The recent ruling (February 06, 2024) by a federal court in Arizona vacating the registration of dicamba products for over-the-top (OTT) applications in dicamba-resistant soybean and cotton will potentially put many farmers in a bind when it comes to controlling weeds in 2024 (Dicamba Availability in Question After Court Vacates EPA Registrations - dtnpf.com). Since the ruling, several state and national soybean associations have asked the U.S. Environmental Protection Agency (EPA) to issue an order to allow farmers to use existing stocks of XtendiMax, Engenia, and Tavium for OTT applications. They have also asked the EPA to appeal (Ag Groups Plead With EPA’s Regan to Allow Use of Existing Dicamba Stocks - dtnpf.com). Today (February 14, 2024), the EPA announced that existing stocks of XtendiMax, Engenia, and Tavium packaged, labeled, and released for shipment prior to February 06, 2024 can be used OTT during the 2024 growing season (EPA Existing Stocks Order for OTT Dicamba Falls Short of Industry Request - dtnpf.com).
The Wisconsin Cropping Systems Weed Science program has conducted several research trials investigating postemergence (POST) weed control in XtendFlex soybean systems (dicamba-, glufosinate-, and glyphosate-resistant soybean) without OTT applications of dicamba. A compilation of data reports from these trials can be found listed below.
Trials evaluating POST weed control in XtendFlex soybean without OTT applications of dicamba:
XtendFlex Soybean Herbicide Recommendations for ESA Counties (2021)
XtendFlex Soybean Herbicide Recommendations for ESA Counties (2022)
XtendFlex Soybean Herbicide Recommendations without Dicamba (2022)
The loss of dicamba for POST weed control in dicamba-resistant soybean will add further challenges to farmers dealing with glyphosate-resistant weed species like waterhemp. In fields with high levels of glyphosate resistance, glufosinate (Liberty) will be the primary option in many POST herbicide programs. As was the case with glyphosate, the reliance on a single herbicide active ingredient will lead to the development of herbicide resistance over time. A trial was conducted in 2020 and 2021 to evaluate potential tank-mix partners with glufosinate and results were published on our website: Would you consider tank-mix combinations to enhance glufosinate waterhemp control in soybean?
Take Home:
Over-the-top applications of dicamba were not required to achieve excellent season-long weed control in XtendFlex soybean in our trials.
Glufosinate (Liberty) was critical for controlling glyphosate-resistant weed species like waterhemp in the absence of dicamba in XtendFlex soybean in our trials.
Tank mixes of glufosinate with a PPO-inhibitor herbicide (Cobra, Flexstar, Resource) can improve control of some species and help delay resistance to glufosinate.
An effective PRE-emergence herbicide program is strongly recommended to reduce early-season crop competition and weed density in season.
This article was written by Ryan DeWerff (UW-Madison Weed Science Research Specialist & Wisconsin Herbicide Evaluation Program Coordinator) and Rodrigo Werle (UW-Madison Associate Professor & Extension Cropping Systems Weed Scientist).