Increasing cases of multiple herbicide-resistant weeds is a serious concern for soybean growers. The annual impact of herbicide-resistant weeds on growers was once estimated by USB at over $2 billion and has continued to increase as the problem spreads, especially for Palmer amaranth, waterhemp, and horseweed (i.e. marestail). Waterhemp is one of the most troublesome weeds in Iowa soybean production. Waterhemp resistant to six different group of herbicides (HG 2, 4, 5, 9, 14, and 27) has been reported. The multiple resistant (glyphosate, ALS, and PPO-resistant) waterhemp epidemic in soybean production has rendered valuable herbicides ineffective, increased weed control costs, prompted a reversion to tillage, and increased use of herbicides with more adverse environmental impacts. This warrants the implementation of multi-tactic approaches to manage resistant weed seed banks in soybean.
US soybean growers, researchers, and industry representatives can draw from the success of the Australian experience with herbicide-resistant weeds. Australia’s joint effort among the public sector, commodity groups, and industry resulted in ~75% adoption of integrated weed management (IWM) tactics. Australian growers incorporated a suite of non-chemical practices, known as harvest weed seed control (HWSC), into their IWM programs, which allowed them to target multiple herbicide-resistant weed populations at harvest. HWSC strategies involve collection and/or destruction of weed seeds during crop harvest, thus minimizing weed seedbank additions. Weed seeds are intercepted by a combine and separated from the bulk crop residue and grain for subsequent management. The most effective forms of HWSC include impact mills and chaff lining or chaff tramlining. The success of HWSC relies on the propensity of annual weed species to retain seeds until soybean harvest. For example, weed seed retention was greater than 90% for Palmer amaranth, waterhemp, and common lambsquarters up to three weeks after soybean maturity.
Chaff lining technology of HWSC holds a great promise in Iowa soybean production since it relatively inexpensive (investment less than $5000) with minimum modifications to the combine. A majority of the weed seeds are in the chaff fraction of the chaff + straw that get spread out from the rear of the combine. Chaff lining is a HWSC tactic that confines the chaff material between stubble rows during harvest and relies on a mulch effect to prevent or reduce weed seed germination and emergence. These chaff rows are typically established by retrofitting a combine with a baffle to separate the chaff from the crop residue or straw and using a chute at the rear of the combine that collects and places chaff into narrow rows (18-24 inches wide by 6-8 inches deep). Crop residues (the chaff fraction that includes weed seeds) are deposited directly behind the combine in those narrow rows. Based on the first research conducted in 2020-2021 in Iowa (funded by ISA), chaff lining concentrated weed seeds to less than 5% of the field, rather than spreading them across the whole field as with a conventional harvest. The concentration of the chaff material places weed seeds in an environment unsuitable for germination and emergence, if left undisturbed. Chaff lines can then be targeted with additional weed control tactics such as shielded sprayers/herbicides. The process of chaff lining, by its nature, can ultimately help decrease weed seed contamination of soybean grain.
This HWSC technology of chaff lining has not been fully tested yet in US soybean. We were the first one to use a commercial chaff liner attached to the rear of a combine to test the efficacy of this technology in 2020-2021. The focus of year 2 of this project will be on the practical implementation of chaff lining in Iowa soybean production systems and design BMP’s based upon the results of herbicide interactions with chaff lining and weed seed decay/emergence.

1. Validate the efficacy of chaff lining (HWSC technology) on reducing weed germination and seedling emergence (mulch effect) and reducing weed seed banks (enhancing weed seed decay).
2. Examine how soybean yield potential and weather variables across years and herbicide programs influence efficacy of chaff lining (HWSC) for mitigating herbicide-resistant weed seed banks and soybean grain seed contamination.
3. Expand the inference of a weed management Decision Support Tool (DST) and enhance adoption by Iowa soybean producers through extension and outreach efforts.

Extension and outreach for timely dissemination of research-based information is an integral part of the project. The PI has 60% extension appointment. Findings and recommendations will be presented during ISU Research Farm Field Days, ICM Conference, and soybean grower and industry meetings across Iowa. The findings of this research will be available to the growers through extension publications as ICM News/Blogs and ICM Extension bulletins. The PI will work with the ISA farm network for more on-farm trials. Media talks and oral presentations in grower and Ag industry winter meetings (CAS etc.) to enhance adoption of this new IWM concept by Iowa soybean producers.

Update:
This is the second year of this research. The chaff liner concentrated ~95% of the waterhemp seeds that entered the combine into the chaff line at soybean harvest in 2021, indicating a drastic reduction in the spread of weed seeds in the field by the harvest operation. We will collect data on waterhemp emergence, density, biomass, and seed production in the spring/summer of 2022. The interaction of chaff lining with herbicide programs will also be quantified.

