Updated February 1, 2021:
The progressive evolution and spread of herbicide-resistant weeds continues to challenge soybean production and grower profitability. The adoption of new technologies such as soybean varieties with resistance to dicamba, glufosinate (Liberty), and 2,4-D has ultimately improved weed control for some farmers, but has presented further challenges in terms of off-target movement of dicamba to sensitive plants and the potential for weed resistance to all three of these herbicides. The overall goal of this project continues to be the refinement of strategies that can be integrated into regional Best Management Practices (BMPs) for weeds to sustain soybean production and profitability. More specifically, practical field research was conducted with a focus on weed seed management with mechanical devitalization, soil residual herbicide interactions with cover crops, 2,4-D and dicamba injury on sensitive soybean, effectiveness of commercial sprayer cleanout procedures for dicamba, monitoring weeds for resistance to dicamba and glufosinate, and understanding the frequency of adverse weather patterns that prevent safe herbicide applications.
Our long-term goal is to develop management solutions for herbicide-resistant weed species to minimize their destructive impact on soybean production, as well apply the information from this research to improve management of other weeds in soybean that are evolving herbicide resistance or are generally considered problematic. The major research objectives addressed through this project are listed below.
Major Objectives:
1) Evaluate the biology of major herbicide-resistant weeds with respect to physiological and physical factors that contribute to their distribution, with emphasis on seed persistence and management in the soil seedbank.
2) Investigate challenges associated with successful implementation of tactics included in current Best Management Practices for control of herbicide-resistant weeds.
3) Coordinate outreach efforts among the principal investigators and the USB-funded outreach program on “Take Action”.
Research Findings and Deliverables:
The integration of non-chemical weed control methods to supplement herbicide use is necessary to provide greater sustainability of weed control technologies and reduce the risk of failed herbicide applications due to herbicide resistance. Research in Missouri demonstrated that a mechanical mill (Seed Terminator) attached to the end of the chaff flow on the back of combines during soybean harvest can devitalize up to 50% of waterhemp seed, reducing additions to the soil weed seedbank. However, a significant amount of waterhemp seed fell to the ground as header loss and seed shatter. The research also highlights that the Integrated Harrington Seed (iHSD) Destructor was not well-adapted for weed material with higher moisture that would be in soybean fields located in the Midsouth prior to any killing frost. This resulted in future research in Arkansas changing to focus on the Redekop brand seed destroyer instead of the iHSD.
Research was also conducted on control of pigweed species from soil residual herbicides under variable rainfall conditions and in combination with a cover crop. Fierce, Valor, Zidua, and Warrant were commonly more effective for control of both Palmer amaranth and waterhemp, with the active ingredients in Fierce, Valor, and Zidua providing the most consistent control across variable rainfall levels.
Xtend and Enlist soybean are the primary soybean traits farmers are using to combat weeds with multiple herbicide resistance. Our research confirms the resistance-mechanisms for Xtend and Enlist soybean offer no cross-resistance across these auxin herbicides, which means that low-level drift of these auxin herbicide to soybeans with the other traits offers no lesser level of soybean injury than conventional soybeans. When off-target movement of 2,4-D and dicamba does occur, our research demonstrates that multiple exposures of soybean to dicamba can have a cumulative effect on soybean injury and subsequent yield loss. Overall, multiple exposure events to dicamba in the early reproductive stages are the most injurious. Soybeans with multiple exposures to 2,4-D drift resulted in very little soybean response and should only be a concern when relatively high levels of 2,4-D drift occurs. A survey of dicamba concentrations in the last rinse of a commercial sprayer cleanout process showed that greater consistency is necessary to remove dicamba residues and have confidence in removing the threat of dicamba contamination in subsequent applications to sensitive soybean.
The farmer Directors for USB were highly concerned with the potential for a shift in the sensitivity of major weed species to dicamba (Xtendimax) or glufosinate (Liberty) as a result of increased use of these herbicides for control of weeds with multiple herbicide resistance. Seed samples of Palmer amaranth, waterhemp, and horseweed (e.g. marestail) were collected in Fall 2018 and 2019 over eight states and screened in the greenhouse to observe any variability in weed response to these herbicides. To date, Palmer amaranth and waterhemp populations surviving dicamba in our screen appears to be the greatest concern for proliferation into resistance in commercial populations. This is not by any means confirming any weed resistance to these herbicides. However, these findings justify continuing the project and collecting weed seeds again in Fall 2020 to identify any negative trends in weed response to these herbicides.
Our research findings have been disseminated in various formats to a wide audience, with a focus on soybean farmers as the primary target. Mechanisms for outreach included both grower and crop consultant meetings (virtual and in-person), extension newsletter articles, and through activities within the Take Action program that involved the development of fact sheets and webinars. In 2020 three webinars based on our research were presented to live audiences, recorded, and posted within the Take Action website.
1) “Waterhemp - Effective Long-Term Management” Aaron Hager, University of Illinois
2) “Status of On-Combine Seed Destruction Technologies”, Kevin Bradley, University of Missouri
3) “Temperature Inversions and Weather Analysis for Dicamba Off-Target Movement”, Mandy Bish, University of Missouri