3. Know how to manage the collected weed seed.
Bale direct collects all the crop residue directly from the combine and compacts it into large bales suitable for sale.
Which system is best?
1. Decide which system fits your farm best.
Impact mills run the chaff through a mill that pulverizes (destroys) the weed seed, which is then spread across the fields. This technology is usually considered the ultimate in HWSC.
Chaff carts are a tow-behind unit on the combine that collects the weed seed-laden chaff, which can then be placed into piles that are later either grazed by livestock, burnt, or both and sown through the following season. Chaff carts are often chosen for use on mixed cropping and livestock farms in Australia as the chaff is an excellent livestock feed; however, spreading manure back onto fields can allow for further seed spread.
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.
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and 95% of waterhemp seed remained on the plant at the time of soybean harvest in three years of research (A.S. Davis, unpublished data).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.
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).
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.
There are several issues with HWSC that limit its utility in our system at this time. However, it is important to recognize that this is new technology for which the bugs are being worked out. Weed problems are severe enough in Australia that many farmers are willing to tolerate the problems associated with the equipment. Currently there are only two versions of the chaff mills on the market. At least three other companies are involved in designing new equipment; these companies have greater resources available for supporting development than the initial companies. In addition to reducing the cost of the equipment, it is likely that some of the current limitations to the internal chaff mills will be resolved. Until HWSC is more widely available and convenient to implement, farmers must take steps to optimize both the effectiveness of herbicide programs and the suppressive ability of the crop.
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.
Each harvest weed seed control practice has its own benefits and challenges with growers leading the charge, working with a small group of researchers to develop harvester modifications that maximise weed seed control with harvest height and seed retention. For harvest weed seed control to be successful at the farm level the practice needs to be both cost effective and practical to fit in with existing operations.
Research by Walsh et. al., 2014 highlighted that harvest weed seed control tactics are equally effective in reducing weed seed production. The use of chaff carts, narrow windrow burning or the Integrated Harrington Seed Destructor, were compared at 24 sites across Australia with an average reduction in ryegrass of 60% germination the following autumn. This was achieved by removing 70-80% of the seed at harvest through either burning or destruction of weed seeds.
In the southern cropping region, low harvest height has been a barrier to adoption with growers not wanting to slow harvest down, incurring higher fuel costs and reducing harvester efficiency. Growers and researchers have since been looking at tactics that will enhance the efficacy of harvest weed seed control without slowing harvest. One option being adopted is sowing crops at narrower row spacings or higher plant populations. Weeds are then forced to grow taller to compete for light, therefore producing seed higher in the crop canopy. Stripper fronts are also being investigated to gauge any differences with weed seed capture and harvest efficiency, reducing the need to cut low whilst minimising fuel consumption.
Harvest weed seed control (HWSC) practices
Seed retention will change over time with the proportion of retained weed seeds declining the longer harvest is delayed past crop maturity. Therefore, crop and weed maturity will have a significant impact on the success of harvest weed seed control. Harvest height is equally important for harvest weed seed control, with a 15cm cut height preferred to capture 80-90% of the ryegrass seed at maturity – this can be challenging in high yielding cereals or bulky hybrid canola crops.
An online twitter survey was conducted in November 2017 by WeedSmart with 269 growers responding. The results indicated that harvest weed seed control practices are changing, with narrow windrow burning declining at the expense of chaff lining and chaff decks. 32% of growers were planning to use narrow windrow burning in 2017 whilst 26% would be chaff lining and 9% using chaff decks. Chaff carts were stable at 13%, mill technology at 3% and 14% would be doing nothing.
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.
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.