Most soybean growers probably don't pay much attention to winter annual weeds. After all, they'll be killed in spring by herbicides or tillage.
But those weeds could be helping perpetuate soybean cyst nematode (SCN) populations, warns Kent Harrison, an Ohio State University weed scientist.
"In a greenhouse screening, we've identified four winter annuals - field pennycress, shepherd's purse, henbit and purple deadnettle - that may serve as home to nematodes after farmers harvest soybeans in the fall," Harrison reports.
As the study progresses, he and his colleagues may add more weeds to the list. And in other areas, different winter annuals may be the culprits.
The idea behind the research is to disrupt the SCN life cycle.
"Winter annuals are small weeds and don't appear to hamper soybean production," says Harrison. "That makes them easy to ignore. When I show pictures of them to farmers, they usually say, 'Oh, that's what those things are.' "
He and his colleagues first looked at summer annual weeds such as velvetleaf and lambsquarters, but found that none was an SCN host. They then focused on weeds that are active outside the soybean growing season. Some germinate after soybean harvest and re-emerge early the next spring. Others complete a life cycle in early spring.
"Winter annuals may provide a place for SCN to reproduce and overwinter," Harrison explains.
Dave Mowers of Mowers Soil Testing Plus, Toulon, IL, says his staff is seeing an increase in winter annual weeds across several Midwestern states.
"They are showing up especially in no-till and reduced-till fields," says Mowers. "We are encouraging farmers to control them, and the sooner the better."
A low rate of 2,4-D or light tillage easily controls these weeds. "They are pretty wimpy," Harrison states.
"We recommend that farmers scout fields for winter annuals in early fall, during or immediately after corn and soybean harvest. This is especially true for no-till fields," he adds.
"Apply 1/2 to 1 pint per acre of 2,4-D to actively growing weeds. Do so before the weeds go dormant - while the average daily temperature is above 50 degrees. Use the amine or low-volatile ester form. Consider adding 1-2 pints/acre of Roundup in a tankmix with 2,4-D if biennial or perennial weeds are present."
Controlling winter annuals alone won't eliminate SCN, Harrison emphasizes. A full-bore program, built on crop rotation and resistant soybean varieties, is needed.