University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

October 13, 2006

Reduce spring weed problems

Control winter annuals in the fall

Taking steps to avoid resistance

After meeting recently with weed scientists from across the country to discuss the potential for glyphosate-resistant weeds, UNL Extension Weed Specialist Mark Bernards reported that glyphosate-resistant marestail (horseweed) has been reported in several states and there is concern it has spread to Nebraska. Not relying totally on glyphosate and varying treatment options can help stop or minimize the development of glyphosate-resistant winter annuals.

For more information on available options, check out this publication in the Glyphosate, Weeds and Crops publication series: Biology and Management of Horseweed , available online or from your local Extension office. Also visit the Glyphosate, Weeds and Crops Web site.
A bumper crop of winter annual weeds including henbit, horseweed (marestail), pennycress and tansy mustard is forecast for next spring for eastern and central Nebraska. The rain this fall has been ideal for germination and establishment of these weeds. They will overwinter and begin growing early next spring, robbing soil moisture from potential crops. If allowed to grow, these winter annuals will be large and difficult to control by row crop planting time.

Herbicides are most effective on winter annuals when applied in the fall or in the spring before bolting (stem elongation). Once stem elongation begins, plants become progressively more difficult to control. Weather conditions and competing work often interfere with timely spring herbicide applications. Why not avoid the conflict and spray winter annuals in the fall?

Kill winter annuals in the fall when they’re small

Herbicides are more effective and economical to use on winter annuals in the fall than in the spring. Also, there is a longer period of favorable weather and field conditions in the fall than in the spring. The ideal treatment period is from late October through mid November.

Waiting until late October before treatment allows time for weed seed germination. Early fall spraying may allow time for weeds to become established following herbicide application. If you’re concerned that it is too cool to control weeds in late October and early November, remember that these weeds are cool season plants and are not killed by frost. They will be actively growing during mild weather and susceptible to herbicides even after a hard freeze.

How late can you wait to spray? Finish spraying the day before the weather closes in for the winter. What about spring germination of winter annuals — a fall treatment won’t control them? Spring germinating winter annuals are not the problem. The real targets are the winter annuals that germinated in the fall and have a running start in the spring. In Nebraska a few winter annuals may germinate in the spring; however, they will be seedlings at row crop planting time and easily controlled with most herbicides.

Economical fall treatments for these winter annuals include 1.5-2 pt 2,4-D ester, glyphosate at 32 oz/acre, 1pt 2,4-D + 4-6oz dicamba, and glyphosate 24 oz + 1 pt 2,4-D. These treatments are very effective.

Fall treatments provide a good opportunity to address the problem without totally relying on glyphosate and risking the development of resistant weeds.

Alex Martin
Extension Weeds Specialist

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Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.
Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
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