Michigan State University Extension
Michigan Soybean Information - FACT9810
11/16/00
Weed Control in Drilled and 30" Wide-Row Roundup Ready Soybeans
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Soybean Facts
Michigan State University Extension
This research and fact sheet funded by Michigan
Soybean Promotion Committee Making Your Checkoff Pay Off
Winter 1998
By: K. A. Nelson and K. A. Renner
Michigan State University
Department of Crop and Soil Sciences
Weed control costs are one of the inevitable factors that
directly affect profitability for soybean growers.
Research at Michigan State University has shown that
split applications of reduced rates of postemergence
herbicides provide cost effective weed control in drilled
soybeans. The introduction of herbicide resistant crops
has offered new opportunities for annual weed control,
soybean safety, and rotational crop safety for Michigan
producers. This fact sheet provides guidelines for
producers who are interested in weed control programs in
Roundup Ready(R) soybean.
Drilled soybeans have greater yield potential that 30"
wide-row soybeans. Increased soybean yield in 7.5" rows
has been attributed to increased light interception
compared with 30" row soybeans. For recommended seeding
rates for narrow row soybeans see MSU Extension Bulletin
E-2080.
Canopy closure is important for soybean competition with
weeds. The soybean canopy in drilled soybeans closes 30
days earlier than the canopy in 30" row soybeans. Faster
canopy closure in drilled soybeans can enhance weed
control by reducing weed seed germination and suppressing
weed growth by shading. This could result in a broader
spectrum of weeds controlled or result in herbicides
more effectively controlling weeds.
Herbicide application timing is critical for effective
weed control. Early weed identification and records on
weeds present in fields can assist weed management
programs by choosing herbicides that will control the
target weeds. Timing of postemergence herbicide
application is critical. Weeds that are too large at the
time of application may not be controlled. In addition,
weeds can reduce soybean yield if allowed to compete with
the soybean for many weeks before a herbicide is applied.
Weeds become competitive with soybeans at the V3 to V4
stage of development. Soybeans are usually at this growth
stage 4 to 5 weeks after planting in Michigan, depending
on the year. Herbicides must be applied that effectively
control the weeds. Roundup Ultra will control most weeds
when they are 4 to 5" in height (see label). Other
postemergence herbicides are more effective when weeds
are 2 to 4" tall. A second flush of weeds can germinate
later in the season following an early herbicide
application. A second herbicide application or a
cultivation would be needed to control these weeds.
Herbicides with residual activity may prevent the
germination of weeds following a herbicide application.
Weeds that germinate 6 weeks after soybeans are planted
do not reduce soybean yield, however, these weeds will
produce some weed seed.
Add ammonium sulfate to Roundup Ultra for improved
velvetleaf control (Vis. 1). Ammonium sulfate has been
utilized to overcome hard water antagonism with several
herbicides. Roundup Ultra contains an adjuvant so
additional surfactants are not recommended.
Tank mixtures with Roundup Ultra may reduce weed control.
Roundup Ultra at I qt/A was applied alone, with ammonium
sulfate at 17 lbs/100 gal, with 2 oz/A Resource, and with
0.7 oz/A Pursuit DG + I qt/A urea ammonium nitrate. Giant
foxtail, redroot pigweed, eastern black nightshade, and
common lambsquarters were controlled by all treatments.
Resource reduced velvetleaf (Vis. 1) and common
ragweed control one year in 30" wide row soybeans.
Soybean yield was greater in drilled than 30" row
soybeans for all herbicide treatments (Vis. 2).
Standard POST herbicides provided good broad spectrum
weed control in drilled soybeans (Vis. 3)
(Vis. 4)
(Vis. 5).
Full rates of Reflex at 1 pt/A + Pinnacle at 1/4 oz/A +
Fusion at 8 oz/A and Classic at 1/3 oz/A + Pinnacle at
1/4 oz/A + Assure II at 6 oz/A and 1/4+1/4x split
applications of these herbicides were applied to Roundup
Ready soybeans. Classic + Pinnacle + Assure II at a
1/4+1/4x split application rate and Reflex + Pinnacle +
Assure II applied at full or 1/4+1/4x split application
rates yielded similar to the handweeded control in
drilled soybeans. Always use the full recommended
adjuvant rates for the standard POST treatments applied
at full or 1/4+1/4x split application rates. Roundup
Ultra provided the most consistent weed control and yield
compared to standard POST weed control programs. Soybean
yield with Roundup Ultra was equal to handweeded controls
in wide and narrow row soybeans (Vis. 6).
PRE followed by POST herbicide application resulted in
soybean yield similar to the handweeded control in narrow
row soybeans only (Vis. 8). Roundup Ultra at 1 qt/A
provided more consistent common ragweed (Vis. 7) and
giant foxtail control, and greater soybean yields
compared with Prowl at 2.4 pt/A PRE followed by Pursuit
DG 1.44 oz/A + Status 1 pt/A and Canopy 4 oz/A PRE
followed by Assure II at 6 oz/A in wide row soybean only.
Sulfentrazone (sold in a co-pack as Authority
First/Synchrony STS) PRE followed by Roundup Ultra
controlled common ragweed, eastern black nightshade,
velvetleaf, giant foxtail, common lambsquarters, and
redroot pigweed and yielded similar to Roundup Ultra
alone or the handweeded control.
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/
This information is for educational purposes only. References
to commercial products or trade names does not imply
endorsement by MSU Extension or bias against those not
mentioned. This information becomes public property upon
publication and may be printed verbatim with credit to MSU
Extension. Reprinting cannot be used to endorse or advertise
a commercial product or company.
This file was generated from data base S1 on 11/16/00.
Data base S1 was last revised on 11/16/00.
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