Many corn and soybean growers have found it challenging to establish fall-seeded cover crops. A team of University of Missouri researchers may have discovered why.
The researchers conducted field studies over three consecutive years to evaluate the impact of more than two dozen residual herbicides on eight cover crops, including Austrian winter pea, cereal rye, crimson clover, hairy vetch, Italian ryegrass, tillage radish, winter oat, and winter wheat.
Four weeks after emergence, researchers examined each cover crop for stand and biomass. They determined that the severity of the impact produced by residual herbicides depends on the weather, the species of cover crop, and the specific herbicides used.
Pyroxasulfone was found to reduce Italian ryegrass and winter oat biomass by about two-thirds in both the corn and soybean experiments. Flumetsulam-containing products resulted in the greatest carryover symptoms in the corn experiment. In the soybean experiment, products containing imazethapyr and fomesafen resulted in the most severe stand and biomass reduction.
Rainfall was found to be an important variable. During two of the three study years, more rain fell between the time of herbicide application and cover crop seeding. As a result, there was less herbicide carryover and less impact on cover crops.
Researchers found that Austrian winter pea was the most sensitive of the cover crops to residual herbicides remaining in the soil. Cereal rye was the least sensitive, with only four of 27 herbicides adversely impacting its establishment.
“Previous research has shown that cereal rye has several agronomic benefits, including a reduction in soil erosion, weed suppression, and an increase in soil organic matter,” says Kevin Bradley, Ph.D., of the University of Missouri. “Now we know cereal rye also can be effectively established in rotation with corn and soybean crops, even following the use of most residual herbicides.”
Results from this research will help growers choose cover crops that are compatible with herbicide use in the previous crop, enhancing the benefits of incorporating these plant mixtures into crop rotations.