Herbicide resistance has emerged as a significant threat to modern agriculture, rapidly diminishing the effectiveness of chemical weed control. We are witnessing an alarming rise in weed biotypes, such as waterhemp, giant ragweed, and horseweed (marestail), that exhibit resistance to multiple herbicide groups. In Missouri, a waterhemp variant has even been identified with resistance to six different herbicide modes of action. Iowa’s farmlands are similarly affected, with the majority of fields harboring waterhemp resistant to at least three herbicide groups, and populations demonstrating five-way resistance are increasingly common. While herbicides will undoubtedly remain a cornerstone of weed management strategies in the near future, it’s evident that relying solely on them is no longer sustainable. To preserve the efficacy of herbicides and ensure long-term weed control, we must integrate alternative tactics into our current cropping systems. This article will explore proactive strategies, with a particular focus on harvest weed seed control (HWSC), that can significantly reduce weed pressure by targeting a critical vulnerability: the Weed Seed Bank.
Understanding the Weed Seed Bank and Weed Life Cycles
Summer annual weeds pose the greatest challenge in corn-soybean rotations because their growth cycle coincides with that of the crops. The life cycle of these annual weeds involves several key stages. The transitions between these stages dictate whether a weed population expands or contracts within a field (Figure 1). Population dynamics models are invaluable tools for researchers, enabling them to assess how control methods targeting specific ‘choke points’ in a weed’s life cycle influence its long-term success or failure.
Figure 1: The life cycle of annual weeds, highlighting the critical role of the weed seed bank in population dynamics. Understanding these stages is crucial for developing effective and sustainable weed management strategies.
Current weed management practices heavily rely on controlling weeds after they emerge from the weed seed bank using preemergence herbicides, or as seedlings with postemergence herbicides. While these herbicides have been remarkably effective, the escalating issue of herbicide resistance casts doubt on their long-term viability. Population dynamics models emphasize that weed populations are exceptionally sensitive to fluctuations in the number of seeds entering the weed seed bank. Therefore, strategies that effectively curb seed production or prevent seed deposition into the seed bank can exert a substantial impact on the long-term management of weed populations. A population model evaluating the benefits of HWSC in delaying herbicide resistance development revealed that eliminating approximately 50% of weed seeds at harvest could postpone resistance evolution by almost a decade (Somerville et al. 2018). This underscores the critical importance of targeting the weed seed bank for sustainable weed management.
Harvest Weed Seed Control: Targeting the Weed Seed Bank at Harvest
Preventing weed seeds produced by escaped weeds from replenishing the weed seed bank is a highly effective approach to integrated weed management. Historically, early mechanized combines were often equipped with weed seed collectors, designed to prevent weed seeds from being returned to the fields. Practices like hand-weeding (“walking beans”) were also common. However, the advent of modern herbicides and the increase in farm sizes led to a decline in the adoption of these labor-intensive tactics. Harvest Weed Seed Control (HWSC) presents a viable alternative to herbicides that can be seamlessly integrated into contemporary agricultural systems without significantly increasing labor demands. By focusing on the weed seed bank, HWSC offers a proactive and sustainable approach to weed management.
The resurgence of interest in HWSC is largely attributed to Australia’s extensive experience battling herbicide-resistant weeds. Western Australia is often considered the global epicenter of herbicide resistance, grappling with widespread occurrences of multiple resistant weed species. The loss of effective herbicides for key weeds, particularly annual ryegrass (Lolium rigidum) in wheat production, compelled the development of innovative strategies aimed at managing weed seeds before they contribute to the weed seed bank. A 2014 survey of Australian farmers revealed that 82% anticipated using some form of HWSC within the subsequent five years (Walsh et al. 2017), demonstrating the growing recognition of its importance in managing the weed seed bank.
Australian farmers have pioneered various HWSC techniques, including chaff carts, baling crop residues, chaff tramlining, narrow-windrow burning, and weed seed destruction (Walsh et al. 2017). Narrow-windrow burning is the most prevalent HWSC method in Australia (Table 1). This technique involves modifying the combine’s residue management system to concentrate chaff into a narrow windrow, which is then burned. Research indicates that combines can collect 70 to 80 percent of weed seeds, concentrating them within the chaff, and the subsequent burning effectively eliminates nearly all of these seeds. The adoption rate of narrow-windrow burning in Western Australia increased dramatically from 15% of farmers in 2004 to over 50% by 2014. While the direct application of narrow-windrow burning may not be suitable for Iowa’s crops and climate, its widespread adoption in Australia highlights the effectiveness of targeting weed seeds at harvest to manage the weed seed bank.
Chaff mills, devices designed to destroy weed seeds during harvest (Schwartz-Lazaro et al. 2017), are potentially more adaptable to Midwest cropping systems. Chaff, which harbors the majority of weed seeds, is separated from other plant material within the combine and then processed through a rolling cage mill. This mill inflicts damage sufficient to render the seeds non-viable, effectively preventing them from entering the weed seed bank. The original design, known as the Harrington Seed Destructor, was a separate unit towed behind the combine. More recent advancements have integrated chaff mills directly into the combine’s rear section, streamlining the process and enhancing efficiency in weed seed bank management.
