Allelopathy in continuous corn is as hard to pin down as an election-year politician. By definition, allelopathy occurs when organisms produce biochemicals that influence the growth of other plants – such as when corn residue harms young corn plants. Such interference can be measured by researchers in controlled studies, butso far it can’t be replicated in the field. Yet it’s often cited as an underlying cause of the yield drag that many growers experience in continuous corn.

Corn after corn is a challenge, says Scott Kibbie, who grows between 200 and 400 acres of continuous corn near Emmetsburg, IA. “There are years the corn crop is as good or almost as good as the corn on soybean ground. Other years, it’s worse for corn on corn. I can’t put my finger on (the cause).”

In years where he sees a yield drag, it averages between 10-15%.

Brad Stewart, who grows 6,500 acres of continuous corn near Yorkville and Farmer City, IL, didn’t see a yield drag in his continuous corn until the fourth year of corn-on-corn.

“The last two years have been pretty poor,” he says, estimating his yield loss at 10-15%.

After seven years of corn-on-corn, last year was the first with a yield penalty for Scott Landrey. He grows 2,100 acres of continuous corn just south of Springfield, IL. But it was a stomach-churning 20-25%.

“We haven’t been able to pinpoint the cause, whether it’s a lack of nitrogen or more toxins in the soil. We ruled out the heat,” Landrey says.

The fluctuating nature of the yield penalty in continuous corn has led some to question whether allelopathy plays a role at all. While others say it’s clearly there, but it’s weather-dependent and manageable.

The classic allelopathy[KL1] laboratory experiment, says Roger Elmore, Extension corn agronomist and agronomy professor at Iowa State University, is to filter water through corn residue and then use that water to germinate corn seed. The control group gets unfiltered water. In all cases, the control group will perform better than corn germinated with filtered water.

In the field, several factors contribute to the yield reduction, he says, including wet weather, residue, disease pressures and allelopathy.  However, the latter is influenced by the first three factors, which results in the fluctuation many farmers see.

“The range is what’s really interesting,” he says. “It goes all over the board from no reduction to sometimes 28-30% yield reduction.”

Even in controlled research conditions where management techniques and hybrids are consistent from year to year, there can still be sizable yield penalties due to weather conditions, Elmore adds. Outside of controlled studies, even if farmers use the same fields year after year for rotated corn and continuous corn and measure results, few growers will account for changes in hybrid productivity and management practices over time.

“They’re probably doing better with corn following corn than they used to, but there’s good data showing they’re still being penalized yield wise,” he says.

Bob Nielsen, Purdue Extension corn specialist and professor of agronomy, wrote in an email that there is very little evidence that allelopathy (autotoxicity) occurs in the field with continuous corn production. “Over the years, laboratory and/or greenhouse research has detected evidence that such effects are possible, but nevertheless, no one that I know of has conclusively demonstrated that it occurs in the ‘real world’.”

Too much residue?

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.”

Compromise on residue

Many continuous corn growers turn to tillage to help break down corn residue and reduce negative affects on the new crop, but a recent study from Pioneer Hi-Bred suggests that no-till farmers can manage residue and decrease the yield drag in continuous corn without hooking up the chisel plow.

Andy Heggenstaller, agronomy research manager with Pioneer Hi-Bred, says a four-year study near Columbia, MO, reviewed several methods for managing corn residue, but the one that proved most effective was removing a portion of the stover.

“Removing about half of the stover in a continuous, no-till corn system can have similar yield benefits as tillage or rotation with soybean,” he says. "Typically, residue from the previous crop negatively affects the new crop and this is magnified in no-till."

The study, which was published in a recent edition of Crop Insights by Pioneer, looked at five methods for managing stover:

Haggenstaller says the first three methods reduced residue by 9%, 16% and 53%. Row cleaning didn’t remove the residue, merely pushed it from the row area. The only effective treatment, bale and removal, increased yields by 16% compared to the control group.

Removing the stover resulted in increased populations and plants that were more vigorous, in part because less nitrogen (N) was tied up by decomposing stover. This result suggests that no-till growers may be able to reduce N fertilizer when some stover is removed from the field.

Haggenstaller cautioned against interpreting the results too broadly, particularly when looking at methods that didn’t perform as well as expected. Chopping stalks in the fall and row cleaners at planting are common residue management practices that have delivered results in other studies, particularly in northern areas where the season for residue decomposition is shorter.  

“Geography is very important when managing residue in continuous corn,” he said. “But if taking some of the stover off in no-till continuous corn delivers a benefit in central Missouri, it (also) would benefit Iowa or southern Minnesota.”