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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.

Ranking of responsive sites only, in order of yield.

Kent

Fairmount

Arthur

Galchutt

medium texture
low salt
medium carbonate

heavy texture
high salt
medium carbonate
sandy texture
medium salt
low carbonate
sandy texture
high salt
medium carbonate

Yield, bu/acre

Pursuit 43.4 Galaxy 37.9 Galaxy 38.5 Galaxy 33.0
First Rate 40.1 First Rate 35.5 Raptor 35.6 Storm 32.8
Flexstar 38.3 Flexstar 30.8 Pinnacle 34.9 Flexstar 31.0
Basagran 35.3

Cobra

29.7 Storm 34.3 Blazer 29.7
Cobra 35.0 Storm 28.9 Pursuit 33.6 Basagran 29.1
Galaxy 33.3 Basagran 28.7 Basagran 32.7 FirstRate 28.2
Raptor 31.3 Pursuit 27.4

Flexstar

31.0 Raptor 28.1
Pinnacle 29.1 Blazer 24.3 Blazer 29.7 Cobra 27.7
Storm 26.9 Pinnacle 23.6 FirstRate 29.1 Pinnacle   25.0
Blazer 24.9 Raptor 20.3 Cobra 20.9

Pursuit

24.0
LSD 5% 6.4   6.2   6.1   1.0

      

    Reviewing these data, a grower might determine that this is too much to consider and it would
be easier to go with Roundup-Ready beans. However, this year two of our six sites are Roundup-
Ready beans and have a Roundup-Ready treatment added to last years ten treatments. It is interesting
to note that the Roundup-Ready beans treated with Roundup are markedly shorter and more chlorotic
weeks after treatment than an unweeded check. Therefore, it does not appear that Roundup is without
effect, even on beans with tolerance to its chemistry when under chlorosis.

    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
dfranzen@ndsuext.nodak.edu


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