Controlling marestail in Roundup Ready corn
By David G. Hallauer
Meadowlark Extension District Agent
For years, reliance on postemergence applications of glyphosate to Roundup Ready corn have been both economical and effective. Today, it might be a little riskier to put all of our eggs in the postemergence control basket. For starters, the risk of yield loss from early season weed competition can get pretty high if weather or other factors delay application. Control of problem weeds like marestail or waterhemp are no longer adequate with glyphosate alone, either. And last but certainly not least, resistance management alone is a good enough reason to do something other than apply glyphosate after the crop has emerge.
Our list of problem weeds is growing. One of them, marestail, can be a significant problem when corn follows soybeans. This is of particular concern when marestail is left uncontrolled during the soybean rotation year (usually signifying some level of resistance).
Fortunately, several herbicides can be used ahead of corn planting with excellent results according to K-State Weed Management Specialist Curtis Thompson. His best recommendation option is a fall application of 2,4-D or dicamba with atrazine and/or glyphosate, all of which provide excellent control. The use of sulfonylurea herbicides can be effective as well, (if marestail is not ALS resistant—if so, include a growth regulator herbicide in the tank mix).
Couldn’t spray in the fall? March is the next very important application window. Early spring applications should include a dicamba-based product for the most effective control results (2,4-D at the rate of 1 quart per acre of 4 pounds per gallon product can be effective in spring on small marestail, but a pint of dicamba has been more consistent). Dicamba is a little weak on winter annual mustards requiring a tank-mix with another herbicide for best results. Atrazine continues to have good activity on small rosette-stage marestail—much less on large, bolted plants.
For more information, check out the 2014 KSU Chemical Weed Control Guide available at your District Extension Office.