Management of herbicide-resistant weeds across Indiana and the entire Midwestern U.S. continues to be the most challenging in soybean compared to other agronomic crops. Ever since weeds resistant to glyphosate (e.g. Roundup) were first documented over 20 years ago we have progressively been mixing additional herbicides to glyphosate or using combinations of other herbicides. Current Best Management Practices (BMPs) for control of herbicide-resistant weed species includes postemergence mixtures of effective herbicides representing different herbicide mode of action groups. However, recent research and commercial observations have indicated that these combinations may lead to increased volatility of dicamba, or antagonism of one of the herbicides in the mixture leading to failed weed control, increasing the potential for further weed evolution towards herbicide resistance. Combinations of glyphosate and glufosinate with dicamba formulations labeled for use in Xtend soybean have been shown in our research and other research groups to increase the potential for dicamba volatility and off-target movement (OTM). In fact, the combination of glyphosate with dicamba in Arkansas is prohibited in Xtend crops because of this increased potential for OTM and the U.S. EPA is considering future dicamba label restrictions to mitigate this potential. The future commercialization of Xtend Flex soybean contains herbicide resistance traits for glyphosate, dicamba, and glufosinate, enabling the possible combination of glufosinate and dicamba, which has also been shown to increase dicamba volatility in our research.
Commercial experience with the progressive increase in dicamba applications in Xtend soybean over the last several years has revealed some adverse weed control observations that were first noted in research over a decade ago. The activity of glyphosate on certain grass species, such as barnyardgrass, annual bluegrass, and fall panicum, has been reduced (i.e. antagonized) by the addition of dicamba. Some weed scientists have speculated that this reduction if grass activity for glyphosate is leading towards the spread of glyphosate-resistant grass biotypes. Thus, the combination of dicamba with glyphosate may actually lead to faster evolution of glyphosate-resistant grass weeds. Similar research has pointed towards the potential for 2,4-D to antagonize glyphosate on grass species as well. Some of this interaction between the two auxin herbicides may be related to the physiological activity of the respective herbicides in the plants, but may also be related to the method of application required for glyphosate applied in Xtend and Enlist soybean that may lead to reduced glyphosate efficacy. For instance, the use of larger spray droplets may compromise spray coverage on target weeds or the prohibited use of ammonium sulfate with dicamba and glyphosate may reduce the activity on grass species, especially under cool air temperatures.
Are we better off applying auxin herbicides separately from glyphosate? As mentioned previously, some states already have restrictions for dicamba and the U.S. EPA is considering broader restrictions for applying herbicides in tank mix combinations. Obviously, not being able to apply herbicide mixtures would be a huge inconvenience and cost for soybean growers. Some research during the initial development of Xtend and Enlist crops demonstrated a benefit for combinations of 2,4-D or dicamba with glyphosate, even on weeds that were resistant to glyphosate.
The combination of glyphosate with dicamba or 2,4-D commonly resulted in faster herbicide activity on broadleaf weeds, as well as greater overall control compared to the herbicides applied separately. These observations reinforce that glyphosate still has some level of activity, albeit minor, on glyphosate-resistant waterhemp, Palmer amaranth, horseweed (aka marestail), and giant ragweed. This minor level of activity of glyphosate can enhance the overall herbicide activity on key target broadleaf weeds, and is a strong benefit for resistance management efforts. Now that the Liberty Link trait is being stacked with various other herbicide resistance traits in soybean, the interactions of glufosinate with the auxin herbicides and even glyphosate are critical to understand. The overall benefits to weed management and the challenges with applying these postemergence herbicide mixtures needs to be re-evaluated to refine current BMPs for herbicide resistance.
For several years the Indiana Soybean Alliance funded a service for our research group to screen suspected herbicide-resistant weed populations in Indiana which were submitted by growers, extension personnel, and industry representatives. This activity generated a map of Indiana showing confirmed populations of waterhemp, Palmer amaranth, and giant ragweed with resistance to several different herbicide mode of action groups. At this time, we are not proposing to continue this resistance screening service as we have previously. However, our intent is to maintain a base level of activity to address any novel cases of herbicide resistance that may evolve and focus less on resistant biotypes that we have already characterized. This research would include some resistant or problematic weed biotypes that we have previously tested at the whole-plant level, but still need to describe the underlying mechanisms and inheritance of the herbicide-resistance traits in the weed biotypes. Some examples of this include waterhemp resistance to atrazine, glyphosate resistance in giant ragweed, mesotrione (Callisto) resistance in waterhemp, and reduced activity of 2,4-D on Palmer amaranth.
Full characterization of the resistance mechanisms in weeds found in Indiana is imperative for improving management decisions as not all resistance mechanism are created equal. For example, the first form of weed resistance to atrazine in waterhemp was an altered target enzyme, which allowed for robust resistance levels and lack of control with atrazine in any application and cross-resistance to other herbicides such as metribuzin (e.g. Sencor). Conversely, another mechanism for waterhemp resistance to atrazine is rapid plant metabolism of the herbicide to an inactive form. This mechanism results in lack of control when atrazine is applied postemergence, but control of these biotypes when atrazine is applied preemergence, plus no cross-resistance to the herbicide metribuzin. Understanding these resistance mechanisms is important as it would provide a greater understanding of how they may spread commercially, explain the degree of resistance to the herbicide, and identify the best management options.
The broad, long-term objective of this proposal is to reduce the impact of herbicide-resistant weeds on soybean production and profitability while developing best management practices for herbicide-resistant weed species. We realize this cannot be achieved within a set number of years, but rather, adapts to the most pressing weed management challenges annually as weeds continue to evolve methods to escape management practices. Our short-term objectives for this proposal are: 1) Outline the benefits and unintended consequences of applying postemergence soybean herbicides in mixtures when targeting herbicide-resistant weed species, and 2) Identify the underlying mechanism(s) for herbicide-resistant weeds in Indiana to allow for improved management recommendations for these species. Overall, this research will help guide the development of effective weed control strategies for industry professionals and farmers to enable more sustainable weed management and soybean production. Information generated by these projects will be disseminated to weed management practitioners through various outlets.