When corn prices were high, a lot of U.S. corn farmers started planting continuous corn, meaning that they planted corn-on-corn for more than three years. During that time, many of those same farmers began seeing a 20-30 bushel per acre drop in yields.
This became referred to as the Continuous Corn Yield Penalty (CCYP), and researchers at the University of Illinois have released the results of a long-term study on continuous corn and found out why it happened.
The three major factors turned out to be reduced nitrogen availability, the accumulation of corn residue, or stover, and weather.
Research leader Laura Gentry, a soil scientist and professor at the University of Illinois, co-authored the study. She spoke during a recent webinar about the results and what it means for corn farmers who want to plant corn year after year.
“The one factor we can control is residue management with partial stover removal, but nutrients are available in stover,” Gentry said. “The question we had was, What’s going to happen when the stover is removed? So we offset the stover removal by adding plant biomass.”
In a continuous corn, strip-till system with Bt hybrids, the study found that more plants with a corresponding increase in inputs like fertilizer and fungicides increased corn yields by an average of 19 bushels per acre both years — which were very dry — when half of the corn stover was removed. However, the improvement of 19 bushels per acre was not enough to offset the CCYP of 25 to 49 bushels per acre.
Gentry said that continuous corn fields have less nitrogen available for succeeding crops because a lot of it remains tied up by residue from the first crop.
“More nitrogen fertilizer is required to account for the nitrogen that is being held in slowly decomposing residues and the associated soil microbial biomass, which is busy decomposing that residue,” she said.
The study compared continuous corn with a corn-soybean rotation, which is common in the Midwest but not in Texas. Corn farmers here usually rotate corn with cotton or grain sorghum. At the Stiles Farm Foundation in Thrall a few years ago, studies on plots planted in continuous corn showed about a 5 percent yield reduction over those in rotation with another crop.
Stiles Farm director Archie Abrameit said farmers here who plant corn for more than two years in a row usually make two or three modifications in soil insecticide application and seed treatment.
“You tend to have more of a problem with pests if you don’t rotate,” he said.
Abrameit said studies of continuous corn compared to corn planted in rotation with another crop are hard to decipher because so many variables are at play, including weather and economics.
“A lot of what we read about corn is geared to the Midwest,” he said. “We have to make our own modifications. We have to consider the background, the soil types and how they respond to different practices. There are a lot of variables.”
The Illinois study found that the average CCYP penalty increases the longer the land is planted in corn, rising 268 percent from years three through seven. Continuous corn is also more affected by “big weather” events like the drought.
Abrameit said that local corn farmers are aware of the risks associated with continuous corn but some will make adjustments for it if they think it will pay off.
“At times of abnormally high grain prices, a five percent reduction in yields might be worth it,” he said. “A lot of these decisions come down to economics.”