Research Highlights

Creating Awareness of Soybean Tolerance with Pre-plant Dicamba Application

By Carol Brown

For farmers everywhere, keeping weeds at bay in a crop field is an ongoing battle. Herbicides are a useful weapon in this fight but need to be handled with care. The herbicide dicamba is effective on weeds when used properly and usage restrictions are followed.

A research project led by Greg Endres, North Dakota State University Cropping Systems Specialist, explores the relationship between low-dose dicamba with non-Xtend^® soybeans, dry beans and sunflowers. The project, supported by the North Dakota Soybean Council, is measuring plant injury and tolerance when dicamba is applied before planting.

“We’ve had people ask us questions about management of this herbicide and its effect on soybeans and other broadleaf crops,” Endres says. “We are conducting this research in response to their inquiries and the results could help with management decisions.”

Dicamba label restrictions include waiting to plant soybeans until after an inch of rainfall and a minimum of 14 days following pre-plant dicamba application. Geographic restrictions include not using dicamba in pre-plant situations if rainfall is less than 25 inches annually.

Endres and his research team conducted tests on low-dose dicamba application at the Carrington and Minot Research Extension Centers (REC) as well as an NDSU research site at Prosper. They applied dicamba at 4 oz. of product per acre, then planted soybeans within seven days and greater than 14 days after application. Plants were monitored for emergence and development during the season, as well as density and injury. At Carrington, soybean seed yield was also measured. Also at the Carrington site, the plots were under irrigation, while the other two sites were dryland. Soybeans were grown under a no-till system at Minot and conventional tillage at Carrington and Prosper.

Results from the first year of the study showed significant soybean plant injury with the earlier planting date across all of the locations. At Carrington, the injuries were significant, but the lowest of the three sites. At Prosper, plant injury was as severe as 73 percent. At Minot, plant injury was as high as 87 percent. Endres says that plant stand with dicamba treatments was not reduced compared to untreated checks at Carrington, but significant stand reductions occurred when planting soybeans within seven days of dicamba application at Prosper and Minot (38 percent and 68 percent, respectively).

“To summarize, generally soybean plant injury was high with both planting dates at each of the locations,” Endres says. “There was significant injury even with the second planting date. Interestingly though, sunflower appeared to be quite a bit more tolerant than the legume crops, which surprised us.”

Because of excessive weed competition due to restrictions within the study and poor crop development, soybean yield was not measured at Minot or Prosper. But at Carrington, Endres saw no yield reduction, and yields were greater than 70 bu/acre across all of the plots including those with dicamba treatments. He attributes this to having adequate rainfall and irrigation throughout the season that allowed the soybeans to overcome the early-season plant injury.

The choices of herbicides are limited for complete weed extermination prior to planting, especially in no-till situations. Endres says farmers are looking for a product that’s effective, low-cost and has some potential residual to suppress later weed seed germination. Dicamba is effective on horseweed (marestail) and wild buckwheat, which are early emerging weeds that are troublesome in no-till systems. The herbicide also works well on several other broadleaf weeds including common lambsquarters and pigweeds. 

The results from this past crop year have been recorded into a database, and the research is being conducted again in crop year 2022 with some fine-tuning, Endres says.

“We want to build awareness of these tests for producers and applicators, to let them know the consequences of plant growth with these strategies,” he says. “Dicamba label restrictions are in place for good reason.”

Published: Jul 4, 2022