Purdue University Weed Scientist Glenn Nice says farmers need to take precautions to prevent carryover of mesotrione, an active ingredient in several corn herbicides, into the following year's soybean crop.

Mesotrione provides excellent pre-emergence control of black nightshade, lambsquarter, pigweeds, annual smartweeds, velvetleaf and waterhemp, says Nice. It also provides excellent postemergence control of these weeds and many others.

"In rare cases it has been reported to carry over into soybean," he says.

In 2004 while updating the "Weed Control Guide for Indiana and Ohio," Nice and his colleagues found a few cases of mesotrione carryover injury to soybean in northeast Indiana. A majority of the injury was found on sandy soils with low organic matter and low cation exchange capacity (CEC). Another common theme was the soil pH, which was often below 6.0.

"Mesotrione is broken down in the soil by microbial activity," Nice says. "Any soil quality that would inhibit microbial activity, such as dry conditions or changes in pH, could potentially lead to a decrease in the breakdown of herbicides. Doubling up on rates due to overlapping or improper calibration of spray equipment would also lead to a high potential for carryover."

Soybeans with carryover injury typically show bleaching of the tips and margins of new growth on the plant.

While in most cases soybeans appear to grow out of the injury without greatly lowering yields, Nice says it is still important to take precautions to prevent carryover. Farmers need to read the herbicide labels and follow application rate, timeline and rotation restrictions.

"One herbicide's rotation restriction for planting soybean is 10 months, and, for the most part, this is sufficient to assure that the herbicide has broken down enough to have no impact on the following year's soybeans," Nice says.

More information about mesotrione carryover can be found in a free Purdue Extension publication, Mesotrione Carryover into Soybean (pdf).