The first of March is almost here so time to start thinking burndown.The mild winter Tennessee has experienced to date has resulted in a relatively low mortality rate for the fall germinated horseweed (marestail). Horseweed that has germinated in the fall is historically more difficult to control than spring germinated horseweed. We will also begin to get spring germination of this weed. New horseweed 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 (horseweed) burndown for over a decade now. It is still widely used though most are utilizing higher rates in recent years to get consistent horseweed control. Another good option is to tankmix it with a Sharpen product (see below) which has given us more consistent control of horseweed 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 horseweed with rates of 2,4-D less than 32 oz/A (1lb ai/A). Even with this rate, if the horseweed was well established from the previous fall, control can be inconsistent. Tankmixtures of 1 oz/A of Sharpen with a pint of 2,4-D in soybean can be a more consistent option for established horseweed. The plant back to soybean from a 2,4-D application is 7 days at the 16 oz/A rate and 15 days for 32 oz/A. 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 horseweed in our research.
Sharpen and the premix of Sharpen and Outlook (Verdict) can be effective horseweed burndown options. Utilized in a tankmix with glyphosate we have 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 oz of dicamba or 1 pt 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 horseweed. Please be mindful of the plant back to soybean (7 days 2,4-D, 14 days dicamba) with these treatments.
Gramoxone (paraquat) can also be utilized as an early pre plant burndown for horseweed. Gramoxone as a burndown option will provide more consistent horseweed control if applied near sunset. Regardless of timing, in our research utilized alone in an early burndown situation, it has proven to be more inconsistent than the options listed above. I really like it applied right behind the planter to help finish off any horseweed that may be recovering from the early burndown and to make sure no Palmer amaranth is emerged before the crop. Gramoxone control of horseweed and pigweed can be improved if tankmixed with metribuzin in soybeans and Cotoran and/or Caparol before cotton.
Though Liberty can be an effective burndown option for horseweed, I would not recommend it for two reasons. First under cool spring conditions Liberty will not provide good horseweed control. Second, the options listed above would be much more economical.