Control Marestail When They are Small

Mark VanGessel, Extension Weed Specialist;

One of the most common questions is how to control marestail for no-till soybeans. Treating them early is the key to success. Control of marestail when its 4 to 6 inches is more likely than when it’s over 10 inches tall. Furthermore, when marestail is tall, it typically is closer to planting and herbicide options are limited. Remember, most of the marestail is resistant to glyphosate and we are seeing more populations that are resistant to ALS herbicides such as chlorimuron (an active ingredient in premixes such as Canopy, Envive, Authority XL, or Trivence) or cloransulam (an active ingredient in FirstRate, Surveil, or Sonic). So the most consistent options include 2,4-D at 1 qt/A or Sharpen applied to small plants. These rates and products have specific replant restrictions requiring early applications.

Also, early treatment of the horseweed provides time to retreat prior to planting if Mother Nature does not cooperate and initial control is not as effective as you had expected.

Should you include a residual herbicide with these early burndowns? I generally don’t see the benefit to using the residuals so early. The residual herbicides will only provide residual control for 3 to 4 weeks. So, you may have a field that is clean at planting, but weeds will begin to emerge shortly after planting. On the other hand, if the residual herbicide is applied at planting, or shortly before planting, then the soybeans will have good weed control for the first 3 to 4 weeks after planting. This is a serious consideration for some weeds, such as Palmer amaranth or morningglory species, that are difficult to control once they get more than 3 inches tall.

In most situations, improving weed control is a matter of application timing rather than trying to be creative on herbicide combinations.

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