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ISSUE 5   June 1, 2000


HERBICIDES, CHLOROSIS AND SOYBEANS

    In May 25, 2000 Crop and Pest Report, information on the sources of chlorosis was listed. Another concern of soybean growers is the impact of herbicide stress on soybeans. A study is on-going to quantify the impacts of herbicides on soybean yield. Locations with chlorotic soybeans were examined in 1998 and 1999. The results of herbicide treatments on these soybeans indicates that herbicides may further stress already stressed soybeans, with significant yield differences between herbicide treatments. Using the chart given in the previous Crop and Pest Report, soybeans growing on higher salt soils seem to perform most poorly when treated with harsh contact herbicides or with herbicides having systemic activity.

    The 1999 results supported the findings of the 1998 study. Some of the harsher contact herbicides such as Blazer tended to be less harsh at some locations. This suggests that when conditions are favorable, these contact herbicides might not affect the stressed soybeans as much as other times. However, it also suggests that unfavorable environments may increase the stress on soybeans from these herbicides and should be used with the understanding of the risks involved in their application. The 1999 results also show that Roundup has some affect on soybean yields and was in the middle group of herbicides for yield. The following is a summary of yield differences at six locations.

1998

Rothsay

Fairmount

Arthur

Galchutt

med.texture
3.5 % OM
low salt

heavy texture
3.5 % OM
high salt

sandy texture
4%OM medium salt

sandytexture
2.5 % OM
high salt

Yield, bu/acre

Pursuit

43.4

Galaxy

37.9

Galaxy

38.5

Galaxy

33.0

FirstRate

40.1

FirstRate

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

 

1999

Rothsay

Colfax N

sandy texture
3.5 % OM
high salt

heavy texture
4.5 % OM
high salt

Yield, bu/acre

Blazer

34.1

Blazer

25.4 a

Pursuit

32.6

Basagran

24.7 a

Flexstar

31.2

Storm

19.6 a

Basagran

30.1

Galaxy

17.3 b

First Rate

29.5

First Rate

14.5 b

Roundup

28.9

Cobra

12.8 b

Pinnacle

28.9

Pursuit

12.1 b

Galaxy

27.4

Flexstar

11.5 bc

Storm

27.0

Pinnacle

11.3 bc

Raptor

21.3

Raptor

7.4 c

Cobra

20.0

   

LSD 5 %

4.9

 

6.6

    Herbicides are an important tool for controlling weeds and maintaining high yield environments. Application of post-emergence broadleaf herbicides to chlorosis stressed soybeans is a special case for the herbicides tested in these trials. Certainly, if there are several herbicide choices for controlling weeds and one of the less stressful herbicides could be used on chlorosis stressed soybeans, yields could be improved by using that product. However, there will be situations when the less stressful herbicides should not be used because of the spectrum of weeds that must be controlled. In these cases, some assessment of the affect of the weeds on final yield or ability to harvest should be made. In cases where few weeds exist and the aim of herbicide control is maintenance of a low-weed environment, perhaps hiring hand-weeders would be a better option than applying a harsh herbicide. For fields with high weed pressures, the decision might be more difficult, with the options being to abandon the field or accept a yield loss through the use of the herbicide.

    To avoid undue losses, it is important for growers to head of problems during the seed and field selection process. Screening fields to avoid high salt problems and selecting varieties not on chlorosis ratings that may have no meaning in our area, but instead on proven track records or performance in our area is extremely important. Herbicide stresses would not be expected to be as great on adapted varieties grown on more favorable soils.

Dr. Dave Franzen
NDSU Extension Soil Specialist


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