Is There a Yield Penalty From Postemergence Soybean Herbicide Injury?
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Each year, soybean producers in the Midwest treat millions of soybean acres
with postemergence herbicides for weed control. Many of these herbicides have
the potential to injure soybeans. Usually, the level of injury experienced from
these herbicides depends on the type of herbicide, the soybean growth stage
at application, planting times, and the environmental conditions present before
and after the herbicide application. Often, soybean plants that are injured
by postemergence herbicide treatments recover quickly. However, recovery from
postemergence herbicides may be reduced under certain environmental conditions,
such as low soil moisture and high temperatures.
In any case, when soybean producers observe this injury on their soybeans, they
frequently ask whether the injury they experienced on their soybeans will equate
to a reduction in soybean yield at the end of the season. This question has
been a particularly important one with the advent of herbicide-resistant soybeans,
in particular Roundup Ready Soybeans, since most producers have not experienced
much injury from applications of Roundup Ultra in this system.
To answer the question of whether injury from postemergence soybean herbicides
affect yield, the Soybean Research and Development Council, comprised of the
Illinois and Iowa Soybean Program Operating Boards, worked in conjunction with
the University of Illinois, Southern Illinois University, and Iowa State University
to conduct research trials on evaluating the effect of postemergence herbicides
on Roundup Ready soybean yield. These research studies were conducted over a
variety of environments in Illinois and Iowa from 1997 to 1999. The implications
of early and late planting, and early and late application timings of Pursuit,
Blazer, and Roundup Ultra (three herbicides with different sites of action),
were investigated. The research areas were maintained weed free throughout the
growing season to eliminate any yield loss due to weed competition.
What Type of Injury Occurred?
Three different distinct injury symptoms were observed from the postemergence
herbicides. The extent and severity of the injury from these herbicides was
greatly influenced by the individual local environments. Visual evaluations
of soybean injury within a week after application were generally between 0 and
34 percent for Pursuit. The injury symptoms consisted of purple leaf veins and
slightly stunted plants. Injury ranged
between 5 and 55 percent for Blazer, with symptoms consisting of necrotic and
chlorotic speckling on the leaf surface.
Injury was infrequent and inconsistent with Roundup Ultra; however, the injury
that was observed was a slight chlorosis that appeared on the newly emerging
leaves. With all three postemergence
herbicides, injury symptoms were non-existent by 21 days after herbicide application.
In cases where there was severe soybean injury, there were no consistent correlations
between planting date and time of herbicide application.
Did Injury Reduce Soybean Yield?
For the three-year, multi-location study, more than 95 percent of the (288)
postemergence herbicide applications did not reduce soybean yield, regardless
of the herbicide or level of soybean injury observed. Although soybean yield was rarely reduced by herbicides, a closer
look at the conditions where yield loss occurred may help identify risk factors
that producers should consider when using postemergence soybean herbicides.
Significant yield reductions ranging from 7 to 23 percent only occurred in 3.5
percent of the treatments. These few instances of yield reduction occurred most
often following late herbicide applications to late-planted soybeans. As mentioned
before, low soil moisture and higher temperatures can often reduce recovery
from postemergence herbicide injury. These conditions are more likely to occur
when applications are made later in the season on late-planted soybeans.
Management Decisions
Injury from postemergence soybean herbicides is not a good predictor of soybean
yield loss. For example, soybean injury from Blazer was occasionally greater
than 30 percent, yet no yield reductions were associated from those applications.
However, delaying the soybean planting date reduced soybean yields from 52 to
47 bu/A, when averaged over all locations, years, application timings, and herbicide
treatments.
There are a number of factors that likely influence the potential for postemergence
herbicide injury to cause soybean yield reductions. These high-risk factors
include late planting dates, late herbicide applications, and poor environmental
conditions for soybean recovery, such as low soil moisture and high temperatures.
If no visual injury develops from the postemergence herbicide, there is a minimal
risk of soybean yield reductions. However, just because herbicide injury is
observed, soybean yields are not necessarily reduced.
Based on the results of this extensive research, the risk of soybean yield loss
from labeled applications of postemergence herbicides is low. Soybean yield
loss is more likely to occur from allowing weeds to compete too long with the
soybeans or from failing to adequately control weeds. Other types of soybean
stress from diseases, insects, or nematodes also decrease soybean yields and
reduce the ability of the soybean to recover from herbicide injury.