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One more reason to control volunteer corn

volunteer corn in soybean field at harvest
TAKE IT OUT: If you’re going after broadleaves and the herbicide isn’t good on volunteer corn, entomologists suggest using a grass herbicide to take out the corn so it doesn’t grow all season.
Entomologist says it’s worth eliminating volunteer corn in soybeans to protect against insect resistance.

Modern corn heads do a much better job of harvesting lodged and down corn than corn heads of the past. Nevertheless, if you harvested corn late and some was lodged, odds are at least some ears and kernels were left behind. Corn left in fields rotating to soybeans in 2018 could become volunteer corn this season.

Depending on your herbicide program, that may or may not be a problem. Suppose you have volunteer corn here and there, and the program you’re using to go after tough broadleaves doesn’t take it out effectively. Is that a problem?

Any volunteer corn allowed to escape and grow with the crop will compete with soybeans and can affect yields. Weed control specialists proved that years ago. But entomologists say there is another important reason for making sure you don’t leave volunteer corn behind and let it escape.

“You don’t want to set up a situation where corn rootworm could feed on those volunteer corn roots as larvae, survive and develop any type of resistance to Bt-rootworm control traits,” says John Obermeyer, Purdue University Extension entomologist.

How it could happen
Many people plant triple-stacked hybrids today with Bt resistance to corn borer and corn rootworm, Obermeyer says. Volunteer corn, the next generation after hybrid corn, will contain diluted doses of the protein in the roots meant to kill corn rootworms.

“The amount of protein in volunteer corn roots is small enough that if rootworm eggs were laid there and larvae hatch, typically around June 1, they will feed on the roots and survive,” Obermeyer explains. “Once they become beetles, if they mate with other beetles which also survived the very low dose, their progeny could begin to express resistance to the corn rootworm protein.”

That means allowing volunteer corn to stay in the field when rootworm larvae hatch could contribute to development of resistance over time, he says. “It’s another very strong reason to make sure that you control volunteer corn in a timely manner with your herbicide program.”

Corn borer story
Why is Obermeyer less concerned about corn borer resistance stemming from volunteer corn? “It’s a different situation because the Bt protein that controls corn borer is expressed plentifully in the aboveground green tissue,” he says.

“The rootworm Bt protein in the roots isn’t expressed in a hybrid at nearly as high a rate as Bt protein in green tissue for corn borer. By the time the rootworm protein is diluted in the generation after the hybrid, it isn’t lethal to rootworm larvae.”   

Rootworm resistance
The reason rootworm resistance developed originally has nothing to do with volunteer corn, the entomologist notes. Instead, farmers in some western Corn Belt states planted continuous corn with the same Bt trait for rootworm control year after year. Eventually, enough of the rootworm population could tolerate it and survived and mated, which led to generations of rootworm larvae that could withstand feeding on the protein.

So the best option to combat another resistance mechanism is to take out volunteer corn with herbicides, Obermeyer says.

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