By Harold Watters, Ohio State University Extension, Champaign County
Several years ago I attended a Montgomery County Extension Weed Tour and saw the marestail problem there. I saw their plots, saw the attempts to clean up the weed with glyphosate in Roundup Ready soybeans and recognized that it was virtually impossible.
Today our soybean fields look as bad in Champaign County (and surrounding areas in western Ohio) as I remember those Montgomery County fields. What our Extension weed specialists, Mark Loux, Jeff Stachler (was still here then), and Tony Dobbels learned was that once you have glyphosate resistant marestail plants, we can only kill about 50% or less with a glyphosate application. Other post products that once upon a time worked on marestail, such as First Rate or Classic lost their effectiveness in the mid-1990s for the most part. So a planned post program today to kill marestail in soybeans is foolish.
How do we manage marestail? And that’s what we must do — manage the problem with plans that start before planting. I know we did not have a particularly large window to get pre-applied herbicides on this season, but for the yield loss and the likely increase in resistant weed seed supply we will generate this year, it would have been worth the seven-day wait to include a 2,4-D application.
The following remarks are straight out of the Ohio & Indiana Weed Control Guide, written by Mark Loux and others. The primary goal of a horseweed management program in no-till soybeans should be the effective control of emerged plants prior to planting, and residual control of plants that emerge for 6 to 8 weeks after planting. This strategy will reduce the need to treat marestail with postemergence herbicides.
The following principles are important in horseweed control programs:
1) 2,4-D ester should be included in herbicide treatments if at all possible;
2) herbicides should be applied when horseweed plants are no more than 4 to 6 inches tall;
3) herbicides applied in the fall will control emerged horseweed, but may not adequately control spring-emerging plants; and
4) spring applications should include a residual herbicide to control later-emerging plants.
Where it is not possible to use 2,4-D ester, the most effective treatments are:
1) saflufenacil + glyphosate or Ignite; and
2) Ignite + metribuzin.
Residual herbicides should be included in preplant burndown treatments. The most effective residual herbicides are those that contain active rates of two modes of action, such as Gangster, Authority XL, Valor XLT, Envive, Sonic/Authority First, Authority MTZ, and a combination of Canopy DF plus metribuzin. These products can help control emerged plants when combined with one of the burndown treatments listed above (do not combine Sharpen with herbicides containing Valor or Authority/Spartan). An application of Valor or metribuzin can provide adequate residual control.