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 (Amaranthus palmeri S. Watson), smooth pigweed (Amaranthus hybridus L.), common lambsquarters (Chenopodium album L.), jimsonweed (Datura stramonium L.), common cocklebur (Xanthium strumarium L.), and johnsongrass [Sorghum halepense (L.) Pers.] up to three weeks after soybean maturity (Schwartz-Lazaro et al. unpublished).

Chaff lining technology of HWSC holds the greatest promise for 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 (8-12 inches). Crop residues (the chaff fraction that includes weed seeds) are deposited directly behind the combine in those narrow rows. Thus, chaff lining concentrates weed seeds to <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 will be the first one to use a commercial chaff liner attached to the rear of a combine to test the efficacy of this technology. The focus of this project will be on the practical implementation of chaff lining in Iowa soybean systems and design BMP’s based upon the results of herbicide interactions with chaff lining and weed seed decay/emergence.

1. Determine 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 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)

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:
Field experiments were initiated in fall of 2020 in Central Iowa for the first testing of chaff lining as a HWSC method in soybean to manage herbicide-resistant waterhemp seed banks. At each location, we used a custom-designed chaff liner installed to the rear of a commercial combine with a 35-feet wide header. We expected to see different soybean yield scenarios across locations, which will have a direct relationship with the amount of soybean chaff collected and subsequent weed control (suppression of emergence).

The chaff liner included a baffle to separate the soybean chaff from the straw and a chute to divert the weed seed-bearing chaff fraction (from the sieves) into a narrow windrow (18-20 inches wide by 6-8 inches deep), which was left to rot or mulch while the straw fraction was chopped and spread out in the field at soybean harvest. The chaff material exiting through the chaff liner chute was collected by placing three aluminum trays per plot as the rear of the combine passes over the trays in the field. Any pigweed seeds lost in the straw fraction (thresher loss) exiting through the straw chopper was captured by placing aluminum trays behind the combine. Samples were also collected from plots without chaff lining for comparison. Pigweed seeds were separated from the chaff material or straw and counted (seeds. m-2).

In 2021, we are retrieving two seed packets per plot at five different timings starting in April through July. We will be monitoring waterhemp emergence in eight permanent 0.5 m2 quadrats (four each in the chaff line and non-chaff line areas of a plot) at biweekly intervals this summer. Chaff samples (3 samples per treatment) will also be taken at the seed packet retrieval timings to determine the chaff residue decomposition (C:N ratio) over the subsequent soybean growing season, which will ultimately influence pigweed emergence (cumulative and pattern) and fate of seeds. Each plot was split into four sub-plots to determine the interaction of chaff lining by herbicide programs on waterhemp control in the subsequent crop, which is corn in one location and soybean in the other location in 2021. The corn or soybean crop was planted (30 inch spacing) with a commercial planter, with minimal disturbance/spread of the chaff lines observed this spring. Herbicide programs include: nontreated control (no herbicide), PRE program only, POST only, and PRE fb POST. Besides emergence counts (density) and seed viability, we will collect data on waterhemp percent control inside and outside chaff lines as a result of herbicide treatments at biweekly intervals. Crop yields will be determined at harvest in the fall of 2021.

Update:
Field experiments were initiated in fall of 2020 in Central Iowa for the first testing of chaff lining as a HWSC method in soybean to manage herbicide-resistant waterhemp seed banks. At each location, we used a custom-designed chaff liner installed to the rear of a commercial combine with a 35-feet wide header.

Experiments were conducted in a split-plot design with four replications. Main plot and split-plot factor were established in soybean phase of the rotation in 2020. The main plot factor consisted of two herbicide programs (HPs) used as postemergence (POST); marginal and aggressive HP. No preemergence (PRE) herbicides were applied in soybean. The marginal HP included herbicides from two sites of action. In contrast, aggressive HP included herbicides from three sites of action. The two HPs were used to create two levels late-season of waterhemp infestation, which might influence efficacy of the chaff liner due to different amounts of crop-weed chaff/residue passing through the combine. Whole plots were 35 feet wide by 200 feet long. The width of the plots was equivalent to the header-width (35 feet) of a commercial combine. A 10-feet wide alleyway between the whole plots was left to prevent any possible movement of waterhemp seeds from one plot to another at the time of soybean harvest.

The chaff liner included a baffle to separate the soybean chaff from the straw and a chute to divert the weed seed-bearing chaff fraction (from the sieves) into a narrow windrow (18-20 inches wide by 6-8 inches deep), which was left to rot or mulch while the straw fraction was chopped and spread out in the field at soybean harvest. The chaff material exiting through the chaff liner chute was collected by placing three aluminum trays per plot as the rear of the combine passes over the trays in the field. Any pigweed seeds lost in the straw fraction (thresher loss) exiting through the straw chopper was captured by placing aluminum trays behind the combine. Samples were also collected from plots without chaff lining for comparison. Pigweed seeds were separated from the chaff material or straw and counted (seeds. m-2). Corn was planted in 2021.

RESULTS:
Effect of Soybean Herbicide Programs on Waterhemp
Soybean herbicide programs (HP) had a significant effect on waterhemp density at the time of soybean harvest in 2020. However, it did not influence waterhemp seed production (120,000 seed m-2 average). The marginal HP, which included herbicides from two sites of action (Table 2) had a waterhemp density of 7 plants m-2. In contrast, waterhemp density in aggressive HP, which included herbicides from three sites of action, were 4 plants m-2.