Update:
Waterhemp seed retention
Across the site-years, waterhemp seed shattering did not start until four weeks before soybean harvest. Almost 90% of seeds were retained on the plant until two weeks before soybean harvest. Waterhemp seed retention declined to 70% by the time of soybean harvest which occurred on first to second week of October in 2020 and 2021. These results indicate that a significant proportion of waterhemp seeds is likely to be retained on the plants around the typical dates of soybean harvest in Iowa.
Efficacy of the Chaff liner at Soybean Harvest
Data from pan samples indicated that chaff liner concentrated greater than 95% of waterhemp seeds into the chaff line at the time of soybean harvest. For instance, in year 1 of the study, the number of waterhemp seeds inside the chaff line was 75,000 seeds m-2. In contrast, the number of waterhemp seeds outside the chaff line was 200 seeds m-2. These results indicated that the chaff liner was very effective in concentrating waterhemp seeds into the chaff line. However, all of waterhemp seeds retained on the plant may not enter the combine due to seed-shattering losses associated with a combine-harvest process. For example, waterhemp seeds could shatter before entering the combine when shaken by the combine header during harvest. Additionally, once waterhemp seeds are inside the combine they may not separate out completely from soybean grains and can enter the grain tank instead of the chaff line.
Effect of Soybean Chaff line on Waterhemp Next Season
Waterhemp density was significantly higher inside the chaff line vs. outside the chaff line during the next growing season. This was because of the high proportion of waterhemp seeds that entered the combine were concentrated into the chaff line during soybean harvest. In the following year, waterhemp density was 67 to 76% higher inside the chaff line vs. outside the chaff line in the month of June and July in the following growing season.
Although the chaff line had a higher waterhemp density, rate of waterhemp emergence was lower inside the chaff line than outside the chaff line. The delayed waterhemp emergence inside the chaff line was most likely due to lower temperatures inside the chaff line compared to outside the chaff-line during the early-growing season. Presence of a crop residue on the soil surface can decrease soil temperatures and temperature fluctuations which can significantly reduce waterhemp emergence rates.
In addition to the lower emergence rate inside the chaff line, waterhemp aboveground biomass was also lower inside the chaff line than outside the chaff line in the following growing season.. However, the rate of waterhemp biomass accumulation was slower outside the chaff line during the early growing season. Waterhemp plants inside the chaff line accumulated 90% of the maximum biomass one week later than plants outside the chaff line.
Interaction Effects of the Chaff line and Herbicides on Waterhemp
Application of either PRE only or PRE fb POST herbicides inside or outside of the chaff line reduced waterhemp density by =90% compared to the untreated check at 6 WAP of corn in the next season. PRE only herbicides had significantly higher waterhemp density inside the chaff line than outside the chaff line. Additional POST herbicide treatment on the chaff line was much needed due to a higher concentration of waterhemp seeds and a high amount of soybean chaff inside the chaff line, which could have partially intercepted PRE herbicides. For example, it has been reported previously that high weed density can decrease the net amount of PRE herbicides absorbed by the individual plants.
An addition of a PRE herbicide treatment influenced waterhemp emergence rate more inside the chaff line than outside the chaff line. For example, inside the chaff line vs. outside the chaff line, waterhemp in the PRE herbicide treatment took one week longer to achieve 50% of the maximum emergence compared with the untreated check. Similarly, addition of PRE herbicide inside the chaff line increased time required to achieve 10 and 90% of the maximum emergence by at least one week compared to the untreated check. Whereas, addition of the PRE herbicide outside the chaff line did not influence time required to achieve 10 and 90% of the maximum emergence. This delayed emergence inside the chaff line compared to outside the chaff line was likely due to a synergistic effect between PRE herbicides and soybean chaff residue.
Management Implications
Results from this research indicated that chaff lining as a harvest weed seed control method in soybean has a high potential to manage waterhemp seedbanks in soybean-based rotations of Iowa and the Midwestern U.S. This research indicates usefulness of chaff limning across soybean fields with different levels of waterhemp infestation. The chaff liner was extremely effective in concentrating waterhemp seeds into the chaff lines, thereby drastically reducing the spread of herbicide-resistant seeds across the field (A Best Management Strategy). Implementing chaff lining in soybean-based cropping systems of the Midwest will reduce selection pressure exerted by herbicides and can potentially reduce herbicide use and cost. A one pass herbicide program provided season-long control of waterhemp outside the chaff line, resulting in lower herbicide use overall. Moreover, concentrated weed seeds inside the chaff line provides an excellent opportunity to practice site-specific weed control tactics such as flaming, banded herbicide application, shielded precision spray, and robotic weed control. These will be investigated in future research at ISU.

• The chaff liner concentrated more than 95% of waterhemp seeds that entered the combine into a narrow band at the center of the 35-feet header (Figure 1); hence, reducing the weed seed spread across the field at the time of soybean harvest.
• Waterhemp emergence in the subsequent corn crop (soybean-corn rotation) was almost 75% higher inside vs. outside the chaff line area of the corn plot. This was because of the higher density of seeds concentrated in the chaff lines.
• Waterhemp had a delayed emergence and accumulated significantly lower biomass inside the chaff line vs. outside the chaff line area in plots.
• The interaction of soybean chaff line by preemergence residual herbicide (Acuron®) applied in corn was synergistic, thereby further delaying waterhemp emergence (more than 2 weeks) in the chaff line vs. outside the chaff line area of the plots.
• Additionally, a postemergence herbicide application was much needed to control late-emerging waterhemp inside the chaff line in the next growing season; however, there was no advantage of adding a postemergence herbicide outside the chaff line area of the corn plot because of the very low density of waterhemp that emerged outside the chaff lines.
• These results indicate that chaff lining as a harvest weed seed control method in soybean would be a complimentary strategy to enhance the efficacy of herbicide programs for weed control and potentially reduce herbicide use (per acre) and cost.

Herbicide-resistant weeds cause additional $40 to $75 to soybean growers; therefore, implementation of this relatively inexpensive HWSC technology will definitely have a significant economic impact by reducing herbicide cost and by preventing soybean yield losses and weed seed contamination of the soybean grain (increased profitability). Iowa soybean producers have shown interest in adopting this technology based on demonstration and dissemination of results from the project conducted in 2020-2021 (year 1).