For the proposed project in 2022, we will use a Redekop® Seed Destructor unit installed at the rear of the John Deer S680 combine. Field experiments will be conducted in the fall of 2022 at two locations in Central Iowa at soybean harvest. The fields with natural infestations of glyphosate-resistant waterhemp have already been selected and planted to soybean in May of 2022. Experiments will be conducted in a randomized complete block design with four replications. The desired late-season waterhemp densities will be achieved using PRE and POST herbicide programs during the 2022 soybean growing season. Soybean yields will be recorded at harvest in 2022. Treatment will include: HWSC (seed destructor unit turned on) vs. no HWSC (seed destructor unit turned off). The Redekop® seed destructor unit attached to commercial JD combines can be switched on and off easily without any modifications or delay in the harvest operation. Each plot will be 35 feet wide (width of the header) by 100 feet long.

Waterhemp seed retention before harvest: Late-season waterhemp (survivors) density in each plot will be evaluated in the fall of 2022. Six female waterhemp plants in each plot will be selected. Plants will be covered with pollination bags and two trays will be placed under each female waterhemp plant at seed set. The trays will be emptied and seed will be cleaned from other debris in the trays at a weekly interval until soybean harvest. At harvest, plants will be clipped and seeds remaining on plants will be collected and cleaned. Percent shattering loss will be calculated by the cumulative amount of seeds in trays compared to the total seed production.

Header loss: At soybean harvest in fall 2022, six aluminum pans (6 by 12 inches) will be placed between soybean rows beneath the female waterhemp plants in each plot. As the combine header passes over the pans, pans will be removed and number of seeds collected (after separating the debris) will be counted to account for header loss as a percent of total seeds retained by the plant.
Weed seed kill efficacy: At soybean harvest in fall 2022, the efficacy of the Redekop seed destructor will be determined by collecting the chaff material (containing weed seeds that passed through the high-impact mill) in a large mesh bag tied directly behind the mill, without altering the air flow. Weed seeds retained in the straw fraction (thresher loss) will be captured by using a tarp behind the straw chopper of the combine (seed destructor and no HWSC plots). Waterhemp (and other weed species if present) seed will be separated from the soybean chaff/straw material and counted (no/m2). For the samples collected from the HWSC (seed destructor) plots, percent weed seed kill by the high-impact mill will be visually assessed under a digital microscope and divided into four categories based on the amount of physical destruction (% visible injury). The four categories will include: no destruction (0% visible injury), minimal destruction (10% injury), moderate destruction (60% injury), and severe destruction (90% injury). Seeds will be germinated at 25 C (optimum temperature) for 4 weeks in an incubator and any non-germinated seed will be tested for viability using a standard crush test or tetrazolium test. Weed seed samples from No HWSC plots will be used for comparison to determine the efficacy of the seed destructor in reducing waterhemp (and other weed species if present) seed viability and percent germination.
Waterhemp seed bank assessment in 2023: Four permanent 0.5 m2 quadrats will be established in each plot (with or without HWSC) in the spring of 2022. Waterhemp (and other weed species present at the site) emerging in each quadrat will be marked separately to establish three different emergence cohorts in 2022. Fields will be planted to corn or soybean in 2022. Data on pigweed emergence period (beginning and end dates), emergence counts, and plant height will be monitored from the 0.5 m2 quadrats at biweekly intervals and a final assessment of pigweed density in late fall. Waterhemp biomass and seed production (by cohort) will also be measured in the four 0.5 m2 quadrats in each plot. Weed biomass will be quantified by clipping weeds at the soil surface; those samples will be oven dried and weighed. Weed seed production (no. m-2) will be estimated to determine the effect of herbicide by HWSC interactions on the weed seed bank reduction. Soybean grain yield will be determined.

