Michigan State University Extension
Michigan Soybean Information - FACT9709
11/16/00
Weed Control in Roundup Ready Soybeans
Soybean Facts Winter 1997
Michigan State University Extension
This research and fact sheet funded by Michigan
Soybean Promotion Committee Making Your Checkoff Pay Off
K.A. Renner and J.M. Lich
Department of Crop and Soil Sciences
Michigan State University
The recent development of the Roundup Ready Soybean,
a soybean genetically engineered to be resistant to
postemergence applications of Roundup Ultra, will
expand the utility of Roundup Ultra in conventional
and no-till soybean production.
Annual Weed Control
Roundup Ultra at 1 pint/Acre provides excellent
control of cocklebur, jimsonweed, common ragweed,
wild mustard, and the foxtails. Roundup Ultra at 1.5
pints/Acre controls redroot and smooth pigweed,
smartweed, black nightshade, large crabgrass, and
barnyardgrass. Roundup Ultra should be applied to
black nightshade, barnyardgrass, and large crabgrass
before they exceed 4 inches in height.
Common lambsquarters and velvetleaf are more
difficult to control. Two pints/Acre of Roundup
Ultra has provided more consistent control than
lower application rates.
Roundup should be applied for annual weed control
when the weeds are four to five inches in height. Do
NOT let the weeds compete with soybeans for longer
than five weeks or soybean yield may be reduced from
the early season weed competition. Roundup Ultra,
the new Roundup formulation, contains nonionic
surfactant and Monsanto does not recommend adding
additional surfactant. However, a nitrogen source
can still be added. The addition of ammonium sulfate
or 28% liquid nitrogen has improved control of
crabgrass, lambsquarters, and velvetleaf in
university research.
Perennial Weed Control
Roundup Ultra is labeled for control of many
perennial weeds. Perennial grasses like quackgrass
are controlled by Roundup Ultra at 2 to 4
pints/Acre. Yellow nutsedge will be controlled for
one season by applications of Roundup Ultra at 2
pints/Acre. Perennial broadleaf weeds such as Canada
thistle, field bindweed, and hemp dogbane require
higher application rates of Roundup Ultra, applied
when the plants are in flower, to effectively
control the underground root system of these weeds.
Therefore, applications of 2 pints/Acre of Roundup
Ultra in Roundup Ready soybeans will control these
perennial broadleaf weeds for one season but do not
expect eradication into the following years.
If perennial broadleaf weeds are a problem in the
field, the grower could apply 1.5 to 2 pints/Acre of
Roundup Ultra when annual weeds are 4 to 5 inches
tall and then apply 2 pints/Acre of Roundup Ultra a
few weeks later when most of the perennial broadleaf
plants have emerged from all the "pieces" of the
underground root system. In a no-till system, the
emergence would be more uniform than in a
conventional-till system. If making a single
application of Roundup Ultra for control of both
annual weeds and perennial broadleaf weeds, don't
delay the application longer than 6 weeks after
soybean planting because: (1) the annual and
perennial weeds will already be competitive with
soybeans and (2) some annual weeds such as
crabgrass, lambsquarters, black nightshade, and
velvetleaf are more difficult to control when they
exceed four inches in height.
Tank Mixtures with Roundup Ultra
The Roundup ready label will not recommend tank
mixtures. In university research, tank mixtures of
Roundup with Basagran or Resource increased
velvetleaf control. However, common ragweed control
decreased when Resource was tank mixed with Roundup
Ultra. Velvetleaf control was not improved by the
addition of Pursuit, Pinnacle, or Classic. Michigan
State suggests applying Roundup Ultra at 2
pints/Acre plus a nitrogen source to 4 inch
velvetleaf. Resource can be applied sequentially for
velvetleaf control if needed.
The Roundup Ready Label
Roundup Ultra can be applied from soybean cracking
through full flower in Roundup Ready soybeans.
Roundup Ultra can be applied more than once during
this time period but the total amount of Roundup
Ultra that can be applied during this time period is
2 quarts/Acre. The label will suggest 1.5 pints/Acre
on weeds less than 6 inches tall with a footnote to
apply this rate to 4 inch black nightshade. The
label will suggest sequential applications of
Roundup Ultra in 30-inch row soybeans if needed.
Drilled soybeans will close the canopy faster and
fewer weeds will emerge following the Roundup Ultra
application. In university research, weed control
was better in drilled soybeans when compared to
30-inch row soybeans because Roundup Ultra, like
many other postemergence soybean herbicides, does
not have soil residual to stop later germinating
weeds.
Other Issues
Apply Roundup Ultra when winds are below 5
miles/hour. Corn and sugarbeets are very sensitive
to off target Roundup Ultra. Be sure that sprayers
are cleaned out thoroughly following Roundup Ultra
applications. Be sure fields are well marked when
the Roundup Ready seed is planted.
Roundup Ultra will provide soybean growers a new
postemergence weed control option in soybeans. It is
the best option growers have for managing perennial
weeds in the soybean crop. Roundup Ultra
applications in Roundup Ready soybeans will be a
cost effective weed control program with no concern
for groundwater or surface water contamination or
toxicity to humans or animals. However, it is
critical that Roundup drift NOT occur. The soybean
varieties with the Roundup Ready gene must have the
yield potential and important agronomic
characteristics of the elite lines growers are
currently planting. If these varieties are
available, Roundup Ready soybeans will be adopted by
many farmers for weed management in soybeans.
Copies of the Soybean Research Fact Sheet are available
from:
Michigan Soybean Promotion Committee
http://www.michigansoybean.org
MSUE County Office
http://www.msue.msu.edu/msue/ctyentpg/
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