Cover crops are widely seen as one of the most promising conservation practices, improving soil health while also removing carbon from the atmosphere. But while the number of Midwestern farmers planting cover crops has increased markedly in recent years, 2017 USDA Census data show only about 5% have adopted the conservation practice. The reluctance of the other 95% may be due, in part, to a perception that cover crops require more effort and may also negatively affect summer cash crop yield.

New University of Illinois research integrates field data and advanced mathematical modeling to understand how cover crops affect soil water, nitrogen, and oxygen dynamics, and may compete with summer cash crops.

“Cover cropping requires management. Otherwise cover crops compete with corn and soybean and can cause some yield loss. With proper management, however, farmers could use the right cover crop types and find the optimal growth window to plant and terminate cover crops to achieve benefits and minimize negative impacts on cash crops,” says Kaiyu Guan, founding director of the Agroecosystem Sustainability Center, associate professor in the Department of Natural Resources and Environmental Sciences, and Blue Waters professor at the National Center for Supercomputing Applications at the University of Illinois. He is also senior author on a new paper published in Field Crops Research.

Guan’s insights are based on a sophisticated mathematical model validated by five years of experimental field data collected from multiple sites across Illinois by Maria Villamil, a co-author of the paper and professor in the Department of Crop Sciences at Illinois. The process-based model aims to identify the underlying drivers of cover crop effects on cash crop yield, including cover crop type; termination timing; and soil factors such as water, nitrogen, oxygen, and soil temperature.

“Process-based models validated with field data have multiple advantages compared to field experiments alone,” says Ziqi Qin, doctoral student working with Guan and lead author on the study. “Most field experiments only focus on final variables such as cash crop yield or cover crop biomass, and can take years to conduct.

“Process-based modeling methods can simulate intermediate variables that are difficult to measure in field experiments, such as processes taking place in the soil. Models validated with field-based measurements can help optimize cover crop decisions, such as cover crop types and planting and termination time, through scenario simulations.”

By incorporating intermediate factors, the model explained why cover crops interfere with cash crop yield. Essentially, the two types of crops compete for common resources in the soil, including water, nitrogen, and oxygen. But context matters and the impacts are species-specific.

Soybean yield was unaffected by either type of cover crop, probably because soybeans put their own nitrogen into the soil. For corn, competition for water is heightened in dry years, according to the model, and the later cover crops are terminated, the less nitrogen is available for cash crops.

When the model focused on cover crop type, it found non-legume species, such as annual ryegrass and cereal rye, reduced corn yield by 0.9 to 6.9%. However, the nitrogen-fixing legume hairy vetch didn’t impact corn yield under high-nitrogen conditions. These findings are consistent with field observations across the Midwest and worldwide, and Guan says that lends credibility to his Midwest-centric modeling study.

The model found termination timing can be just as important as species. Late termination of non-legume cover crops – just a day before planting – resulted in more corn yield loss than terminating a month ahead of planting.

But that’s less time for the cover crop to do its work.

“There is a tradeoff between cover crop benefit and cash crop yield. If we terminate earlier, the cover crop won’t affect cash crop yield as much, but it will accumulate less biomass and potentially take up less soil nitrogen. So we have to balance those two factors,” Villamil says.

The model also identified other factors that negatively impacted cash crops, including cooler soil temperatures under cover crop biomass and less soil oxygen availability.

“You have to understand the process, and that part has been missing from other research in this area,” Guan says. “For example, I don't think people fully appreciate the impact of oxygen in the soil, which turned out to be an important factor in our model. And many of these factors change in context of weather, climate, and soil. All these are worth more systematic studies.”

Guan notes programs like the USDA’s Pandemic Cover Crop Program, which reduces crop insurance premiums for farmers who grow cover crops, may incentivize more of the 95% who don’t to get on board with the conservation practice.

“In addition, with the increase in the private carbon credit market, there could be an increase of cover crop adoption in a significant way. We probably will see a surge. So this makes this topic extremely relevant and important,” he says. “We’re here to tell farmers how the science works, and then properly guide them to gain the benefit of cover crops.”