The average continuous corn yield penalty increased nearly threefold from the third year to the seventh year of successive corn crops, according to 5 years’ research in east-central Illinois by University of Illinois’ Laura Gentry, soil scientist, and Fred Below, crop physiologist. The persistence of the yield penalty was regardless of N rates, they found.

“This is at odds with claims of many Corn Belt farmers who argue that continuous-corn yields eventually attain the same level as corn-soybean rotations,” Gentry says.

The causes are still a bit of a mystery. The best predictors of the continuous-corn yield penalty in any given year, the research found, are:

  • N availability. Soil-N mineralization decreases in continuous-corn systems, Gentry says. In very good growing years, when soils mineralize a lot of N, increased N supply can largely offset the yield penalty, she says. “Unfortunately, higher N rates do not eliminate the yield penalty during average or poor growing seasons.”
  • Number of years in continuous corn. The yearly residue accumulation in continuous-corn systems can lower soil temps and delay germination, tie up N, harbor diseases and fungi, and interfere with seed placement, stand establishment and soil microorganisms, Gentry says. “Our research shows a clear escalation in the yield penalty from the third year to the fifth year to the seventh year.”
  • Unfavorable weather is more detrimental to corn grown continuously than in rotation.

bad weather intensifies continuous corn yield penalty

Two of these three factors — weather and soil N mineralization — are beyond farmers’ control. So, for those who decide to raise continuous corn, “managing corn stover has the greatest potential for reducing the continuous corn yield penalty,” Gentry says.