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’.”