Updated January 31, 2022:
Weed management in soybean revolves around weed resistance to herbicides and soybean farmers are encouraged to implement Best Management Practices (BMPs) for greater sustainability. The overall goal of this project continues to be the refinement of strategies that can be integrated into regional BMPs for weeds to sustain soybean production and profitability. More specifically, practical field research was conducted on harvest weed seed control practices, planting ‘green’ into living cover crops to encourage greater biomass for weed suppression, field weed collections to survey the extent that weed populations are progressively moving towards less sensitivity to dicamba and glufosinate (Liberty), and the weather and application factors responsible for the off-target movement of auxin herbicides (e.g. dicamba and 2,4-D). This research targeted a broad geographical gradient to account for environmental factors on these practices and builds on our previous research grants with USB funding by adding factors related to Enlist soybean adoption, a greater focus on harvest weed seed control measures, and furthers testing with cover crops. As with our previous proposals, we continued to work in collaboration with the USB Take Action outreach initiative to deliver our findings and recommendations to growers for these BMPs.
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) Characterize and quantify the environmental, landscape, and application factors that contribute to off-target movement of herbicides that impact environmental stewardship.
4) 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. Our research has demonstrated the use of an impact mill in the chaff stream of a commercial combine can reduce viable weed seed additions to the soil by over 70% for Palmer amaranth and waterhemp. At this time, three commercial models are available in the U.S., but two models (Seed Terminator and Redekop Seed Destroyer) performed well in all field conditions. The results of this research have been delivered through multiple methods, including two Take Action webinars.
If potentially herbicide-resistant weeds are present during harvest, a farmer could also consider the practice of windrowing the chaff from the combine that would concentrate weed seeds and either allow for decomposition or predation in the chaff pile, which has been implemented in Australia for herbicide-resistant weeds. If nothing else, these chaff lines would allow for greater focus of future weed management practices on these strips. The refinement of chaff lining and understanding the geographical or climate influences on degradation of weed seeds will be of interest for our 2022 field research.
Planting soybean into living ‘green’ cover crops have been reported as a means to allow more time for cover crop growth, resulting in greater plant biomass to suppress weeds. Our research to date with cereal rye supports that concept, but didn’t eliminate the need for controlling weed escapes with herbicides. Unfortunately, the greater cover crop biomass from delaying spring termination and soybean planting also resulted in cereal rye interfering with soybean planting, growth, and grain yield. This research will be conducted again in 2022 to improve soybean establishment and validate the weed suppression observed in 2021.
Since fall 2018 our project has been screening Palmer amaranth, waterhemp, and horseweed (i.e. marestail) populations across several soybean production states for potential herbicide resistance to Liberty and dicamba herbicides. The palmer amaranth populations tested over the years have demonstrated a progressive increase in survival from field use rates of dicamba. Thus far, Palmer amaranth resistant to Liberty in Arkansas and dicamba resistance in Tennessee has been confirmed. Waterhemp populations have also exhibited variable responses to these herbicides in our testing. To date, our research has contributed towards the confirmation of waterhemp resistant to dicamba in Indiana and Tennessee. We are shifting our focus in 2022 to include 2,4-D in our greenhouse screens to reflect the use of Enlist soybean commercially. Horseweed will no longer be involved with our screening since testing has not yielded any populations with suspected resistance to these herbicides, as well as the observation that commercial control of horseweed has increased with the adoption of Xtend, Liberty Link, and Enlist soybean.
Our research on off-target-movement of herbicides was developed out of concern from farmer Directors within USB. This research has shown that topography and water bodies (e.g. pond, stream) can influence the formation of air temperature inversions that can lead to herbicide movement following application. Vapor movement of dicamba and 2,4-D choline can occur, but the extent of volatility and the degree of injury to sensitive plants such as soybean is more of a concern with dicamba. Various herbicide combinations and adjuvants can increase the volatility of these herbicides and the herbicide labels for dicamba have evolved to reflect these concerns, such as the requirement for a volatility reduction agent (VRA) to be added to all dicamba applications in 2021.
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. Information generated through the research in this project has been disseminated by investigators through presentations (276), newsletters and written documents in various forms (99), and through social media activity (559 individual postings). Altogether, our outreach activity has produced over 2 million individual contact points in 2021.