Seed & Chemical
Australia’s fight against herbicide resistance
By Donna Fleury
Three prototype HSD systems being tested. Photo by Michael Walsh.
In Australia, the development of multiple herbicide resistance in some of the most serious annual weeds has been the catalyst for the development of new agronomic practices. Researchers and industry have developed new non-chemical weed control techniques focused on weed seed capture and destruction during commercial grain crop harvest.
“Herbicide resistance in problematic weeds is extensive across the Australian crop production zone,” explains Dr. Michael Walsh, research associate professor at the University of Western Australia, in Crawley, Western Australia. “It is particularly severe across the western Australian wheat production region (10 million hectares) where 98 per cent of annual ryegrass populations are resistant to at least one mode of action herbicide.” The majority of populations are now multi-resistant (i.e., have multiple resistance mechanisms), with the resistance problem consistently severe across all cropping systems and crop types.
The biggest problem weeds infesting Australian cropping fields are annual ryegrass, wild radish, wild oats and brome grass. Walsh explains that these annual species all have high genetic diversity, boast prolific seed production, can establish high population densities and have relatively short-lived seed banks. They also retain a significant portion of their seeds at maturity, meaning that many seeds remain attached to the upright plant and are collected during the grain crop harvest. Walsh and his colleagues have developed alternative weed control strategies or harvest weed seed control (HWSC) systems used during commercial grain harvest operations to minimize fresh seed inputs to the seedbank and lower overall weed populations.
“The clear message now emerging from our research is that all feasible and practical means need to be used to drive weed populations to the lowest possible levels in crop production fields,” explains Walsh. “Very low weed populations are not just about avoiding or managing herbicide resistance, but more about improved crop production systems. When weeds are not dictating the cropping practices, the production system becomes much more flexible and profitable. More specifically though, we have learned that adding HWSC at the end of the growing season to target weed seeds perfectly complements herbicide-based weed control programs to deliver very low crop-weed densities.”
HWSC systems significantly reduce weed seed
Walsh and his team have developed and tested HWSC systems in Australia including narrow-windrow burning, chaff carts, bale direct and the Harrington Seed Destructor. These HWSC systems target the weed seed bearing chaff material during commercial grain harvest. The research program, part of the Australian Herbicide Resistance Initiative (AHRI), also provides growers with best practices for adopting and implementing these systems (http://www.ahri.uwa.edu.au).
Narrow-windrow burning is currently the most widely adopted HWSC system in Australia and is used by about 70 per cent of crop producers in Western Australia. This simple, effective and inexpensive system uses a grain harvester mounted chute to concentrate all of the chaff and straw residues into a narrow-windrow (500 to 600 millimetres, or 20 to 24 inches). “These narrow windrows are burned after harvest, with weed seed kill levels averaging 70 to 80 per cent and as high as 99 per cent for both annual ryegrass and wild radish in wheat, canola and lupin chaff, and straw residues,” says Walsh. “Narrow windrows are ideal because they burn hotter and longer, killing the weed seeds and minimizing the area burned, which keeps residue on the fields to minimize erosion risk.”
Chaff cart systems consist of a chaff collection and transfer mechanism attached to a grain harvester that delivers the weed seed bearing chaff fraction into a bulk collection bin. The collected chaff must be managed properly to prevent returning the weed seeds to the field. The chaff is usually dumped in heaps in a line across fields to be burned or used for livestock feed. A Bale Direct System consists of a large square baler directly attached to the harvester that constructs bales from the chaff and straw residues exiting the grain harvester. Although both are efficient systems, the post-harvest management requirement for chaff and the lack of markets for baled materials has currently limited the adoption of these systems.
The Harrington Seed Destructor (HSD) was developed in 2007 by an innovative Australian crop producer, Ray Harrington, as a system to process the weed-seed bearing chaff during the harvest operation. The HSD technology went into commercial production in 2012 and comprises a trailer-mounted cage mill, with chaff and straw transfer systems, and a diesel motor as a power source that is hooked to the rear of the combine. Evaluation of this system under commercial harvest conditions by AHRI over a number of seasons determined that HSD destroyed at least 95 per cent of annual weed seeds during harvest. The cost of purchasing an HSD system is approximately $240,000 (AUD).
“We have established estimated costs for these systems here in Australia; however, they may not necessarily be the same in other countries such as Canada because of the differences in cropping systems and production capacities,” explains Walsh. Based on a typical 4,000 ha cropping program in Australia, the costs for using HWSC systems per ha are roughly as follows (these numbers do not include the cost of nutrient removal):
- Narrow-windrow burning $2/ha
- Chaff cart $6/ha
- Bale Direct $18/ha
- HSD $16/ha
Research results confirm value of HWSC
Peter Newman, an AHRI colleague, evaluated the combined impact of herbicides plus HWSC over 10 consecutive seasons from 2002 to 2012, and found that targeted low weed densities were only achieved in fields where both early-season herbicides and HWSC were routinely practised. The research, conducted on fields where annual ryegrass densities were very high (35 to 50 plants per square metre), compared trials with herbicide treatments alone and trials with both in-crop herbicide treatments and late-season HWSC treatments. The goal was to reduce annual ryegrass populations to less than one plant per square metre. The annual ryegrass populations in the study were not herbicide resistant to the herbicides used in these studies.
As expected, effective herbicide treatments reduced in-crop annual ryegrass populations to less than 10 plants per square metre within five consecutive growing seasons, with populations averaging four plants per square metre for the rest of the study. The combined treatments of early-season herbicides and HWSC reduced annual ryegrass populations from an average of 35 plants per square metre in 2002 to 0.5 plants per square metre in 2011.
“Our research results confirm that the real value of HWSC systems is as part of a system that includes early-season weed control practices on weed seedlings, such as herbicides, and HWSC on late-season mature seed-bearing weeds to lower weed populations and minimize seedbank contributions,” says Walsh. “Low weed densities in cropping systems not only provide flexibility in crop choice, seeding time and herbicide use, they also play a critical role in sustaining herbicide resources for the ongoing control of crop weeds.
“Restricting weed population densities to very low levels also reduces the potential for resistance evolution to our remaining highly valued herbicide resources,” he adds. “Herbicide preservation is essential for sustaining future crop production so the addition of HWSC and other control strategies is absolutely necessary in supporting the ongoing efficacy of herbicides.”
Walsh says he believes the HWSC systems have potential as a new non-chemical weed control tool not only in Australia, but also in other major crop producing countries with similar crop weed populations, such as Canada, the U.S., Spain, Italy and Argentina. “The HWSC system is a tool to help achieve herbicide sustainability, to improve diversity and to help avoid exclusive reliance on herbicides for weed control,” he notes.
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