Volume
2 Issue
5
June 21,
2005
Timing POST Herbicide Applications in Soybeans
In a survey published by the Minnesota
Department of Agriculture, it was reported that 84 percent
of soybean acres in 2003 were treated with glyphosate.
Although the glyphosate system has simplified weed control
for many growers, proper timing of herbicide applications
is still important in optimizing soybean yields.
Weeds and crops can co-exist for a period of time before
competition for light, water and nutrients leads to crop
yield losses. The challenge is to time postemergence
(POST) herbicide applications before yield losses occur,
while at the same time achieving optimal weed control.
The
length of time that weeds can remain with the crop without
affecting crop yields is defined as the critical period of
weed control. Critical periods of weed control occur early
in the season with early-emerging weeds, and later on in
the season with late-emerging weeds.
The
critical period of weed control early in the season for
soybeans generally ranges from two to four weeks after
planting, but can be shorter or longer depending on a
number of factors. Weed population characteristics (i.e.
species, density, emergence patterns), cropping practices
(i.e. population, row spacing, tillage), and environmental
conditions all influence the critical period of weed
control.
Increased
weed densities and weeds that emerge early in the season
or with the crop shorten the critical period of weed
control. A foxtail that emerges with the crop would be
much more competitive than a foxtail that emerges four
weeks after the crop, for example.
Weed species can also differ greatly in
competitiveness. For example, research conducted by the
University of Nebraska found that four foxtail plants were
equal in competitiveness to one velvetleaf plant.
Cultural practices such as row spacing can also
influence the critical period. In another study conducted
by the University of Nebraska, the critical period in
soybeans in 7.5”, 15”, and 30” rows occurred at the V3,
V2, and V1 soybean stages, respectively. Soybeans planted
in narrow rows were more competitive with weeds than were
soybeans planted in wider rows, and yield effects due to
weed competition were not seen until later growth stages.
Weed height is a critical factor when making POST
herbicide applications. Recommended glyphosate rates vary
depending on weed height by species, so be sure to check
the label based on your conditions. Weed height affects
not only the level of control, but also influences
potential yield impacts, with larger weeds being more
competitive than smaller weeds. Just because glyphosate
may give you a wide window of control for many weed
species, doesn’t mean you should necessarily take
advantage of it if you want to prevent yield losses due to
weed competition.
Many factors affect the critical period of weed control,
but probably the most important factor is weed density. If
your soybean field looks like a solid green carpet of
weeds, you should target a POST application very soon. On
the other hand, if you can easily see the crop rows and
bare soil between them, you have more time to make a POST
application, keeping in mind maximum weed heights at
application as stated on the herbicide label.
It is very difficult, if not impossible, depending your
weed population, to achieve optimal weed control and
soybean yield with a weed management program consisting of
one POST glyphosate application. Research conducted by the
University of Minnesota in 2004 across six locations
confirms this point. Optimal yields were obtained in a
glyphosate system by using sequential applications or a
preemergence herbicide followed by a POST application.
Although not included in this study, a timely cultivation
can also be effective in the quest for optimal yields and
returns.
Lizabeth Stahl
Regional Extension Educator Crops
U of MN Regional Center, Worthington
|
