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Corn+Soybean Digest

Stay In Rotation

Growing soybeans after soybeans lowers yield Remember the great feeling that came with your last field of 50-bu/acre soybeans? If you plant continuous soybeans, crop experts say, all you may have left is that memory.

Low crop prices are leading more farmers to plant soybeans after soybeans, according to reports from the eastern Corn Belt. Unfortunately, environmental conditions caused more disease last season, says Anne Dorrance, Ohio State University plant pathologist. Farmers who plant those fields back into soybeans are leaving themselves open to trouble.

Studies show that anytime you plant continuous soybeans - or continuous corn or wheat, for that matter - you run a risk of up to a 30% yield reduction. Jim Beuerlein, Ohio State crop scientist, says the yield loss for the first non-rotated year of soybeans averages 10%. If you go a third year, he says, you get an additional 7-9% drop.

"Eventually, you're usually out 20-25% of your yield," Beuerlein says. "If soybean cyst nematodes (SCN) become a problem, the yield loss can be greater than 50%."

Growers are taking the risk that they'll have no yield reduction, or at least minimal loss. They're reporting fewer wheat plantings and saying planting soybeans after soybeans is necessary when prices are so low. In Ohio, soybean acreage totals 4-4.5 million each year, of which 1-1.5 million are in continuous soybean production. That number is expected to increase this year.

In west-central Ohio, Shelby County extension agent Roger Bender sees a gradual movement away from wheat. With wheat prices down and the market for straw shrinking, he says, wheat isn't as attractive as it used to be. "Plus, we had some years with disease problems in wheat," Bender says. "That memory lingers, so we see more guys going to just corn and beans."

Add to that the fact that it costs less to plant soybeans than corn, and you get one rationalization for the increase in continuous soybean acres. That helps explain the increased prevalence of soybean diseases.

SCN is prevalent now in eastern Corn Belt states like Ohio and Indiana. Dorrance says she's also seeing an increase in sclerotinia white mold, brown stem rot and phomopsis. The latter, a form of seed decay, is most prevalent when there's more moisture late in the growing season. This past year, Dorrance says, conditions were perfect for phomopsis.

"As beans were drying down, we had a month of rains," she says. "So these fields have very high inoculum levels from the pathogens' survival on soybean straw. During rotation, there's time for other organisms to break down the crop residues."

Rotation is a key to managing all of those diseases and giving your soybeans a chance to meet their yield potential. Severe problems with white mold or SCN can require longer rotations, but Dorrance says a good rotation is wheat-corn-soybeans or even wheat-corn-soybeans-soybeans.

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