1. Evaluate the efficacy of seed destructor as a novel, IWM tactic to destroy (seed kill) waterhemp seeds and reduce soil weed seed banks.
2. Examine how herbicide programs influence efficacy of weed seed destruction (HWSC method) for mitigating herbicide-resistant waterhemp seed banks and soybean grain seed contamination.
3. Expand the inference of a weed management Decision Support Tool (DST) and extension and outreach events to enhance adoption by soybean producers

Extension and outreach for timely dissemination of research-based information is an integral part of the project. The PI has 60% statewide 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 is working with the ISA On-farm network for further expansion of these HWSC on-farm trials. Media talks and oral presentations in grower and Ag industry meetings will be delivered to enhance knowledge and adoption of this novel IWM concept by soybean producers in Iowa.

Update:
Field experiments were established in soybean fields in Gilbert, IA and Dayton, IA in fall of 2022. The first objective of this study (Objective 1) was to evaluate the efficacy of the Redekop Seed Destructor (Figure 1) in destroying waterhemp seeds at the time of soybean harvest. The Redekop seed destructor unit (currently priced at $75,000) was installed at the rear of a John Deer S680 combine.

The success of the harvest weed seed control methods relies on the propensity of some weed species like waterhemp and Palmer amaranth to retain a majority of seeds by the time the soybean crop is harvested. 75-80% of waterhemp seeds were still retained by the plants at the time of soybean harvest after accounting for shattering losses to the ground (September through the date of harvest). Out of those seeds that were retained by the plants at harvest, approximately 30% were lost as the combine head passed through those plants in the field (header loss; Figure 2). Additionally, 10% of the waterhemp seeds entering the combine can be lost at the grain tank (grain tank contamination) or escape through the combine thresher (chopper) during the harvest operation.

To determine the seed kill efficacy of the Redekop seed destructor, waterhemp seed samples exiting the combine were collected in 1 m2 trays placed on the ground with the seed destructor unit turned on vs. off (treatments). Those samples were cleaned and weed seeds were counted. More than 90% of pigweed seeds exiting the seed destructor unit had a moderate to severe physical damage when viewed under a digital microscope (Figure 3). Those seeds failed to germinate and were considered non-viable. These results from the first run of this study indicated that the Redekop seed destructor was effective in reducing viable weed seed returns to the soil by more than 90% compared to a conventional harvest operation.

Future efforts need to be focused on minimizing the weed seed inputs to the soil because of the combine header loss (reel contacting the waterhemp plants) to more effectively utilize this harvest weed seed control technology in soybean. Early harvest dates of soybean would also be beneficial to minimize natural shattering of waterhemp seeds with an objective to capture a majority of weed seeds and destroy them with the seed destructor.

Waterhemp seed bank decline: To quantify a decline in viable weed seed bank (Objective 2), four permanent 0.5 m2 quadrats will be established in each plot (with or without HWSC) in the spring of 2023. Waterhemp (and other weed species present at the test sites) emerging in each quadrat will be marked separately to establish three different emergence cohorts in 2023. Fields will be planted to corn or soybean in 2023. Data on pigweed emergence period (beginning and end dates), emergence counts, and plant height will be monitored from 0.5 m2 quadrats at biweekly intervals and a final assessment of pigweed density in August. Waterhemp biomass and seed production (by cohort) will also be determined in the four 0.5 m2 quadrats in each plot. Weed biomass will be quantified by clipping weeds at the soil surface; those samples will be oven dried and weighed. Weed seed production (no. m-2) will be estimated to determine the impact of this harvest weed seed control technology in reducing weed seed banks in soybean-based production systems of Iowa.