The effectiveness of chaff mills is influenced by factors such as seed retention on weeds until harvest and the mill’s seed destruction capability. Seed retention varies significantly among weed species. For instance, chaff mills would be less effective for managing giant foxtail, as most of its seeds fall from the plants before harvest. However, research conducted by weed scientists at the University of Illinois revealed that a substantial proportion of waterhemp seeds, ranging from 72% to 95%, remained on the plant at soybean harvest across three years (A.S. Davis, unpublished data). This high seed retention makes waterhemp a prime target for HWSC strategies aimed at reducing the weed seed bank. It’s important to note that in years with delayed harvests, such as the 2018 season in north-central Iowa, the value of HWSC for waterhemp might be diminished. Tests across a wide range of weed seeds have demonstrated that chaff mills can achieve over 95% seed destruction rates, indicating that seed retention on plants is often the limiting factor in the overall effectiveness of this weed seed bank management tactic.
By physically destroying weed seeds through combine modifications, HWSC directly reduces the return of seeds to the soil weed seed bank. This proactive approach effectively lessens herbicide selection pressure, thereby mitigating the evolution of herbicide-resistant weeds. However, implementing this technology comes with associated costs. Early versions of chaff mills were priced around $120,000, but industry experts anticipate that mass production could reduce costs by at least 50%. Beyond the initial equipment expense, chaff mills also present operational considerations. Current models require approximately 80 to 100 horsepower, which can lead to a 12-20% reduction in combine capacity (Anonymous 2018). The increased power demand for weed seed destruction also translates to an average increase in fuel consumption of about 0.4 gallons per ton of grain. Farmers need to weigh these economic and operational factors against the long-term benefits of weed seed bank reduction and herbicide sustainability.
Current experience with chaff mills in corn and soybean systems in the Midwest is limited, leaving some uncertainty about their performance under these specific conditions. Green stems from crops and weeds can negatively impact mill performance by obstructing chaff flow. The University of Arkansas, the only institution in the United States with an integrated chaff mill, reports better performance in corn compared to soybean. The challenges in soybean are likely linked to the presence of green material frequently encountered during soybean harvest. Addressing these operational challenges will be crucial for optimizing chaff mill technology for effective weed seed bank management in diverse cropping systems.
Another concern associated with chaff mills is the rapid wear of mill components. Manufacturers estimate a rotor lifespan of approximately 800 hours, but Australian users often report lifespans less than half of that. Rotor replacement costs exceed $2000. The low cutting height during soybean harvest could potentially exacerbate rotor wear compared to corn harvest due to increased soil and debris intake within the soybean residue. Extending the lifespan and reducing the maintenance costs of chaff mill components are important areas for ongoing development and refinement to enhance the practicality and economic viability of this weed seed bank management technology.
While many Iowa farmers may not currently perceive weed management as a pressing enough issue to justify the investment and adjustments associated with HWSC, it is crucial to learn from the experiences of Australian farmers. Australian agriculture turned to HWSC out of necessity, after facing widespread weed resistance to nearly all available herbicides. The accelerating rate of herbicide resistance development and the limited introduction of new herbicide modes of action suggest that Midwestern agriculture may face a similar predicament in the foreseeable future. To proactively prevent further herbicide losses, a shift in perspective towards weed management is essential. Reducing the size of the weed seed bank must be recognized as equally important as protecting immediate crop yields. Investing in weed seed bank management today can safeguard herbicide effectiveness and ensure sustainable weed control for the future.
Although limitations currently exist regarding the widespread utility of HWSC in Midwestern systems, it is important to acknowledge that chaff mill technology is still in its early stages of development. The severity of weed problems in Australia has driven farmers to embrace these technologies despite existing challenges, highlighting the potential of HWSC for weed seed bank management. Currently, only two versions of chaff mills are commercially available, but at least three other companies are actively engaged in developing new equipment, bringing greater resources and innovation to the field. In addition to cost reductions, ongoing development efforts are likely to address current limitations and improve the practicality of integrated chaff mills. Until HWSC becomes more readily accessible and convenient, farmers should prioritize optimizing herbicide programs and enhancing the competitive ability of crops to suppress weeds and minimize weed seed bank replenishment.
References
Anonymous. 2018. Residue management at harvest. Weed seed options. Kondinin Group. Online https://weedsmart.org.au/wp-content/uploads/2018/06/RR_1802_weedsmart.pdf
Schwartz-Lazaro, L.M., J.K. Norsworthy, Walsh, M.J., and Bagavathinnan, M.V. 2017. Efficacy of the Integrated Harrington Seed Destructor on Weeds of Soybean and Rice Production Systems in the Southern United States. Crop Sci. 57:2812-2818.
Somerville, G.J., S.B. Powles, M.J. Walsh and M. Renton. 2018. Modeling the impact of harvest weed seed control on herbicide-resistance evolution. Weed Sci. 66:395-403.
Walsh, M. and Newman, P. 2007. Burning narrow windrows for weed seed destruction. Field Crops Res. 104:24-30.
Walsh, M., Ouzman, J., Newman, P., Powles, S., and Llewellyn, R. 2017. High levels of adoption indicated that harvest weed seed control is now an established weed control practice in Australian Cropping. Weed Technol. 31:341-347.
Prepared for the 2018 Integrated Crop Management Conference. Ames, IA.
