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Bruce Burdick
1109 S. Birch St.
Albany, MO 64402
660-726-5610
Email:
BurdickB@missouri.edu
Hundley-Whaley Center phone
660-726-3698
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EFFECT OF ROUNDUP APPLICATION TIMING UPON CORN YIELDS AT HUNDLEY-WHALEY FARM
Study C9MN1610
by Donald E. Null, Regional Agronomy Specialist
SITUATION: Producers considering the use of a Roundup Ready corn weed control program need
information as to the correct time to apply their herbicides. Most people involved in making recommendations,
recommend a pre-emergence herbicide, followed by a post application of Roundup.
OBJECTIVE; To determine the optimum application time for Roundup on Roundup Ready corn and to
determine the effect of application timing on crop yield response.
METHODS: The study was conducted at the Hundley-Whaley Farm at Albany. The study was made up of 10
treatments. It was designed as a randomized complete block with six replications. DeKalb DK626RR was
planted on June 7, 1999 into a conventionally prepared seedbed which had been in soybean production in 1998.
Planting rate was 25,500 seeds per acre in 36" wide rows. Fertilizer was broadcast applied to the soil surface
after planting at a rate of 180-0-90. The nitrogen source was ammonium nitrate.
The treatments in the study, along with application dates are in the table below.
| Trt#
| Treatment(1)
| Time
| Date
| Other Times
| Dates
| Corn Height
| 1
| Weedy check |
| 2
| Roundup Ultra, 1 qt
| 2" weeds
| June 21
| none
|
| 5" corn
| | 3
| Roundup Ultra, 1 qt
| 4" weeds
| June 30
| none
|
| 12" corn
| | 4
| Roundup Ultra, 1 qt
| 6" weeds
| July 6
| none
|
| 24" corn
| | 5
| Roundup Ultra, 1 qt
| 9" weeds
| July 7
| none
|
| 26" corn
| | 6
| Roundup Ultra, 1 qt
| 12" weeds
| July 9
| none
|
| 28" corn
| | 7
| Roundup Ultra, 1 qt
| 2" weeds
| June 21
| 2" regrowth
| July 6
July 20
| 5" corn
24" corn
48" corn
| | 8
| Roundup Ultra, 1 qt
| 4" weeds
| June 30
| 2" regrowth
| July 6
July 20
| 12" corn
24" corn
48" corn
| | 9
| Roundup Ultra, 1 qt
| 6' weeds
| July 6
| 2" regrowth
| 24" corn
34" corn
48" corn
| July 13
July 20
| | 10
| Harness Xtra, 3 qts
Roundup Ultra, 1 qt
| Pre
2" Weeds
| June 7
| 2" regrowth
| July 6
July 20
| 0
24" corn
48" corn
| |
(1) All Roundup applications included ammonium sulfate at 17 pounds/100 gallon of spray solution
The dominant weeds in the study were cocklebur (heavy pressure), velvetleaf (moderate
pressure) and giant foxtail (light pressure). These weed pressures existed in the plots not treated with a
pre-emergence herbicide. The pre-emergence Harness Xtra in treatment #10 allowed a light pressure
of cocklebur and velvetleaf to break through.
The last Roundup application was made July 20 to corn that was 4 foot tall (V8). The corn
was 90-95% canopied at that date.
The season at Hundley-Whaley could be described as a very wet spring period followed by a
very dry summer and fall.
The corn was machine harvested on Oct. 18. Grain moisture ranged from 21 to 26%.
RESULTS: All Roundup treatments provided complete control of weeds in all treatments within 7-10
days following application.
Figure #1 (below) shows the corn yield and end-of-season weed control ratings of treatments
2-6. These were treatments that only received one application of Roundup at different times through
the spring season. These data suggest that if only one treatment was made, delaying it until the weeds
were 6" tall provided the highest corn yield. The data also suggests that the reason the first two
applications performed poorly was due to new weed flushes, following the initial Roundup
application.
Actual yields for the corn in Figure #1 was 54 bu/acre for the 2" only weed control treatment;
85 bu/acre for the 4" weed treatment; 111 bu/acre for the 6" weed treatment; 105 for the 9" weed
control treatment and 92 bu/acre for the 12" weed treatment. The no-treatment, weedy check provided
a corn yield of 13 bu/acre. The LSD (P = 0.05) was 25 bu/acre.
Figure #2 shows the corn yields of treatments 7, 8 & 9. This data agrees with other research
data that has been generated by Western Corn Belt Weed Scientists. It indicates that corn is not very
tolerant to early season weed compet-ition. The highest yielding treatment was treatment #7. Treat-ment #10, (Pre-emer-gence Harness Xtra followed by post Roundup) yielded 129 bu/acre and this
yield was not statistically different from the highest yielding treatment. The yield trend however
suggests that any weed
competition beyond 2"
weeds is detrimental to
crop yield.
We were
surprised, and quite
pleased with the top yield-
ing treatment in the study,
considering the late
planting date and the dry
summer weather.
Planting date had been
delayed due to the
excessively wet weather in
the spring.
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