Waterhemp seed shattering did not start until September 3 or four weeks before soybean harvest in both HPs. The percent seed retention over time did not differ between the HPs. Overall, 90% of seeds were retained on the plant until September 19 or about two weeks before soybean harvest. Waterhemp seed retention declined to 70% average by the time of soybean harvest on October 1, 2020. 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 (USDA Crop Progress 2020).

Efficacy of Chaff lining at Soybean Harvest
Data indicated that the chaff liner concentrated >99% of waterhemp seeds into the chaff line at the time of soybean harvest. 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 waterhemp seeds retained on the plant may not enter the combine due to seed-shattering losses associated with a combine-harvest process (Ruttledge et al. 2018). 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 in Corn Phase of the Rotation
Waterhemp density was significantly higher inside the chaff line than outside the chaff line during the corn phase of the rotation in 2021. This was because >99% of waterhemp seeds that entered the combine were concentrated into the chaff line during soybean harvest in 2020. In 2021, waterhemp density was 67% higher inside the chaff-line (123 plants m-2) vs. outside the chaff line (40 plants m-2) at 6 weeks after corn planting (WAP). Similarly, waterhemp density was 76% higher inside the chaff line (213 plants m-2) vs. outside the chaff line (52 plants m-2) at 12 WAP.

Although the chaff line had a higher waterhemp density, rate of waterhemp emergence was lower inside the chaff line than outside the chaff line. For example, waterhemp inside the chaff line took one week longer (48 days) to achieve 50% of the maximum emergence (t50) compared to waterhemp outside the chaff line (42 days). Similarly, waterhemp inside the chaff line initiated emergence 5 days later (37 days after corn planting, DAP) than the waterhemp outside the chaff line (32 DAP). Moreover, it took 63 days to achieve 90% of the maximum emergence by waterhemp inside the chaff line compared to only 54 days by waterhemp 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 (data not shown). Presence of a crop residue on the soil surface can decrease soil temperatures and temperature fluctuations, which can significantly reduce the waterhemp emergence rate.

In addition to the lower emergence rate inside the chaff line, waterhemp aboveground biomass per unit area at 12 WAP was also lower inside the chaff line vs. outside the chaff line. Waterhemp aboveground biomass at 12 WAP was 63% greater outside (43 g m-2) than inside the chaff line (16 g m-2). However, the rate of waterhemp biomass accumulation per unit area was slower outside the chaff line during the early growing season. For example, waterhemp inside the chaff line accumulated 10% of the maximum biomass one week earlier (40 DAP based on the t10 value) than waterhemp outside the chaff line (47 DAP). Nonetheless, the trend was reversed over time, Waterhemp inside the chaff line accumulated 90% of the maximum biomass one week later (77 DAP) than waterhemp outside the chaff line (68 DAP).

Results from this study indicate that chaff lining as a harvest weed seed control method in soybean has a high potential to manage waterhemp seed bank in soybean-corn rotations of the Midwest. The chaff liner was extremely effective in concentrating waterhemp seeds into a narrow-row of soybean chaff (chaff line); hence, preventing the spread of resistant weed seeds. Additionally, the efficacy of chaff liner in concentrating waterhemp seeds was not affected by the waterhemp density at the time of soybean harvest. This indicates usefulness of the chaff liner across soybean fields with different levels of waterhemp infestation. This novel research suggests that implementing chaff lining in soybean-based cropping systems of the Midwest will reduce selection pressure exerted by herbicides used in soybean-corn rotations.

Waterhemp escapes are becoming very common in soybean fields of the Midwestern United States due to continued rise in herbicide-resistant (HR) populations. In a conventional harvesting system, weed seeds are also harvested with the crop and spread back into the field. Harvest weed seed control (HWSC) methods such as chaff lining concentrate weed-seed-bearing crop chaff into a narrow-row (chaff line). These chaff lines (18-20 inches wide) are kept undisturbed during the following growing-seasons, assuming the chaff line will create an environment less-favorable for weed seed germination and survival. Field experiments were conducted in a soybean-corn rotation over two years (2020-2021) at ISU Curtiss Farm near Ames, IA and a grower field in Roland, IA to quantify the efficacy of chaff lining to manage waterhemp seeds at the time of soybean harvest. About 70% of waterhemp seeds were retained on the mother plant at the time of soybean harvest in 2020. The chaff liner concentrated >99% of the waterhemp seeds that entered the combine into the chaff line.

Although waterhemp density was 76% average higher inside the chaff line than outside the chaff line, aboveground biomass was 63% average lower inside the chaff line than outside the chaff-line at 12 weeks after planting corn (2021). Similarly, waterhemp inside the chaff line had a delayed emergence than waterhemp outside the chaff line. Due to a low density of waterhemp outside the chaff line, there will potentially be a reduction in overall herbicide use. However, the high density of waterhemp within the chaff lines needs to be effectively managed with a PRE followed by POST residual herbicide program in the subsequent corn crop. Moreover, concentrated waterhemp density inside the chaff line provides an excellent opportunity to practice site-specific weed control tactics such as flaming, banded/shielded herbicide application, targeted tillage, and robotic weed control. These results suggest that chaff lining, a non-chemical tactic, should be implemented in soybean-based cropping systems of Iowa and the Midwest to target waterhemp seeds at harvest and reduce selection pressure exerted by herbicides, thereby managing herbicide-resistant weed seed banks.

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).