As escalating corn prices have encouraged many farmers to switch to growing corn continuously, they wonder why they have been seeing unusually high yield reductions over the past several years. The University of Illinois conducted a six-year study that identified three key factors affecting yield in continuous corn (CC) systems.



“Prior to this study, the most common management recommendations for continuous corn production were to apply an additional 45 lbs. of nitrogen/acre and reserve your best crop land for it,” says U of I Soil Scientist and lead author Laura Gentry. “Very little was known about the agents or mechanisms causing reduced yields in continuous corn systems.”


Although corn can be cropped continuously, it is widely accepted that there is a yield reduction compared to corn rotated with soybean (CS). This difference is referred to as the continuous corn yield penalty (CCYP), which is generally in the range of 20-30 bu./acre. The 2012 growing season marked the third consecutive year of unusually high CCYP values in the U.S. Midwest, often with corn yields that were 30-50 bu./acre less than corn following soybean.

The researchers conducted the experiment from 2005 to 2010 in east-central Illinois, beginning with corn produced in a third-year CC system or a CS rotation, at six N fertilizer rates. The study investigated:

  1. How the yield penalty changed with time in CC
  2. Under what conditions increasing the nitrogen (N) fertilizer rate reduced the penalty
  3. What causes the penalty?

Each year, they determined an “agronomically optimum N rate” and corresponding yield value for each rotation (CC and CS). On average, corn yield at the agronomically optimum N rate for CC was 167 bu., compared to 192 bu./acre for CS – a CCYP of 25 bu./acre. CCYP values ranged yearly from 9 to 42 bu./acre.

Matias Ruffo, a co-author of the paper and Worldwide Agronomy Manager at The Mosaic Co., says, “To explore the causes of the CCYP, we tested a number of different weather- and yield-related measurements for their relationships with the CCYP. We found that with just three predictors, we could estimate the CCYP with almost 100% accuracy.” The predictors were: 1) unfertilized CC yield, 2) years in CC, and 3) the difference between CC and CS delta yields.

 

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The researchers found that the best predictor of the CCYP was unfertilized CC yield. In years when unfertilized CC yields were relatively high, the yield penalty was low, and vice versa. Unfertilized CC yield is an indicator of how much N the soil is supplying to the corn crop, either from residual fertilizer N or from decomposition of previous crop residues and other organic matter (N mineralization).