By: Aaron Viner 

The decision on when to terminate cover crops may affect the overall yield of the upcoming cash crop.

Katja Koehler-Cole, a professor of agronomy at the University of Nebraska, said research is showing that a later termination may result in a lower overall yield. However, that doesn’t mean early termination is right for your field.

Koehler-Cole has been researching the allelopathic effect of cereal rye on corn. Allelochemicals are released into the soil by all plants through the plant roots, washed off leaves or shoots by rainfall or released by decaying residues.

“When we look at weed spieces, many of them have small seeds which can make them vulnerable to alelopathic chemicals,” Koehler-Cole said. “These chemicals suppress germination and very early growth. They won’t kill an established plant.”

Despite the possible natural herbicide, the allelopathic compound from some cover crops may have a negative effect on the crop moving into the season. In her research, Koehler-Cole found corn germination was sometimes affected by some of the winter cereals, and in some cases showed corn root length was also reduced.

Iowa State extension crop specialist Mark Licht said determining the effect of allelopathic chemicals is difficult to research. Many natural effects are happening in the early growth stagest, which makes singling one out difficult.

Terminating cereal rye two weeks ahead of planting will often avoid negative effects, but many farmers are able to have success planting green into the cover crop.

Less favorable conditions may also include less soil nitrogen and concerns of cover crops carrying pathogens over to the corn seedlings.

Licht said while there are signs of yield loss, blaming it all on allelochemicals is not always the right answer.

“I would challenge anyone to say definitively it’s allelopathy versus pythium versus nitrogen immobilization,” Licht said. “If you are talking about planting into waist high rye, you might have nitrogen immobilization. We have to be careful with saying it’s allelopathy versus something else because it could be all three of them.”

If doing pre-plant terminations, Koehler-Cole suggests applying nitrogen as a starter. Existing cover crops may take much nitrogen from the field, leading to the “sickly” look as corn emerges from rye.

“Terminate several weeks before you plant and then you should be safer,” she said. “Then the compounds will have been degraded into the soil.”

For those planting green, Koehler-Cole suggests waiting until at least emergence before terminating the cover crops, and ensure what is being used for termination will not hurt the corn or soybeans.

“The idea would then be to terminate once the corn or soybeans have emerge because then the crop should be past the vulnerable stage,” she said.