Weed control in fall kills winter annuals, speeds spring planting.

Missouri

When asked, "When's the best time to control those dense stands of winter weeds showing up in no-till fields?" weed scientist Bill Johnson has an immediate and emphatic response.

"Go scout the fields right now!" said the University of Missouri Extension specialist. "If there are dense infestations, you may need to start managing those weeds." Many growers weren't aware of winter weed problems until planting time this past spring, Johnson said. "Winter weeds are becoming a major issue.

"It was inevitable that as growers used the popular Roundup Ready technology year after year, the weed populations would shift." An estimated 70% of the soybean acreage in Missouri is treated with the glyphosate herbicides.

Henbit and chickweed now flourish in harvested fields during the winter months. Producers who remember the solid blanket of small purple flowers over their fields this past spring will know about henbit and the look-alike purple dead nettle.

Thick stands of weeds delay soil warming in the spring. That can delay planting, Johnson said. The winter weeds also remove excessive amounts of soil moisture if spring planting is delayed. They can create drought-like conditions, leading to poor crop emergence.

A heavy crop of winter weeds can interfere with spring tillage and planting, especially if planting is delayed by wet weather.

Last spring, when planting was delayed by wet weather, winter weeds attracted black cutworm moths, said Wayne Bailey, MU entomologist. "They started laying eggs on the weeds for a first generation of the pest that attacks corn seedlings."

The glyphosate herbicide on soybeans controls the weeds early in the crop year, Bailey said. But, the compound has no residual activity. That gives weeds a chance to start growing in the fall.

A shift in weed populations can attract new pests. MU entomologists are watching an increase in the common stalk borer. "Stalk borer hasn't been a big problem, but with more winter weeds we're seeing more of them."

Johnson sees an increasing need for fall-applied herbicide in some situations but not all. In particular, weeds can delay spring planting on poorly drained soils. "The winter weeds, if sprayed now, are relatively easy to control," Johnson said.

"The benefit of a fall herbicide depends greatly on the spring weather and drying characteristics of a field. If planting is early, the fall application can replace a spring burndown application."

A wide variety of weed control products are available, Johnson said. Most contain an old-reliable herbicide, 2,4-D. In winter weed trials at Columbia and Novelty, MO, most herbicides gave control of 85% or more.

Johnson recommends that producers go into the fields now to scout for areas where the small weeds are emerging. MU has a new manual with color pictures to help identify weeds commonly found in no-till fields. Request "Early Spring Weeds of No-Till Crop Production," NCR614, ($3). Telephone orders: (573) 882-7216 or 1-800-292-0969. It is also available at local University Extension Centers.

Johnson urges waiting to spray until the soil temperature drops just below 50 degrees. "It's just the reverse of what we recommend in the spring.

"You want to spray when it's warm enough to get a good kill. But, spray after the ground is cool enough to prevent a re-emergence of another crop of weeds."

Johnson and Bailey will be discussing the shifting weed and insect populations at the MU Crop Management Clinic, Nov. 28-29, in Columbia.

Extension specialists and crop protection company representatives will preview research and profit-making tips in a wide range of topics. The conference covers weeds, but also insects, diseases, fertility, and environmental issues.

Details can be obtained from Johnson at johnsonwg@missouri.edu or 573-882-2001.

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