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Corn+Soybean Digest

Small Grains Fake Out White Mold

Tim Maloney is not exactly in winter wheat country, but he likely will increase his wheat seeding. That's because the Janesville, WI, farmer, researcher and crop consultant has found that small grains, when used as cover crops, can reduce white mold and boost yields in soybeans.

Yup, you read that right. Winter wheat and other small grains show promise for putting a hit on white mold. By forming an early season canopy, they create an environment for the fungus to germinate prematurely. By the time soybeans bloom — the period when they typically contract white mold — the white mold fungus is pretty well spent.

Soybean acreage has boomed in Wisconsin and other northern Corn Belt states over the past 10 years. That area has also been a hotbed for white mold in years when conditions were right for the disease.

Much of the soybean ground was no-till drilled because that's what produces high yields — except when white mold strikes. White mold likes the closed canopy and moist conditions of narrow rows. But it hasn't been as much of a problem when beans follow small grains.

For example, Steve Tesarik, who farms with his brother Mike and father Ken near Whitelaw, WI, has some acreage in a corn-winter wheat-soybean rotation. He has seen this small grain effect firsthand.

“We've found that when we no-till drill soybeans after wheat, we never have white mold,” Tesarik reports. “If we plant the same soybean variety on the same day in a field with a corn-soybean rotation (no wheat), we do have white mold if it's a white mold year.”

University of Wisconsin plant pathologist Craig Grau observed a similar situation with wheat and oats in research fields. He theorizes that small grains could be prompting white mold sclerotia to germinate and spend themselves, resulting in less threat to the next soybean crop. However, he knows that most Wisconsin farmers do not grow wheat because it's not as economically competitive as a corn-soybean rotation. And, most oats are grown as a nurse crop for alfalfa.

Nevertheless, Grau thinks that a small grain crop grown ahead of soybeans may promote formation of a closed small grain canopy that would encourage white mold sclerotia germination before soybean flowering. That thinking prompted several research projects, some with farmer/consultant Maloney.

In 1998 and 1999, Maloney compared spring-seeded oats followed by no-till drilled soybeans to soybeans not preceded by a cover crop. The oats were chemically killed in early June.

Good growing conditions in the spring of 1998 led to sclerotia germination and high apothecia counts in the cover crop plots before burndown. It appeared the oats provided an environment favorable to “spending off” apothecia, says Maloney. Soybeans with an oats cover crop in 1998 yielded 73.8 bu/acre vs. 69.2 bu/acre for beans without a cover crop.

In 1999, apothecia counts were high early in the year in the cover crop area and high later in soybeans without a cover crop. But yields were similar in both.

For 2000, Maloney expanded his comparisons to include winter wheat, seeded the previous fall, and spring-seeded barley. Wheat was chemically killed at or soon after soybean planting. Oats and barley were killed in early June.

In all three cases, the small grain cover crop encouraged early white mold sclerotia germination and production of apothecia that came well ahead of soybean flowering. By contrast, the soybean plots without a cover crop didn't experience apothecia until they canopied in July, but counts were not high.

Soybeans with a winter wheat cover crop yielded 61.9 bu/acre, those without a cover crop totaled 54.3 bu/acre. Beans with an oats cover crop produced 53.4 bu/acre, and those with a barley cover crop yielded 50.3 bu/acre.

In 2001, growing conditions were hot and dry and white mold was not a threat. Nevertheless, soybeans with a wheat cover crop yielded 45.5 bu/acre, followed by 41.5 bu/acre with no cover crop and 38.5 bu/acre with an oats cover crop. Oats likely reduced soil moisture before being burned off.

University of Wisconsin trials by Grau in 2001 have shown a yield standoff between soybeans grown after a wheat cover crop and beans grown without a cover crop. He did not test oats or barley.

“In our most recent testing, there was no white mold, probably due to hot weather during soybean flowering,” Grau notes. “At least the wheat did not have a negative effect on yield.”

There hasn't been much white mold pressure in Wisconsin since small grain research began in 1998. In years with more extensive white mold, the small grain strategy would likely be more dramatic.

“Based on our research to date, winter wheat looks most promising as a cover crop,” says Maloney. “Besides higher yields, it controls winter erosion and can be killed during normal spring burndown.”

He doesn't think producers will like an oats-soybean combination. It'll cause a trip across a field in early June to kill the oats, plus extra wheel tracks in beans, he says.

Maloney says wheat seeding costs $8/acre for seed at 2 bu/acre and $10/acre for drilling.

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