Probably most applicable for Midwest cropping systems are chaff mills, devices that destroy weed seed during harvest (Schwartz-Lazaro et al. 2017). Chaff, which contains the majority of weed seed, is separated from other plant material as it moves through the combine. The chaff is run through a rolling cage mill that damages seed sufficiently to render them non-viable. The original design, the Harrington Seed Destructor, was a separate unit pulled behind the combine. More recently, chaff mills are integrated into the back of the combine.
Factors that influence the effectiveness of chaff mills are how long seed are retained on weeds and the ability of the mill to destroy seed. Seed retention varies widely among species. Chaff mills or other forms of HWSC would have little value for managing giant foxtail since most seeds fall from plants prior to harvest. Weed scientists at the University of Illinois found that 72, 92, and 95% of waterhemp seed remained on the plant at the time of soybean harvest in three years of research (A.S. Davis, unpublished data). The value of HWSC for waterhemp would be diminished in years with late harvest, such as in north central Iowa in 2018. Tests with a wide range of weed seeds have shown that greater than 95% of seed entering the mill is rendered non-viable, thus retention on the plants is the limiting factor in the effectiveness of this tactic.
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.
The interest in HWSC has been driven by Australia’s struggle with herbicide resistant weeds. Western Australia is recognized as the herbicide resistant weed capital of the world due to the widespread occurrence of multiple resistant weeds. The loss of effective herbicides for several important weed species, especially annual ryegrass (Lolium rigidum) in wheat production, has forced the development of innovative approaches to managing weed seed before they enter the seedbank. A survey of Australian farmers in 2014 found that 82% of respondents expected to use some form of HWSC within 5 years (Walsh et al. 2017).
Australians have developed several HWSC techniques, including chaff carts, baling of crop residues, chaff tramlining, narrow-windrow burning, and weed seed destruction (Walsh et al. 2017). Narrow-windrow burning is the most widely used HWSC practice in Australia (Table 1). This strategy involves altering how the combine manages crop residue during harvest (Walsh and Newman 2007). Relatively simple modifications are made to concentrate the chaff in a narrow windrow that is later burned. Research has shown that 70 to 80 percent of weed seed is collected by the combine and concentrated in the chaff, and nearly all of these seeds are killed by fire. The use of this practice in Western Australia has increased from 15% of farmers in 2004 to more than 50% of farmers in 2014. Due to differences in crops and climate, this tactic may not be effective in Iowa, but its widespread adoption demonstrates the value of targeting weed seed at harvest.
Currently there is limited experience using the chaff mills in corn or soybean, thus it is unclear how well they will perform in our system. Green stems of crops and weeds negatively impact mill performance, frequently blocking flow of the chaff through the mill. The University of Arkansas has the only integrated chaff mill in the United States, and they report that it has worked better in corn than in soybean. The problems in soybean likely are associated with the green material frequently present during soybean harvest.
Summer annuals are the dominant weed problem in the corn-soybean rotation since these pests establish and mature at the same time as the crops. The annual life cycle has several distinct stages, the success at transitioning between these stages determines whether a weed increases or decreases within a field (Figure 1). Population dynamic models allow researchers to investigate how control tactics targeting different ‘choke points’ influence the long-term success or failure of a weed.
Herbicide resistance remains an ongoing challenge for Australian grain growers but the industry is continually innovating to minimise the risks. Non-chemical tools are becoming mainstream practice so that growers and advisers can deal with herbicide resistance by reducing weed seed banks and protecting chemistry.
Harvest weed seed control (HWSC) practices
One of the most popular weed management tactics being adopted in recent years is harvest weed seed control (HWSC). This process takes advantage of seed retention at maturity by collecting weed seeds as they pass through the harvester. Problematic weeds such as annual ryegrass, brome grass and wild radish retain 77-95% of their seed above a harvest cut height of 15cm at maturity, creating an ideal opportunity for seed collection.
Harvest weed seed control adoption
Research has recently commenced to gauge the impacts of chaff lining and chaff decks on the rotting of weed seeds under different crop types. Preliminary data suggests poor seed survival under canola or barley chaff because of an allelopathic effect; however in wheat there was high ryegrass seed survival underneath the chaff row which is unexplained. Michael Walsh from Sydney University and John Broster from Charles Sturt University are currently working to quantify the value of rotting under chaff line and chaff deck systems.