February, 1999
Roundup Application Timing
by Donald E. Null, Regional Agronomy Specialist
University Outreach and Extension, NW Region
Situation: There are several questions relative to Roundup
application timing. They include the following:
1. Since Roundup provides no soil residual activity, can
Roundup applications be applied too early?
2. Due to the excellent efficacy provided by Roundup, can
burndown treatments be delayed without hurting weed control
and soybean yields?
3. Are crop yields and weed control efficacy reduced by delaying
Roundup applications too long? Several experiments at
Hundley-Whaley were designed and conducted during 1998 to
help answer these questions.
Study Results: Study 3823 had the full labeled rate of
Roundup (32 oz/acre) applied at three different times to wide-rowed
soybeans. The early-post treatment was made 23 days after
planting (DAP) to 2-6" tall weeds. The mid-post treatments were
applied 29DAP to 2-12" weeds. The late-post treatments were
applied 35DAP to 2-18" tall weeds.
Figure 4 Study 3823
The results of these treatments can be seen in the end-of-season
and crop yield data in Figure 4. The results indicate that giant
foxtail control was enhanced by delaying Roundup applications.
Common waterhemp, cocklebur and volunteer corn weed control were
all at their highest level, along with crop yield, with the
35DAP, late-post applications.
Study 4109 was designed to determine the effect of
application timing on weed control and soybean yield. Asgrow
AG3601 soybeans were drilled into a conventionally prepared
seedbed on May 28. Heavy rains in early June had an adverse
effect upon crop establishment, and this could have impacted the
overall results of the trial since plots with a poor stand of
soybeans did not provide the canopy required for late season weed
control from early and mid-post Roundup treatments. Single
Roundup treatments in this study were confined to one early-post
treatment, applied 21DAP on June 19 to V1 soybeans and cotyledon
to 5" tall weeds, and one R-1, beginning bloom treatment made
45DAP on July 13 to R1 soybeans and 4-30" tall weeds.
Another treatment was made up of four Roundup applications. One
quart of Roundup was applied early-post, mid-post, late-post and beginning
bloom. This treatment was to show the performance of soybeans
when no weed pressure was allowed to compete with the crop
throughout the growing season. Weed pressure consisted of heavy
cocklebur pressure and moderate pressure of giant foxtail,
ivyleaf morningglory, common waterhemp and shattercane.
Figure 5 Study 4109
Figure 5 shows the soybean yields from these three treatments. Late July
weed control ratings showed the early-post Roundup treatment
controlling 50-80% of the weeds in the treatment area. In
contrast, the R-1 Roundup treatment was providing 95-100% control
of all weeds except morningglory. The morningglory control
rating was estimated to be a little over 50% in this treatment.
Of course, the four applications of Roundup treatment was
providing essentially 100% control of all weed species.
While there was no clear statistically significant yield difference
between the R-1 and multiple Roundup applications, the data does
support the notion that producing season long, weed-free soybeans
does provide the highest soybean yields.
Study 3016 was planted to Asgrow AG3701 soybeans on May 20 in
wide rows into a conventionally prepared seedbed. Weed pressure
in this test site was very heavy and consisted primarily of giant
foxtail and wild sunflower. Roundup application timing ranged
from early-post (2-4" weeds) on June 5 to mid-post (4-8" weeds)
on June 16. These treatments were observed on July 13 and a very
noticeable difference was apparent between the early-post and
mid-post treated soybean height and canopy development.
Measurements were taken and the results can be seen in Figure 6.
On July 13, the treatments with early weed control resulted in
soybeans that were 8 inches taller than the soybeans that had
delayed weed control. The 36" wide-rowed, early weed control
plots were estimated to be 87% canopied which was significantly
higher than in the delayed weed control plots. It was difficult
for me to believe that these differences would not result in yield
differences too.
Figure 6 Study 3016
We did take these plots to yield and the graphics in Figure 7 show
the results of the treatments. The early application of Roundup
to 2-4" weeds allowed a heavy second flush of weeds that
seriously affected crop yield in an adverse way. A second
application to 2" regrowth also allowed other weed flushes to
emerge and compete with the crop. The overall best weed control
and the highest yielding soybeans were the result of treatments
that delayed Roundup applications to 4-8" weeds with a second
application to either 3-4" tall or 4-8" tall second flush weeds.
Figure 7 Study 3016
As a rule, this year's data are different than the data from
1997. Last year, delaying Roundup treatments past 28DAP resulted
in yield reductions at Hundley-Whaley. The difference is thought
to be in the weather variations between the two years. The
weather last year was dry from mid-July to mid-September. This
year there was no shortage of moisture during the growing season.
Dry weather following an early Roundup application would be
expected to result in fewer second flush weeds emerging. And the
early control of weeds would provide less weed competition and
more soil moisture for the soybean plants.
