A more likely cause for the yield drag in continuous corn could be too much residue left on the ground, which will cause the soil to remain cool and moist and affect germination, he adds.

“Stover management is the crucial piece of the puzzle. Growers who do well with continuous corn in the eastern Corn Belt are generally those who aggressively manage the stover. “


Residue is definitely the chief issue, agrees Fred Below, professor of crop physiology at the University of Illinois.

Removing the residue is the critical step to managing allelopathy, he says. He and

colleague Laura Gentry, a visiting research assistant professor,just finished a seven-year study on continuous corn.

“The penalty on average is 25 bu./acre in Illinois,” he says, “due to residue accumulation.”

Residue contributes to the continuous corn yield penalty in three ways, Gentry adds. It’s physical, biochemical and autotoxic.

Physical is the residue’s interference with the growth of the next year’s corn crop by reducing the temperature of the soil early in the growing season or holding soil moisture, she says. Biochemically, the residue can inhibit the growth of the next year’s corn crop through nutrient, especially N, immobilization. The last effect of residue is the autotoxic component.

“Of those three things—autotoxicity is the hardest to quantify,” Gentry says. “We aren’t really sure what the chemicals are that are being released by the corn plant. (Allelopathy) is like the ‘black box.’ If you have yield loss you can’t account for with physical and biochemical explanations, it’s probably that.”

Their research shows that each year in continuous corn, the penalty worsens for seven years, then plateaus out.

“It doesn’t go away, as many growers think, but it doesn’t get any worse,” Below says.

Stewart is familiar with the principles of allelopathy, but says, “we really think it comes back around to managing the residue and getting it broken down as soon as we can.”

After harvest, he tills the fields to get the ground “as black as we can” and to jump-start stover decomposition. In the spring, he cultivates just enough to level the ground before planting, uses a trash-whipper in front of the planter to eliminate any remaining residue, and adds an in-furrow fertilizer to boost germination. Choosing the right hybrid is absolutely critical, he adds.

Kibbie says breaking down the stalks is key to decreasing the yield drag in his fields. Most years, he applies ammonium sulfate to break down the residue after harvest, then in the spring uses a starter fertilizer in the furrow to kick-start germination.

Although the starter fertilizer – a mix of N and micronutrients – is his secret weapon against allelopathy, Kibbie says farmers shouldn’t simply increase the amount of fertilizer on the field. Choosing the right hybrid, managing stover and planting in fields with good drainage are critical steps, as well.

“Drainage is more critical with corn-on-corn; I don’t know why. There could something in the stalks that when they get wet and start to decompose that hurts the new corn.  It (allelopathy happens), but I think a lot of the yield drag depends on Mother Nature.”