Soybeans - Herbicides, Chlorosis and 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 1999

Rothsay		Fairmount		Arthur		Galchutt
med.texture 3.5 % OM low salt		heavy texture 3.5 % OM high salt		sandy texture 4%OM medium salt		sandy texture 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

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