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.
Identifying weak links in a weed’s life cycle
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and 95% of waterhemp seed remained on the plant at the time of soybean harvest in three years of research (A.S. Davis, unpublished data).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.
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.
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.
Chaff lining funnels the chaff and weed seeds into narrow rows behind the combine, where the residue is left to overwinter. The weed seeds are exposed to natural elements that can lead to weed seed decay and predation. Typically a follow-up herbicide application is required. – Chaff lining is usually considered a good entry-level HWSC option.
An excellent way to stop weeds in their tracks is to collect these weed seeds at harvest and either destroy them or deposit them in a known location where they can be monitored and controlled later. Soybean, wheat, and other crops harvested with a grain header are ideal choices for harvest weed seed control (HWSC). Other crops such as cotton and corn need further equipment development to make HWSC a viable option.
HWSC is being rapidly adopted in Australia and other countries around the world. There are six systems currently being used on Australian farms and they have been initially developed by farmers.
If you are considering adding harvest weed seed control (HWSC) to your weed control program there are excellent resources on the WeedSmart website to help guide you through the initial decisions and the implementation of this important weed control tool.
Research has demonstrated that all are very effective weed seedbank management tactics for a range of weed species, achieving over 80 percent control and for some nearly 100 percent.
There are six systems currently used to collect and manage weed seed at harvest. They can be grouped according to the way crop residue is managed: chaff only or chaff + straw.
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.
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
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.
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 cannot be used in isolation for weed management; growers and advisers should implement a range of diverse weed management practices to drive weed numbers down. Defined as the ‘Big six’ (www.weedsmart.org.au/the-big-six), these management practices include diverse rotations, mix and rotating herbicides, crop competition, double knocks, crop topping/hay to stop seed set and harvest weed seed control. The ‘big six’ complements best practice agronomy such as calendar sowing combined with effective pre-emergent herbicide packages.