View uploaded report

Update:
Field experiments were established in soybean fields in Gilbert, IA and Dayton, IA. The objective of this one-year study was to evaluate the efficacy of the Redekop Seed Destructor in destroying waterhemp seeds at the time of soybean harvest in 2022 and reducing viable weed seed bank in following year (2023). The Redekop seed destructor unit (currently priced at ~$80,000) was installed at the rear of a John Deer S680 combine.
The success of harvest weed seed control methods relies on the propensity of some weed species like waterhemp and Palmer amaranth to retain most seeds by the time the soybean crop is harvested. Almost 60 to 75% of waterhemp seeds were still retained by the plants at the time of soybean harvest after accounting for shattering losses to the ground. The late harvest can potentially account for greater losses due to seed shattering. Out of those seeds that were retained by the plants at harvest, approximately 30% were lost as the combine head passed through those plants in the field (header loss). Additionally, 10% of the waterhemp seeds entering the combine can be lost at the grain tank (grain tank contamination) or escape through the combine thresher (chopper) during the harvest operation.
To determine the seed-kill efficacy of the Redekop seed destructor, waterhemp seed samples exiting the combine were collected in 1 m2 trays placed on the ground with the seed destructor unit turned on vs. off (treatments). Those samples were cleaned and weed seeds were counted. More than 90% of pigweed seeds exiting the seed destructor unit had a moderate to severe physical damage when viewed under a digital microscope. Only a small proportion of seeds escaped with fully intact seeds.
Fifty seeds from each category were germinated at 32/22 C day/night temperatures (14/10 hour periods). Seed germination was observed and recorded for 28 days. At the end of observation period, non-germinated seeds were tested for viability using the imbibed seed crush test. A majority of moderate to severe physically-damaged seeds failed to germinate and tested non-viable. Furthermore, seeds with slight damage are less likely to persist in soil seedbank due to increased seed mortality factors. These results indicated that the Redekop seed destructor was effective in reducing viable weed seed returns to the soil compared to a conventional harvest operation.
The early-season waterhemp density in 2023 season did not differ in the presence or absence of weed seed destructor in 2022 soybean. Further evaluations of waterhemp seedling density, plant height, and biomass/seed production during the summer/fall of 2023 could not be performed as the study was terminated before completion.
Using weed seed destructor over one season is unlikely to have a significant effect in reducing seedling emergence in the following year. No observed differences in weed seedling emergence can most likely be attributed to the residual seed bank from previous years, which could mask the effect of seed destruction. Additionally, seed shattering losses occurred before or at the time of soybean harvest. Those seeds were left on or near the soil surface under no-till until the following growing season. Therefore, the seed destruction efficacy and seed bank needs to be quantified over multiple growing seasons to accurately determine and model the decline in weed seed bank and subsequent reduction in herbicide inputs and costs.
Future efforts need to be focused on minimizing the weed seed inputs to the soil because of the combine header loss (reel contacting the waterhemp plants) to more effectively utilize this harvest weed seed control technology in soybean. Early harvest dates of soybean would also be beneficial to minimize natural shattering of waterhemp seeds with an objective to capture a majority of weed seeds and destroy them with the seed destructor technology.

View uploaded report

This research suggest that weed seed destruction using a Redekop high-impact mill is a powerful non-chemical weed control tactic with potential to kill greater than 90% of waterhemp and Pamer amaranth seeds that passes through the combine at soybean harvest. This harvest weed seed control tactic can be implemented successfully to mitigate herbicide-resistant weed seed banks in soybean-based production systems and reduce reliance on herbicides and costs in the long-term.

Herbicide-resistant weeds cause additional $45-$75 per acre to soybean growers, which translates into more than $100 Million loss annually in soybean in the state. Furthermore, cost of developing a new herbicide site of action is estimated to be $286 Million and takes on an average of 11 years (USB). Although there will be an investment of $70,000 for a seed destructor unit, implementation of this HWSC technology with >95% seed kill efficacy will potentially have a significant economic return (increased profitability) within ~5-6 years by reducing herbicide cost, reduced weed seed bank, and by preventing soybean yield losses and weed seed contamination of the soybean grain. The ultimate goal would be to preserve the utility of existing herbicide resources and enhance long-term sustainability and profitability of soybean production in Iowa.