How do you rid your soybean fields of a pest that’s been around for the last 30 years? Try rotating varieties.

Variety rotation is being studied by University of Arkansas Division of Agriculture plant pathologists as a way to decrease the soybean cyst nematode population in fields.

“We’re looking at rotation to see if we can control the soybean cyst nematode through rotations of varieties having different resistant types,” says Bob Robbins, UA plant pathologist and nematology professor. Resistant types, he adds, are the original source of the cyst resistance.

According to Terry Kirkpatrick, who’s conducting the research with Robbins, many times producers don’t want to rotate crops each year because, economically, they may do better growing soybeans.

“We’re trying to give farmers some guidance and keep them in the soybean business if they don’t want to rotate their crops, but at the same time control their cyst nematode problem,” explains Kirkpatrick.

The studies work like this: the first year, the farmer can plant whichever resistant variety type he chooses. The second year, he would plant another resistance variety type. He might sacrifice a little on yield, but this should start reducing the nematode population. The third year, the farmer plants the third resistant variety type.

The three-year rotation cycle is then repeated.

Robbins says the test varieties are planted in early May or June, and soil samples are taken at this time. From the soil samples, the cyst nematode population is determined. During harvest, another cyst nematode population count is taken, as well as soybean yield.

“We take the beginning and ending population counts to get a reproductive index,” Robbins says. “We’re looking for the number to go down.”

So far, Robbins says, the populations seem to be decreasing somewhat using the three-variety rotation. He explains that the rotations don’t have to reduce the populations a lot; if the numbers keep going down gradually, it will be a big help to farmers.

“Another question we’re studying is what happens if farmers choose to continually plant the multiple resistance variety,” says Kirkpatrick. “We want to know if, over time, we will actually develop a new race of nematodes because we forced the pest to change and adapt to the multiple resistance varieties.”

Kirkpatrick says a good parallel is Roundup-resistant weeds. “When Roundup came out, we used it on everything and now we’ve got resistant pigweed and horsetail coming in.”

Robbins and Kirkpatrick are conducting their research with funding from the Arkansas Soybean Promotion Board.

e-mail: efortune@uaex.edu