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.