On February 3, Monsanto announced its commercial launch plans for soybean varieties resistant to the herbicides dicamba and glyphosate (designated Roundup Ready 2 Xtend varieties). This announcement followed China’s decision to import grain from these varieties. Many weed management practitioners hope this new technology package will provided improved control of problematic weed populations, including those with evolved resistance to glyphosate and herbicides from other site-of-action families. The weed science program at the University of Illinois has evaluated this technology for several years, conducting field research that has helped us better understand the technology and how it might be best utilized in Illinois soybean production systems.
We believe the technology can be a useful new tool for weed management, but are less confident that soybean farmers can realize its full utility during the 2016 growing season. Currently, there are no federal or state labels for any dicamba-containing product that allow applications at soybean planting (preemergence) or after the soybean crop has emerged (postemergence). Furthermore, there is uncertainty about whether approved federal and state labels will be granted in time to allow application of dicamba-containing products on these varieties during the early portion of, or perhaps even much of, the 2016 growing season. Without approved labels, applying a dicamba-containing product to these soybean varieties would constitute a violation of both state and federal laws.
Some have posed the question of whether or not dicamba can be applied prior to planting dicamba-resistant soybean varieties. The answer is “yes”, but remember this type of application must follow the herbicide label guidelines regardless of the soybean variety planted. For example, following the application of Clarity for control of existing vegetation prior to planting (including soybean varieties resistant to dicamba) and one inch of accumulated precipitation, a waiting interval of 14 days is required for up to 8 ounces of Clarity and 28 days for up to 16 ounces. This use pattern is governed by the herbicide label, not by the soybean variety planted.
Herbicide-resistant weed populations continue to be a common occurrence across most areas of Illinois. Waterhemp and horseweed (marestail) are the two most common herbicide-resistant weed species in Illinois, and observations during 2015 suggest these species are likely to remain prevalent in 2016. More than 1300 waterhemp samples (representing 236 fields) were submitted to the University of Illinois Plant Clinic in 2015 for herbicide resistance screening. The sheer number of samples submitted suggests herbicide-resistant waterhemp continues to be a significant management challenge for farmers. Waterhemp plants and/or populations resistant to herbicides from more than one site-of-action group are increasingly common, and we do not foresee this changing. Recent survey data indicated resistance occurred in close to 90% of the fields sampled, and multiple resistance to glyphosate and PPO inhibitors was confirmed in 54% of the fields sampled.
Soybean producers planning to rely on dicamba and dicamba-resistant soybean in their 2016 weed management programs for control of waterhemp populations resistant to both glyphosate and PPO inhibitors are encouraged to consider utilizing alternative strategies. The Enlist soybean trait technology and the complementary Enlist Duo herbicide formulation have received regulatory clearances, but without export approval to China it remains unclear how widely available these varieties will be in 2016. Alternative strategies to manage weed populations with resistance to multiple soybean herbicides include rotating fields to a different crop, or planting soybean varieties resistant to glufosinate (i.e., Liberty Link) and utilizing glufosinate as a postemergence herbicide. Please keep in mind, however, that regardless of the crop planted, the variety selected, or the herbicide applied, the most sustainable solution to the challenges of herbicide-resistant weeds is an integrated weed management system that utilizes both chemical and non-chemical tactics to eliminate weed seed production throughout the growing season.