The Continuous Corn Yield Penalty: Understanding the Agents and Mechanisms.

Continuous corn (CC) acreage will inevitably increase in the U.S. if demand for corn (Zea mays L.) increases as projected.  It is widely accepted that yields are reduced when corn is grown continuously versus in rotation with soybean (Glycine max L.) although causes for the yield reduction are unclear.  The objective of this study was to elucidate the source(s) of the continuous corn yield penalty (CCYP).  The experiment was conducted from 2005-2010 in east-central Illinois beginning with 3^rd year CC or a corn-soybean (CS) rotation at six nitrogen (N) fertilizer rates.  Averaged across all years, yield at the agronomic optimum N rate for CC was 8.84 Mg ha^-1 and for CS was 10.20 Mg ha^-1, resulting in a CCYP of 1.36 Mg ha^-1 (25.6 bu a^-1); CCYP values ranged yearly from 0.47 to 2.23 Mg ha^-1 (8.9 to 42.0 bu a^-1).  Using a regression model, three significant predictors explained greater than 99% of variability in the CCYP dataset: Unfertilized CC yield (0NCCYD), years in CC (CCYRS), and the difference between CC and CS delta yields (DELTADIFF).  The strongest predictor, 0NCCYD, indicates net soil N mineralization and demonstrates that in CC systems N mineralization decreased and/or N immobilization increased.  CCYRS was strongly and positively correlated with CCYP indicating that the CCYP became greater with more time in CC monoculture, a conclusion that is counter to that of many farmers in the U.S. Corn Belt.  We believe that CCYRS measures the negative effects of accumulated corn residue in CC systems.  Finally, DELTADIFF reflects yearly weather patterns that disproportionately reduce yields for CC relative to CS systems.  Examining the relationship of each predictor with the CCYP suggests that the primary causative agents of the CCYP are weather, nitrogen availability, corn stover, and their interactions.