Study examines the impact of residual herbicides on fall-seeded cover crops

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Cover crops are attracting widespread attention for the benefits they provide in crop rotations, especially for soil health. However, many growers of corn and soybean crops know it can be challenging to establish fall-seeded cover crops. An article featured in the journal Weed Technology provides important insights into why. The authors have discovered that residual herbicides applied as part of a no-tillage system or to control glyphosate-resistant weeds can inhibit cover crop growth.

A team from the University of Missouri 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.

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