Adding Wheat To Corn-soybean Rotations Helps Control Pests

With prices for corn and soybeans leveling off, wheat could be poised for a comeback. Because of its repositioning on a more level economic playing field, wheat offers a financially competitive option for a third crop in a corn-soybean rotation.

“From a biological standpoint, having more diversity in the rotation is always good for the system as a whole,” says Joel Ransom, North Dakota State University Extension agronomist.

Benefits from rotating

Wheat offers significant threefold benefits when grown with corn and soybeans. Wheat aids in the management of glyphosate-resistant weeds in both crops. It also helps control important pests in both the corn and the soybeans.

In the bean crop, the soybean cyst nematode takes an extra hit when wheat is grown between soybeans and corn. Adding wheat to an otherwise two-year rotation extends the break between soybean crops. This serves to further diminish otherwise persistent nematode populations.

“The soybean cyst nematode makes a cyst that persists in the soil, and the cyst is filled with eggs that release worms,” says Ransom. “The worms can only survive on soybean plants and closely related weed species. If there are no soybeans around, the nematode population is reduced.”

In a two-year rotation between soybeans and corn, the break to corn, of course, kills some of the nematodes. Lengthening the break by adding wheat further depletes the number of viable worm eggs.

Breaking the cycle

To corn’s benefit, extending the rotation by adding wheat helps to control the corn rootworm. Formerly, a one-year break from growing corn gave the time needed to provide control. The rootworm in some areas of the U.S. has genetically adapted, however, learning how to survive a two-year rotation.

In the life cycle of the rootworm, the adult female is attracted to a corn plant and feeds on the silk. The rootworm then drops to the ground and lays eggs.

In the past, the one-year break from corn by growing soybeans gave the time needed to starve the rootworms, reducing populations. Recently, variant species of rootworms have biologically adapted to the two-year rotation.

“These types of rootworms have ‘learned’ that farmers grow soybeans after the corn,” says Ransom. “During the soybean year, they remain dormant in the soil and emerge then in the second year to feed on the corn crop following the soybeans.”

Lengthening the break between corn and soybeans by adding wheat to the rotation starves the emerging rootworms.

“A GMO trait in corn offers some control, but there have been reports of resistance being built up in the rootworm,” says Ransom. “This is a pernicious pest, so you need to use as many control strategies as you can. Effective rotation is a good alternative.” Improved weed control is another benefit of adding wheat to a corn-soybean rotation.

“Some weeds have developed resistance to the glyphosate used in controlling weeds in both corn and soybeans,” says Ransom. “By adding wheat to the rotation, you can use a different herbicide for weed control. That reduces the rate at which weeds are developing resistance to glyphosate.”

Wheat is best placed in the rotation as the crop to follow soybeans, with corn then following the wheat.

“Wheat is susceptible to Fusarium head blight,” says Ransom. “Head blight overwinters on stalks of wheat and on corn, as well. Because wheat is susceptible and corn is not, it’s better to grow wheat after the soybeans.”

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