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Can wheat/soybeans slow nematodes?

One year of wheat double-cropped with soybeans did a better job of reducing reniform nematode populations in cotton as a year of corn, according to a Mississippi cotton producer

Rick Parsons, who farms 1,200 acres of cotton, 1,200 acres of soybeans and a little less than 1,000 acres of wheat in Vance, stresses that his experiment with a wheat/soybean and cotton rotation to control reniform isn't scientific. “We don't have enough years to know exactly what's going on.”

But the evidence clearly showed significant reductions in reniform nematode numbers after the wheat/soybean crop.

Parsons started rotating corn on about half his cotton acreage in the mid-1990s, “because the land had been in cotton so long, and at the time, the price of corn looked good and I wanted to build organic matter. Corn really helps.”

For a couple of years, the rotation program of one year in cotton and one year in corn worked well. “We were making tremendous yields behind corn.”

Then in 1999, cotton yields faltered on one field behind corn. “We had a great stand,” the producer recalled. “We kept the plant size down. We liked the way the internodes looked and we had a good boll count.

“But when we started picking that cotton, it just didn't turn out. We didn't pick but 900 pounds, irrigated. We should have been making 1,000 or 1,200 pounds. I couldn't figure out what happened.”

A friend suggested that Parsons may have a reniform nematode problem. The producer didn't think he did since the field had been in corn for two or the previous four years and corn is a non-host for the reniform; but he decided to check it out anyway.

In the fall of 1999, his consultant Mike Sartor, with Sartor's Ag Consulting, sampled selected 7.5-acre grids in the field. “I was shocked,” Parsons said of the results. “We had 40,000 to 60,000 reniform nematodes per pint of soil in some places and the threshold (in the fall) is 5,000.”

At that point, Parsons had made up his mind that he would have to go to two years of corn to control the reniform nematode. That's when he said he “stumbled” onto the wheat and soybean option.

The producer had already gone to a wheat/soybean rotation on the farm a couple of years earlier, but that decision was based purely on the wheat/bean crop's profitability versus corn. In 1999/2000 wheat and double-cropped soybeans were planted in the high reniform nematode field.

In the fall of 2000, Sartor sampled the field again for reniform nematodes and much to everyone's surprise, populations had declined dramatically. One grid that had 59,000 reniform nematodes per pint of soil in 1999 had only 1,487 in the fall of 2000. Results were similar across the other grids sampled.

“It just knocked the fool out of the nematodes,” Parsons said.

In addition, in the extremely dry year of 2000, all of Parson's cotton following wheat and soybeans made almost 1,200 pounds, all irrigated under a center pivot.

So what happened in the field with the reniform nematodes? For one, Parsons planted a moderately resistant soybean variety, Hyperformer 574, which Mississippi State University entomologist Gary Lawrence says played a big role in reducing nematode numbers.

The effect of the wheat/soybean crop on reniform nematodes, “needs more study,” Lawrence says. The nemtatologist plans to sample Parsons' field after the producer plants cotton in the field, to start his own database for the field. Unfortunately, research funds for a broader project on the role of wheat/soybeans were not approved this year.

Lawrence added that after wheat is disked in following harvest, it forms substances which are nematicidal. And some cover crops like rye and vetches are known to reduce nematode populations.

University of Arkansas nematologist Bob Robbins wasn't surprised by the reduction in numbers on the Parsons field, noting that wheat is a non-host for reniform nematodes and that three of the four crops planted in the field over a three-year period were non-hosts — corn, wheat and moderately resistant soybeans.

Sartor adds that he's finding more and more reniform nematodes in his customer's fields these days.

“I've routinely taken soil samples for the last 25 years. Starting in about 1992, we started picking up the reniform nematode. From 1993 to 1996, the populations have jumped.

“They really fluctuate in population throughout a field,” the consultant said. “On a field in Belzoni (Miss.), one part of the field showed less than 1,000 per pint of soil and another part of the field showed 30,000 per pint of soil.”

Sartor and a growing number of nematologists are now starting to believe that once nematodes infest a field, they're there to stay. Non-host crops simply force them down into the soil where they remain dormant until a host crop is planted. “I believe we're going to be fighting nematodes from now on. We have to learn how to manage them.”

Rotation is the best option for control of reniform nematodes, and corn is still considered the best crop for rotation, noted Sartor. “If I have a field where the reniform population is up above 15,000, I'll recommend corn or another non-host crop for two years in a row instead of one.

“If it's less than that, or if a farmer has to put cotton on it for economical reasons, I recommend five pounds of Temik at planting. Then we come back with two 8.5 ounce applications of Vydate CLV 7-9 days apart beginning at pinhead square.”

Vydate can also provide some control of plant bugs, which are increasing in damage about that time of the season. It's also cheaper than coming back with a sidedress application of another five pounds of Temik, another option for reniform nematode control. “When you do that, you're getting up to $35 an acre.”

Parsons believes Temik was keeping reniform nematode numbers down when he was using a higher rate several years ago. “The problem didn't start showing up until I dropped my rate of Temik.”

Parsons isn't sure that raising the rate of Temik now will help control an established reniform nematode population. But at this time, samples indicate that the reniform nematode is not at threshold levels on any of his fields.

This coming season, Parsons will be able to make even more observations about how various rotations impact reniform nematodes. “I'll have cotton behind corn, cotton behind wheat and cotton behind cotton.”

e-mail: [email protected]

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