By Larry Steckel, Extension Weed Specialist
The mild winter Tennessee has experienced to date has resulted in a relatively low mortality rate for the fall-germinated marestail (horseweed). Marestail that has germinated in the fall is historically more difficult to control than spring-germinated marestail. We will also begin to get spring germination of this weed. New marestail germination will likely start next week and could run into May like last year.
Below are some of my thoughts on burndown strategies. Dicamba has been the standard “go to” herbicide for marestail burndown for over a decade now. It is still widely used, though most are utilizing higher rates in recent years to get consistent marestail control. Another good option is to tankmix it with a Sharpen product (see below), which has given us more consistent control of marestail and does help from a resistance management perspective.
The herbicide 2,4-D can be about as effective as dicamba on horseweed provided the rate is high enough. When applied alone or with glyphosate, I have never had consistent control of marestail with rates of 2,4-D less than 32 ounces per acre (1 pound active ingredient per acre). Even with this rate, if the marestail was well established from the previous fall, control can be inconsistent.
Tankmixtures of 1 ounce per acre of Sharpen with a pint of 2,4-D in soybean can be a more consistent option for established marestail. The plant-back to soybean from a 2,4-D application is 7 days at the 16-ounces-per-acre rate and 15 days for 32 ounces per acre. The plant-back to cotton, regardless of rate, is 30 days.
Latigo, which is a premix of dicamba and 2,4-D, has also been an effective burndown option for marestail in our research. Sharpen and the premix of Sharpen and Outlook (Verdict) can be effective marestail burndown options. Utilized in a tankmix with glyphosate, we’ve found horseweed control with Verdict, which is an EC formulation, to be a little more consistent across environments than Sharpen, which is a SC formulation.
Verdict always seems like it is in tight supply. If that is the case, a three-way premix of glyphosate plus Sharpen with 4 ounces of dicamba or 1 pint of 2,4-D has been as effective as Verdict. In fact, that three-way tank mix in our research has been the most consistent control option for marestail. Please be mindful of the plant-back to soybean (7 days for 2,4-D, 14 days for dicamba) with these treatments.
Gramoxone (paraquat) can also be utilized as an early pre-plant burndown for marestail. Gramoxone as a burndown option will provide more consistent marestail control if applied near sunset. Regardless of timing, utilized alone in an early burndown situation, it has proven to be more inconsistent in our research than the options listed above. I really like it applied right behind the planter to help finish off any marestail that may be recovering from the early burndown and to make sure no Palmer amaranth is emerged before the crop.
Gramoxone control of marestail and pigweed can be improved if tank mixed with metribuzin in soybeans and Cotoran and/or Caparol before cotton. Though Liberty can be an effective burndown option for marestail, I would not recommend it for two reasons. First, under cool spring conditions, Liberty will not provide good marestail control. Second, the options listed above would be much more economical.