Updated December 12, 2021:
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.