Australia and Harvest Weed Seed Control


With the help of Dr. Michael Walsh, Director of Weed Research for the University of Sydney, I received a Nancy Roma Paech Visiting Professorship in Agriculture from the University of Sydney Institute of Agriculture. The professorship covered most of my expenses for a seven-week visit to Australia starting in late October of last year. I gave five seminars on farming systems in Eastern Washington during my stay, but my primary goal for the trip was to establish a joint research project with Dr. Walsh and learn more about Harvest Weed Seed Control (HWSC). Listen to episode 47 of the WSU Wheat Beat Podcast to hear my interview with Dr. Walsh about HWSC.

The Australians have developed wide-spread herbicide-resistant weed populations. In response to this situation, and in an effort to maintain highly productive reduced tillage systems, they have developed and adopted HWSC. Harvest weed seed control takes advantage of the biological attribute of seed retention at maturity in some annual weed species. In Australia, annual ryegrass retains about 80% of its seed at harvest, wild radish about 99%, brome grass about 77%, and wild oat about 84%. These retained weed seeds can enter the combine at harvest time and rather than spreading them back out over the entire field, basically helping them to spread, they can be processed in various ways to reduce their viability and spread.

The majority of weed seeds that enter the combine during harvest, exit in the chaff fraction. Consequently, the chaff fraction is the focus of HWSC systems. Chaff carts, which act as bulk chaff collection bins, were one of the first HWSC used in Australia. However, management of the large volumes of collected chaff is difficult and adoption rates for this system are low.

Narrow-windrow burning is currently the most commonly used HWSC system in Australia. With this system, a chute is attached to the rear of the combine that concentrates the chaff and straw into a 20- to 24-inch windrow. These windrows are subsequently burned, when environmental conditions are favorable. Research in Australia and Eastern Washington has shown that 99% of annual ryegrass seed in the windrow is destroyed by this method. However, this system has several drawbacks including the destruction of crop residues that are critical for collecting and retaining soil water, reducing soil erosion, and promoting soil health. It also results in smoke, which poses health and safety issues.

The bale direct system uses a large square baler attached directly behind the combine that builds bales from the chaff, which contains the weed seeds, and straw exiting the combine. This system requires available markets for the baled material. There are concerns over removing too much crop residue, which is a source for both carbon and plant nutrients, and spreading weed seed as bales are moved on roads and highways.

The development and integration of impact mills, such as the integrated Harrington Seed Destructor, into the rear of combines, has shown great promise for processing the chaff during harvest to sufficiently control weed seeds. Currently, the cost of purchasing and operating this equipment is prohibitively high for many growers, but as development continues and costs come down, this may be the preferred HWSC system.

Chaff lining in harvester.
Chaff lining, where the chaff and weed seeds are laid down in a narrow line behind the combine. Weed seeds in the chaff struggle to survive in this inhospitable environment.
Chaff tramlining.
Chaff tramlining, where chaff and weed seeds are placed in the wheel track.

During my visit to Australia, I was involved with on-farm research looking at chaff tramlining and chaff lining. These are two low-cost HWCS systems that are rapidly gaining popularity in Australia. Attachments at the rear of the combine collect and place chaff into 10- to 12-inch rows, either in the wheel tracks (chaff tramlining) or between stubble rows directly behind the combine (chaff lining).

Many Australian farmers are using controlled traffic systems where they drive the same wheel tracks for all their field operations. This requires that all their equipment be set-up with the same wheel widths and that all the equipment is of the same width or factor of the same base width, for example, a 30-ft wide combine head and drill, and a 90-ft wide sprayer. Although controlled traffic allows chaff lines to be placed in the same spot every year, it is not critical to the success of chaff lining, although it is required for chaff tramlining.

Concentrating the chaff in narrow rows creates a hostile environment for weed seed germination and emergence. Those weeds that do emerge and grow are concentrated in the narrow rows where they compete with each other, thus reducing seed production, and affect less of the crop area. Subsequent weed control efforts, if required, can be concentrated on these narrow strips in the field. Chaff tramlining places the seed in the wheel rows where the soil is compacted, making growth and development even more difficult.

In my short visit to Australia, Dr. Walsh and I initiated a field study to look at the effect that stripper fronts (a.k.a. stripper headers or Shelbourne stripper headers) on combines have on HWSC. Stripper fronts harvest just the crop heads and pods, leaving tall, standing crop stubble that reduces wind erosion and evaporation from the soil surface, thus increasing soil water storage for subsequent crop use. Stripper fronts are becoming increasingly popular in Australia, but there are concerns that they may not be as effective at collecting weed seeds as conventional or draper fronts.

I would like to continue the research I began in Australia here in Eastern Washington. The weeds of primary interest to me at this time are: Italian ryegrass, downy brome, and feral rye (a.k.a. volunteer or cereal rye).  I am looking for growers that may be interested in making the necessary modifications to their combines to look at the effects of chaff lining on weed control of one of the above weed species. I am interested at looking at conventional platform, draper, and/or stripper fronts.

If this sounds like something you would be interested in, please contact me at or 509-335-2961