From visiting with retailers the past month, it is very apparent there will be a major shortage of dicamba for burndown this spring. A number of folks from different retailers have stated that they have been allocated only about 20 percent to 25 percent of all the dicamba they sold last year. The cause of this shortfall apparently comes from some overseas suppliers who decided for whatever reason to shut down production. Apparently this problem cuts across companies as even Latigo, a premix of dicamba and 2,4-D marketed by Helena, will be in short supply.
Dicamba-based burndown programs have been the backbone for horseweed (marestail) control in Tennessee for over a decade. It now looks like for many acres we will have to go with a plan B. Other options for burndown of horseweed are 2,4-D, Sharpen, Verdict, Gramoxone and Liberty.
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/A). Even with this rate if the horseweed was well established from the previous fall, control can be somewhat inconsistent.
Tank mixtures 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.
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 a EC formulation, to be a little more consistent across environments than Sharpen which is a SC formulation. The three way premix of glyphosate plus Sharpen with 1 pt of 2,4-D has been a very consistent control option for horseweed.
Gramoxone (paraquat) can also be utilized as an early pre plant burndown for horseweed. However, in our research utilized alone in an early burndown situation, it has proven to be more inconsistent then 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 or Caparol before cotton.
Though Liberty can be an effective burndown option for horseweed, I would not recommend it for three reasons. First under cool spring conditions Liberty will not provide good horseweed control. Second, the options listed above would be much more economical. Third, there is a concern that if much Liberty is used for burndown there may be a shortage of it for the more needed in-crop applications this summer.