ISSUE 12 JULY 22, 1999
CHLOROTIC SOYBEANS AND POSTEMERGENCE HERBICIDE
INTERACTIONS- OBSERVATIONS AND 1998 DATA-
In 1998, in response to observations made by growers, a study was
initiated by myself and Dr.
Zollinger to investigate the unique situation in the region with respect to applying various postemergence
soybean herbicides on plants stressed with chlorosis. There does not appear to be any way to
apply postemergence chemicals in a manner which bypasses the chlorosis-stressed stage of growth.
About the first to third trifoliate leaf stage is the time that weeds are most susceptible to these herbicides,
but it unfortunately also falls within the window of stress if chlorosis is present.
Currently, most growers consider only the cost of a product and its
weed spectrum. Some consideration
is also given to visual effects of the herbicide, but it is generally accepted that little or no injury results,
as limited studies in the central corn belt have shown in the past on non-chlorotic beans.
However, this study is being conducted to determine whether an
additional factor should be considered-
the added stress and possibly yield reducing effects of postemergence herbicides on already stressed soybeans.
Last year and this year, six locations with reasonably uniform
chlorosis were identified in the Valley
and treated with 10-11 different herbicide treatments. Soil and plants from each individual plot were
obtained to determine whether differences within treatments were caused by small-scale spatial variability
of salts or carbonates (determined in an earlier study to be leading causes of our unique chlorosis problem).
When certain reps of treatments were affected by differences in salt level, yields were adjusted accordingly
to be fair to all treatments.
Out of six sites, four showed differences between herbicide
treatments in yield. Yield differences were
as great as 19 bu/acre between treatments. The results of the four sites are displayed in the following table.
Generally, Galaxy seemed the easiest on the beans overall. At the
low salt sites, Pursuit and Raptor
were also rated highly. However, when salts were high, Pursuit and Raptor were in the bottom half of
treatments. First Rate was the only salt and carbonate neutral treatment, but in the sandier textures, the
ratings were low due to increased activity in coarser textured soils. Out of the ten treatments, three
treatments seemed to be generally harder on bean yields- Blazer, Pinnacle and Cobra.
|First Rate||40.1||First Rate||35.5||Raptor||35.6||Storm||32.8|
Reviewing these data, a grower might determine that this is too much to consider and it would
I caution that these data are from one year of research, however, a
second set of data will be collected
this fall from our treatments.
Also, growers will need to consider the effect of weeds on yields if
the field is left untreated. Certainly,
weeds affect yields. It is hoped that these data give growers a good decision making tool to find not only
a herbicide that controls economic infestations of weeds, but also is safest on the crop in our region.
Dr. Dave Franzen
NDSU Extension Soil Specialist