Study 3909 was designed to answer the question of delayed
burndown applications in Roundup Ready soybeans. Asgrow AG3701
soybeans were no-till drilled into a moderately weedy site on May
11. Herbicide treatments each consisted of Roundup (32 oz/acre)
applied at various times following planting, and then again when
new flushes of weeds occurred. The first treatment consisted of
a Roundup application made to a light infestation of just
emerging weeds. Treatment 2 was made May 18 to 1-2" weeds when
the soybeans were just starting to crack through the soil.
Treatment 3 was made May 21 to VC-V1 stage soybeans with 3-4"
tall weeds. Treatment 4 was made May 26 to V1-V2 soybeans with
5-6" tall weeds. Treatment 5 was made May 28 to V1-V2 soybeans
with 7-8" tall weeds. Treatment 6 was made June 1 to V2-V3
soybeans with 9" and taller weeds. The only winter annual weed
in the plot area was corn gromwell, at moderate to heavy
pressure. It was essentially mature by May 21. Other weeds
consisted of giant foxtail, common waterhemp, cocklebur and velvetleaf.
Figure 8 Study 3909
Figure 8 illustrates the soybean yields from the six treatments.
This work indicates that in 1998 there was no soybean yield
reduction by delaying burndown applications on moderately weed
infested Roundup Ready soybeans for up to 20 days following
planting. The untreated weedy treatment in this experiment
yielded 23.9 bu/acre which was 40% of the maximum yield for the test.
Do delayed Roundup burndown applications adversely effect yield?
My response is that "it depends". The data suggest that the
outcome to delayed burndown applications is very weather
dependent. If the weather is dry following planting the answer
is probably "yes". Delaying the burndown application will allow
the weeds to pull water from the soil that might otherwise be
available for crop use. If the weather is wet following planting
then the answer is probably "no". Water is most likely the
limiting factor at this stage of soybean production.
Roundup Rates
Situation: Roundup is known to be a very
efficacious herbicide. Falling commodity prices and increased
seed costs have resulted in some producers considering cost
cutting options that include reducing Roundup rates. Can Roundup
rates be reduced without a reduction in weed control?
Study Results: Several of the studies conducted at the
Hundley-Whaley farm in 1998 employed less than the 32 ounce
minimum rate recommended by Monsanto.
Study 3016 had several different rates of Roundup in the treatment
list, each applied mid-post.
Figure 9 Study 3016
Figure 9 shows the percent weed control evaluations on
July 7. The initial mid-post application was made to 4-8" weeds
on June 16. The second Roundup application was made July 1.
Weed control ratings, following the initial mid-post
applications, did not show a significant difference in
application rates for Roundup.
Study 2711 was designed to look at reduced Roundup rates in
no-till, drilled soybeans. Asgrow AG3701 soybeans were planted
on June 3 at a rate of 195,000 seeds/acre. All treatments were
sprayed with 1 quart of Roundup following planting to kill
emerged weed species. Mid-post applications of Roundup were made
July 7 to 2-6" velvetleaf, 2-10" giant foxtail, 2-8" common
waterhemp, 4-8" ivyleaf morningglory and ½-4" teaweed. Weed
pressure was considered light to moderate for these species. Ten
days later, another Roundup application was made on some of the
treatments to 3" weed regrowth. Table 6 below shows the results
of the August 3 rating for the mid-post treatments. With the
exception of the morningglories, there was very little difference
in weed response due to rates.
Mid-Post Treatment Percent Weed Control
Gft Teaweed MrnGlor Wthmp Bu/a
Roundup, 1 qt/acre 100 98 80 100 57.8
Roundup, 1 qt/acre fb 1 pt/acre 100 98 94 100 60.6
Roundup to 3" weed regrowth
Roundup, 1.5 pt/acre 100 96 71 100 66.6
Roundup, 1.5 pt/acre fb 1 pt/acre 100 98 93 100 59.7
Roundup to 3" weed regrowth
Roundup, 1 pt/acre 100 95 68 100 61.5
Roundup, 1 pt/acre fb ½ pt/acre 100 98 88 100 62.7
Roundup to 3" weed regrowth
LSD 0.05 NS NS 20 NS NS
Table 6 Study 2711
Can Roundup rates be reduced? The data presented here would
indicate that good weed control of many species can be achieved with
less than the full labeled rate of Roundup. There are other
considerations, however. Growing conditions can impact efficacy.
Drought has an adverse effect upon the efficacy of most post weed
control programs.
Weed species such as lambsquarters, giant ragweed, velvetleaf and
morningglories are more likely to need full labeled rates for
control than species like giant foxtail, cocklebur and sunflower.
Size of weed and stage of growth can also impact rates needed.
As a general rule, larger weeds are more difficult to control
with reduced rates of Roundup. Application accuracy can also
impact reduced rate efficacy with Roundup.
In addition, if reduced Roundup rates are considered, past
research would certainly support the use of ammonium sulfate in
the spray solution.
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