 3. The Value ($)
of a bushel of wheat required to offset the costs of various insecticide
treatments.
| Potential Yield
(Bu/Ac) |
Fall Treatment
@ $11/Ac. |
Winter Treatment
@ $6/Ac. |
Fall & Winter Treatments @ $17/Ac. |
| 100 | 7.23 | 3.00 | 7.35 |
| 90 | 8.03 | 3.33 | 8.17 |
| 80 | 9.04 | 3.75 | 9.19 |
| 70 | 10.33 | 4.29 | 10.49 |
| 60 | 12.06 | 5.00 | 12.23 |
| 50 | 14.47 | 6.00 | 14.66 |
| 40 | 18.09 | 7.50 | 18.47 |
| 30 | 24.12 | 10.00 | 24.64 |
There is no consistently successful strategy to reduce losses to BYD virus by trying to control their aphid vectors with insecticidal sprays. While sprays may kill many aphids and reduce the percentage of infected plants, potential yield savings may not pay for the chemical and application. There are many other factors that impact the relative effect of BYDV infections.
BYDV infections developed very late in the 1998-1999 crop, probably because of very low aphid numbers during the fall. The aphids that were present did not arrive until December. The late aphid flight probably resulted from the late summer-early fall drought that affected Kentucky.
The lateness of the aphid/BYDV infections
is illustrated by the fact that the late winter (Feb. 17) application was
just as effective at reducing BYDV symptoms as either of the other two
applications (Table 1). A larger than "normal" portion of the infections
occurred after Feekes GS 3. Because of this, the data presented in Table
3 must be used very carefully. If you consider only Table 3, it appears
that the most appropriate time to make an insecticide application is in
the late winter. While this was true in 1998-99, this may not be the case
in most years. If both aphids and BYDV had been present very early in the
fall, the percentage of infected plants and the relative damage to each
would have been much greater. While late infections may be important in
a year of good prices and low costs, an early fall infection is always
a more important consideration.
Acknowledgments
The authors express their gratitude to
Dr.'s Don Hershman (Plant Pathology) and Lloyd Murdock (Agronomy) for their
review of this publication. We also especially appreciate the time and
work of Dr. Dick Trimble (Ag-Economic) in proofing and challenging our
economic arguments.
Drill calibration is one of the most important steps in obtaining uniform desirable stands and holding down seed costs. Just as no two drills are alike, each seed lot/variety can differ in seed size, purity and germination level. A few minutes spent calibrating your drill with each seed lot can be the difference between economical seed use and replanting!
Drills should be calibrated either in the
field or by counting wheel revolutions that equal 100 to 200-ft of linear
distance. Start by using the drill settings suggested by the manufacturer
for the desired seeding rate (usually listed in pounds of seed per acre).
Operate the drill in a test strip and collect seeds from 3 to 5 drop tubes
to measure the actual seeding rate at a given setting. Record the total
weights from all drop tubes and compare it to the desired weight. Adjust
the drilling setting up or down according to the first run and repeat the
process until you have achieved a seeding rate within 3 percent of the
target rate. Record drill settings that match the desired seeding rate
for a given seed size because this information will be a useful starting
point when different seed lots/varieties of similar size and quality are
used each season.
Table 1 contains a summary of 28 calibration
trials with four different no-till drills that were used in a recent study
to compare drill performance. Note that the range in desired seeding rates
was between 110 and 183 pounds of seed per acre and that all drills were
within 3 percent of the desired rate. The important result of this study
was that the difference in the average error (difference between the target
rate and actual rate) between drills was not significant. The Kentucky
Small Grain Growers Association provided support for this study, which
also lead to the refinement of a spreadsheet (illustrated in Table 2) that
was originally developed by Mike Ellis, a no-till farmer and crop manager
in Shelby County. It calculates calibration weights based on the desired
plant population, row spacing, and seed tag data (seed weight, germination
and purity). It was modified to serve as a record keeping tool to keep
track of seed needs and costs for each variety/lot used in a given operation.
Free copies of this spreadsheet may be
obtained from the authors through county extension offices to help calibrate
drills, compute the amount of seed needed, and compare seed costs at different
seeding rates. Note that with all other factors equal (yield potential,
seed quality and cost), lots/varieties with smaller seeds are a better
buy.
