February, 1999
Soil Applied Herbicides followed by Roundup
by Donald E. Null, Regional Agronomy Specialist
University Outreach and Extension, NW Region
Situation: Companies that manufacture soil applied herbicides are
actively promoting weed control programs for Roundup Ready
soybeans that include their products. Monsanto discourages soil
applied weed control programs to Roundup Ready soybeans. Several
research studies were conducted at the Hundley-Whaley farm to aid
in securing an answer to the question of need for soil applied
products.
Study Results: Study 3513 included comparisons of soil applied
Canopy XL followed by an early post application of Roundup and an
early post application of Roundup alone. The soybeans in this
study were planted on May 28 in wide rows at a rate of 196,000
seeds/acre. The Roundup was applied on June 19 to V1-V2 stage
soybeans at a rate of 1 pint/acre + AMS. Giant foxtail and
waterhemp sizes ranged from ¼-1" tall and cocklebur were ½-4"
tall.
The Canopy XL rate was 6.8 oz/acre and it was providing
nearly complete control of cocklebur and common waterhemp at the
time the Roundup was applied. Only giant foxtail was not being
controlled well by the Canopy XL.
Figure 1 shows the results of these two treatments on July 29,
forty days after the Roundup was applied. The initial weed
control provided by the Roundup was excellent but the lack of
residual herbicides allowed for new flushes of cocklebur and
waterhemp. The giant foxtail pressure was not heavy and little
emerged following the initial Roundup
application.
Figure 1 Study 3513
Figure 2 shows the results of two other treatments
in the study. One was Dual followed by Roundup and the other was
Roundup alone. Roundup was again applied early post but the rate
was 1 qt/acre + AMS. The Dual at the full labeled rate aided in
giant foxtail and waterhemp control but these two weeds were not
the main yield robbing species. In this case cocklebur is
believed to be the weed most impacting crop yields and the new
flushes of this weed did severely impact crop yield.
Figure 2 Study 3513
Unfortunately, there were no treatments with a mid-post
application timing for Roundup in this study. If such treatments
had been included, the results might have been very different.
Study 3710 was another study that looked at soil residual
treatments to aid in full season weed control in Roundup Ready
soybeans. In this study Treflan + FirstRate were incorporated
into the soil on May 19 prior to planting soybeans in wide rows
on the same day. This study did use mid-post Roundup
applications that were applied to V2-V3 soybeans on June 17. The
Roundup rate was 1 pint/acre following the Treflan + FirstRate
and 2 pints/acre where used alone. Figure 3 shows the results of
these two treatments 30 days after the Roundup was applied.
Velvetleaf control and soybean yield were significantly enhanced
by the PPI treatments in this study. Common waterhemp control
was significantly better for the Roundup alone treatment.
Perhaps this was due to the size of the waterhemp that escaped
the Treflan in the Treflan + FirstRate treatment. One pint of
Roundup could have been too low of a rate to take out a high
percentage of the 6" waterhemp that were present at that time.
Figure 3 Study 3710
Study 3823 compared Steel applied pre-emergence, followed by
Roundup, to Roundup alone. Wide rowed soybeans were planted May
27. Roundup treatments were applied at various times ranging
from early-post to late-post. End-of-season ratings were taken,
along with crop yields. Cocklebur, giant foxtail, common
waterhemp and volunteer corn were the most troublesome weeds in
the study, with cocklebur providing very heavy pressure to the
crop. The table below (Table 2) shows the results of the
treatments.
Treatment Percent Weed Control
Gft Cklb Wthmp Vol Cn Bu/a
Steel alone (3 pint/acre) 93 71 12 12 35.2
Steel fb Early-post Roundup 95 77 60 98 46.1
(24 oz/acre)
Steel fb Mid-post Roundup 98 77 73 87 47.2
(24 oz/acre)
Steel fb Late-post Roundup 100 80 93 100 51.0
(24 oz/acre)
LSD 0.05 7 13 19 167.5 7.5
Table 2: Study 3823
Table 2 shows that post application of Roundup greatly benefited
the soil applied Steel applications. There was a trend that
indicated that later Roundup applications were more beneficial
than early applications but the differences were not
statistically significant.
Did the applications of Steel benefit the weed control program?
Tables 3-5 show the Steel followed by Roundup, verses Roundup
applied alone, applied from early through late post. There did
appear to be some advantage to having the soil-applied Steel
applied prior to the Roundup applications, however, with the
exception of giant foxtail control in the early and mid-post
treatments, the differences in weed control and crop performance
were statistically insignificant. Would the data show different
results in a dry year? The answer would probably be "yes", but
for this year these are the results.
Was there any adverse crop response from using the Steel, soil
applied? We observed no reduction in crop seedling growth or
stand establishment due to the soil-applied
herbicide in this study.
Treatment End of Season Weed Control (%)
Gft Cklb Wthmp Vol Cn Bu/a
Steel fb Early-post Roundup 95 77 60 98 46.1
(24 oz/acre)
Roundup Early-post 77 75 65 90 44.2
(32 oz/acre)
LSD 0.05 7 NS NS NS NS
Table 3
Treatment End of Season Weed Control (%)
Gft Cklb Wthmp Vol Cn Bu/a
Steel fb Mid-post Roundup 98 77 73 87 47.2
(24 oz/acre)
Roundup Mid-post 87 73 70 100 40.6
(32 oz/acre)
LSD 0.05 8 NS NS NS NS
Table 4
Treatment End of Season Weed Control (%)
Gft Cklb Wthmp Vol Cn Bu/a
Steel fb Late-post Roundup 100 80 93 100 50.8
(24 oz/acre)
Roundup Late-post 98 87 90 100 53.1
(32 oz/acre)
LSD 0.05 NS NS NS NS NS
Table 5
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