By Eric Jones
The dicamba labels (Xtendimax, Engenia, and Tavium) for over-the-top applications in tolerant varieties have recently been vacated, which effectively means these dicamba products cannot be applied over-the-top of tolerant soybeans nationwide (Case 4:20-cv-00555-DCB). On February 14, 2024, the Environmental Protection Agency (abbreviated as EPA) issued an Existing Stocks Order, which allows for the use of these dicamba products if they were purchased and in possession before the label was vacated (EPA Existing Stocks Order). While the Existing Stock Order allows for the use of these products if in possession, the demand will be high, and the supply may be low. While the label is vacated, the EPA response is yet to be made. Instead of speculating what the outcome of these dicamba labels will be, farmers should start planning weed management plans today. Many chemical and seed inputs have already been purchased for the 2024 growing season. However, even if you have purchased a seed variety with tolerance to a different herbicide such as 2,4-D (or Enlist), the principles for an effective weed management plan remain the same.
While the potential loss of the dicamba label will result in the loss of a useful, effective tool for weed management, this should be a reminder that one herbicide or tactic should not be the focus of a management plan. A strong preemergence herbicide program utilizing multiple herbicide groups should be a staple in a weed management plan. Utilizing multiple herbicide groups will increase the spectrum of weed control and reduce the selection pressure on herbicide-resistant weeds. If the growing season is started off with effective weed control, less reliance will be placed on the postemergence herbicides.
There are still effective herbicides that can be applied postemergence in soybean. While glyphosate (or Roundup) will not control many populations of kochia or waterhemp in South Dakota due to resistance, this herbicide still has utility in controlling other broadleaf and grass weeds. The PPO-inhibiting herbicides (including acifluorfen [Ultra Blazer], fomesafen [Flexstar], and lactofen [Cobra]) will effectively control many broadleaf weeds, but the application should be made when weeds are small. Grass weeds can be controlled with ACCase-inhibiting herbicides (such as clethodim [Select] or quizalofop [Assure]) or glyphosate. Some dicamba-tolerant varieties (such as Xtendflex) also have glufosinate (or Liberty) tolerance. Glufosinate will effectively control many weeds, but timely application on small weeds is crucial. Be sure to check your seed label before applying glyphosate or glufosinate to ensure the seed is tolerant to the herbicide or herbicides. Including herbicide with soil residual activity in the tank should be considered to control later emerging weeds in the growing season. Similar to the preemergence herbicide application, multiple herbicide groups should be utilized. The weed control spectrum is increased and the selection pressure on resistant weeds is reduced when multiple herbicide groups are applied. A more-intensive list of herbicide options for use in soybean can be found in the latest South Dakota Pest Management Guide for Soybeans.
Lastly, while herbicides are a large component of a weed management plan, using other non-chemical tactics will play a role in ensuring effective management. Tactics, such as crop rotation and row spacing, will aid in controlling weeds, but can also provide agronomic benefits as well. While laborious, tactics such as hand weeding isolated weed patches can pay dividends as well. The best non-chemical tactics are the ones that are easily adaptable to current production practices.
Source : sdstate.edu