Updated January 31, 2022:
Research has been conducted over the last two years on the harvest weed seed control practice of using a mechanical mill integrated on a commercial combine to devitalize weed seeds in the chaff stream. The Integrated Harrington Seed Destructor (iHSD) was deemed a failure because the unit would readily plug in weed or soybean material that was slightly green or wet. The Seed TerminatorTM was deemed a success for the second year in a row, and a third unit, Redekop, was evaluated for the first time during Fall 2020 harvest and had promising results. A limitation for all these units would be the amount of ‘header loss’ of the weed seed shattering from the plants and not entering the combine. Research on header loss was conducted last fall at multiple universities across a range of geographies and the seeds and data are currently being processed.

Another harvest weed seed control practice being evaluated is the formation of concentrated chaff lines with the combine to promote weed seed decay in the chaff compost. If weed seeds don’t have accelerated decay in these lines, at least the practice prevents broadcasting the weed seeds across the entire width of the combine pass to help ease weed management in subsequent years. Chaff lines and a cereal rye cover crop were established at multiple research sites in Fall 2020 and plans for spring research activity have been discussed and shared among the investigator group.

A third non-chemical weed control practice of ‘planting green’ was initiated in Fall 2020. A total of eight different field trials were established with a cereal rye cover crop to evaluate the impact of letting more cover crop biomass develop in the spring and planting the soybean crop while the cover crop continues to grow (‘planting green’). The protocol for the research methods this spring for soybean planting, herbicide applications, cover crop termination, and data collection has been shared with the entire research team.

Our greenhouse screens from 2018 and 2019 reveal a significant level of variable sensitivity of waterhemp, Palmer amaranth, and horseweed to both dicamba and glufosinate (Liberty) herbicides. Annual surveys for herbicide resistance in waterhemp, Palmer Amaranth, and horseweed (aka marestail) have been conducted by collecting seed from weeds surviving management in soybean fields prior to fall harvest. 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. Field collections on these weeds were performed in Fall 2020 and are currently being threshed and cleaned for seeds before planting in the greenhouse for evaluation.

The impact of multiple, low-dose exposure events of off-target dicamba and 2,4-D to sensitive soybean was investigated for the last two years. Data analysis was still ongoing over the last quarter and are not final. The preliminary analysis demonstrated that multiple exposures of soybean to dicamba can have a cumulative effect on soybean injury and subsequent yield loss. Soybeans exposed to equivalent rates of 2,4-D (relative to the field use rate) exhibited minimal to no response compared with dicamba. Related research studying the factors that contribute to the off-target movement of dicamba and 2,4-D choline confirmed that the potential for movement of 2,4-D choline via volatility is dramatically less than the commercial formulations of dicamba labeled for use in Xtend soybean. This research also highlights the importance of only mixing ag chemicals that are approved for use with dicamba to prevent a possible increase in dicamba volatility. This research has been discussed by the investigator group over the last quarter and the protocols will be adjusted to include the Volatility Reduction Agents (VRA) now required for all dicamba applications in Xtend soybean.

Over the last quarter several outreach materials have been planned based on research from this project. These materials include updating current Take Action fact sheets and the development of new webinars or video recordings. Two topics are planned to be developed into a webinar or video recording: 1) Air temperature inversion research – Mandy Bish, University of Missouri, and 2) Summary of research on cereal rye for horseweed management – Mark Loux, The Ohio State University.

Updated January 31, 2022:
This project has a primary focus on field research during the soybean production season. As such, field research has been initiated from April through May 2021 for the majority of our experiments and the trials are progressing through the season. Thus, no new research findings or deliverables to improve weed management evolved during this last reporting period. Some preliminary findings should be evident over the next reporting period as weed management tactics are evaluated in the field.

Updated January 31, 2022:
This project has a primary focus on field research during the soybean production season. As such, field research has been initiated this spring and is ongoing for most of the research activity. An early data analysis and summary from the harvest weed seed control study has demonstrated that as much as 50,000 pigweed (waterhemp or Palmer amaranth) seeds per bushel of soybean can be found in combine grain tanks when harvesting soybean fields with weeds that have escaped management during the growing season. Nearly three times more pigweed seed moved through the combine where is could be devitalized by a mechanical hammer mill compared to the pigweed seed lost at the combine header and deposited directly back on the soil. This research highlights that mechanical devitalizers on the back of combines have the potential to dramatically reduce the amount of viable weed seed being added to the soil seedbank. Although weed seed being lost at the combine head to the soil, and the abundance of weed seed in the combine grain tank are two other areas that should be addressed to effectively manage seeds from weeds that survive in-season management tactics.

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.