Rodrigo Werle, Assistant Professor and Extension Cropping Systems Weed Scientist
Shawn Conley, Professor and State Extension Soybean and Small Grain Specialist
The United States Environmental Protection Agency (EPA) announced on October 27, 2020 the approval of a new five-year registration plan for three dicamba products (see: “EPA Announces 2020 Dicamba Registration Decision”); the new registration plan contains additional restrictions aimed at minimizing off-site movement of dicamba1, including:
- Requiring an approved pH-buffering agent (also called a Volatility Reduction Agent or VRA) be tank mixed with over-the-top approved dicamba products prior to all applications to control volatility.
- Requiring a downwind buffer of 240 feet and 310 feet in areas where listed species are located.
- Prohibiting over-the-top application of dicamba on soybeans after June 30
First of all, we believe that having access to dicamba for use POST-emergence on Xtend/Xtendflex soybeans is good news to farmers in Wisconsin struggling to control waterhemp and other troublesome broadleaf weeds such as marestail and giant ragweed. Resistance to Group 9 (glyphosate) and Group 2 (imazethapyr, chlorimuron-ethyl, cloransulam-methyl, etc.) herbicides is widespread in Wisconsin waterhemp populations while resistance to Group 14 (lactofen, fomesafen, fluthiacet-methyl, etc.) is on the rise (see: “Wisconsin Waterhemp Resistance Project – 2019 Update”). Given waterhemp resistance to herbicide Groups 2, 9 and 14, POST-emergence options on soybeans become very limited and farmers struggling with this weed are left to decide between the novel herbicide tolerance trait packages available in the market: dicamba (Xtend soybeans), 2,4-D (Enlist E3 soybeans), or glufosinate (XtendFlex, Enlist E3 and LLGT27 soybeans). Removing dicamba (Group 4) would reduce the effective POST-emergence options and could lead to overreliance on glufosinate (Group 10) and/or 2,4-D (Group 4), which could result in rapid evolution of resistance to such herbicides.
When using dicamba, besides following all label recommendations, farmers should avoid spraying when high temperature and lack of consistent wind are predicted during and 48 hours following a potential application; our research indicates such conditions are conducive to secondary dicamba off-site movement. For additional information see our recent blog post: “Spray solution pH and soybean injury as influenced by dicamba and 2,4-D herbicide formulation and spray additives”. It’s also important that farmers communicate with their neighbors to fully understand their surroundings (such as presence of dicamba sensitive areas, including non-Xtend soybeans) and have a solid plan in terms of who will be responsible for the application (farmer him/herself or commercial applicator) and how will weeds be managed in the buffer areas within their fields before deciding to adopt this technology. It is also likely this is not the end of this story so we encourage farmers to have a plan B (and C) just in case they can’t use dicamba.
Lastly, integrated weed management strategies, including the use of effective PRE-emergence herbicide programs, are imperative for successful and sustainable long-term management of waterhemp and other troublesome weed species regardless of herbicide-tolerance trait and POST-emergence herbicide program of choice. For assistance with PRE-emergence herbicide selection see “Residual control of waterhemp with PRE-emergence herbicides in soybean”.
Drs. Werle and Conley will be providing additional information regarding the impact of EPA’s dicamba announcement to Wisconsin soybean farmers during the upcoming winter meetings. Stay tuned!
1Research conducted by the Cropping Systems Weed Science Program at the University of Wisconsin-Madison was taken into consideration by the EPA to support these additional restrictions.