Don’t Forget about Horseweed

Chris Boerboom, Extension Weed Scientist

I was visiting a no-till field yesterday and noticed a good stand of spring-germinated horseweed (marestail) (photo 1).   Although this spring has been cool and some weeds are growing slower than typical, this is one winter annual weed that needs timely management and shouldn’t be ignored. Horseweed is a bit unusual for a winter annual. Many winter annuals mature early in the spring/summer and their competition is limited. For example, the shepherd’s purse right next to this horseweed was already flowering and will mature and die in early June even if it is not controlled. However, this horseweed will continue to be competitive with soybeans through the whole summer (photo 2).   

Horseweed is controlled in most no-till fields with the burndown treatment. However, I know some people plan on “delayed” burndowns in no-till soybean fields. The “delay” is to combine the burndown treatment with the postemergence glyphosate treatment. This certainly saves an application, but there is a risk with horseweed. By the time this “delayed” application is made, the horseweed is likely to be much larger and more difficult to control. The standard recommendation for seedling and rosette horseweed control is glyphosate plus 2,4-D ester. As horseweed stems start to elongate, they become more difficult to control. Horseweed that is 4-6 inches tall can be still be controlled, but the glyphosate rate should be increased to 1 lb ae/a (48 oz of a 3 lb/gal formulation) along with 2,4-D. After horseweed exceeds 6 inches tall, it becomes difficult to control and the glyphosate rate should be increased to 1.5 lb ae/a.  

Obviously, 2,4-D can only be used preplant in soybeans. This means that larger horseweed targeted with “delayed” glyphosate will be relying on only glyphosate for control in the soybeans.   The resulting horseweed control may not be at a satisfactory level. 2,4-D has a second benefit in this situration. In addition to improving the consistency of horseweed control, 2,4-D adds a second mode of action that should delay the development of glyphosate-resistant horseweed. A key limitation with 2,4-D is that it must be applied 7 days before planting soybeans. If this interval is not available between the burndown treatment and soybean planting, Gramoxone plus Sencor is effective on the seedling and rosette stage.   Herbicides with the ingredients of Classic (chlorimuron) or FirstRate (cloransulam) are also options to use with glyphosate. While ALS-resistant horseweed exists in other states, I am not aware of any in Wisconsin at this time that would make chlorimuron or cloransulam inefffective. 

Photo 1. Horseweed seedlings in no-till.

Photo 2. Mature horseweed plant.