Grain harvesters are, without doubt, the best weed spreading tools available. When Australian farmers and researchers began using the harvest operation to collect and destroy weed seed, it kick-started a revolution in weed management that has global implications.
It is fitting that two researchers, Dr Stephen Powles and Dr Michael Walsh, who have ‘been there from the beginning’, should publish a scientific review paper celebrating the success of harvest weed seed control, or HWSC, as a pillar in integrated weed management in Australian grain production systems.
HWSC is useful against a range of important weeds in Australia and overseas, whether resistant to herbicides or not.
HWSC control fits within a diverse weed management strategy. Crop competition and effective herbicides increase the efficacy and value of HWSC.
If weed density is high when HWSC is introduced, it takes time to drive down numbers. The routine use of HWSC in a weed management program ensures weed numbers are maintained at very low densities.
‘Resistance’ to or ‘avoidance’ of HWSC is possible through early seed shedding and low-growing seed heads, but low weed numbers will stave off resistance evolution in weeds targeted with HWSC. There is potential for the weed spectrum to shift toward weeds with more prostrate growth habits.
Low weed density opens the way to site-specific weed control tactics that could eliminate the reliance on field-wide in-crop herbicide use.
The efficacy of HWSC is reliant on collecting and processing a high percentage of weed seeds through the header.
There are six HWSC systems available for Australian growers to use, all of which effectively collect and destroy weed seed that enters the header at harvest.
The WeedSmart Big 6 integrated weed management program culminates with HWSC to keep herbicide resistance low by breaking the lifecycle of weeds that have evaded other control tactics in-crop.
Chafflining is one of the six HWSC tools available.
Experimental design features
Michael Walsh (left), Associate Professor and Director Weed Research at the University of Sydney and Stephen Powles, Emeritus Professor at the University of Western Australia.
This review paper by Dr Stephen Powles and Dr Michael Walsh reflects on 30 years of HWSC research and innovation, driven primarily by the widespread incidence of annual ryegrass populations with resistance to multiple herbicide modes of action in Australia.
Herbicide resistance became evident in annual ryegrass within 10 to 15 years of farmers adopting conservation cropping systems that preserved soil moisture and increased yield through minimal cultivation. There are now approximately 40 weed species confirmed with resistance to one or more herbicide modes of action in Australian cropping systems. These species vary in their susceptibility to HWSC.
The full impact of HWSC takes time
It takes time for the full effect of HWSC treatment to be seen, particularly when this tactic is often not implemented until after herbicide resistance has caused an explosion in weed numbers.
In a 16-year study of 25 continuous cropping fields in Western Australia, all with high annual ryegrass populations (>50 plants/m2), the HWSC + effective herbicides treatment was consistently better at driving down the weed seed bank than herbicides alone.
After eight years, the full benefit of HWSC was clear to see, with annual ryegrass plant densities consistently below 1.0 plant/m2 in the fields where HWSC treatments were included in herbicide-based weed management programs compared to 5 to 10 plants/m2 in fields where herbicides alone were used.
Influences on HWSC efficacy
HWSC relies on weed seed being retained and held at a harvestable height when the crop is ready to harvest. Weed seed retention is particularly variable, even within species, and environmental conditions such as wind, rain and high temperatures have a significant influence.
During the harvest period, several of the major weed species in Australian cropping systems, such as wild oats, brome grass and barley grass, will shed a portion of their seed prior to crop harvest. Studies in Western Australia showed that by the end of the first four weeks of a wheat crop harvest, over 50 per cent of the weed seed from these species had shed.
Numerous Australian studies have demonstrated the immensely positive influence of crop competition on weed seed retention and seed head height. Many weeds elongate in response to shading in a high biomass crop. Studies have repeatedly shown that increased crop competition increases weed seed retention and seed head height and decreases overall weed seed production.
Next step = HWSC + SSWC
When HWSC is included in a diverse weed management program, the result is often very low weed numbers. It is estimated that ryegrass density is less than 1 plant/m2 across much of Australia’s cropping regions due to optimal herbicide use and the implementation of HWSC.
This has opened up the opportunity to reduce the amount of herbicide applied in-crop through the development of site-specific weed control (SSWC) tactics to target individual weeds. This has the potential to save up to 90 per cent of in-crop herbicide use that is currently applied as a blanket spray across the whole field.
Long-term use of HWSC
Theoretically, weed populations can evolve to avoid HWSC. Continual use of HWSC could favour early shedding biotypes and plants that collapse or snap before harvest, placing the seed heads below harvest height. To date, annual ryegrass (the main target weed for HWSC), has shown no evidence of adaptation that has resulted in avoidance of HWSC.
Changes in weed response to HWSC might be inevitable, but low weed density is the best means to reduce the rate of evolution of any type of resistance. A separate challenge may be a shift toward weed species with more prostrate plant architecture, where seed is produced on lateral tillers or branches.
HWSC should never be implemented as a stand-alone weed management tactic. Its effectiveness relies on robust in-crop herbicide use and strong crop competition.
Adoption of HWSC in Australia
Kondinin Group’s 2019 National Agricultural Survey (229 respondents) indicated that HWSC is now implemented on 75 per cent of Australian grain farms (albeit with a higher adoption rate in Western Australia).
With the tactic now proven to be effective for many major weeds in Australia, it is clear that HWSC adoption has followed regional increases in herbicide resistance prevalence.
In 2010 and 2011, researchers compared chaff carts, narrow windrow burning and impact mills (HSD) at 25 sites in Western Australia, South Australia, Victoria and New South Wales. They found that the three HWSC treatments were similarly effective in reducing annual ryegrass emergence in the following season by 60 per cent, compared with the no-HWSC treatment. These trials also showed that where the weed seed bank is high, HWSC only reduced annual ryegrass emergence by 30 per cent. In contrast, they observed a 90 per cent reduction in emergence when seed bank densities were low.
Potential for HWSC overseas
Having noted the value of HWSC in managing herbicide resistant weeds, and the commercial development of a range of HWSC tools, there is interest in adopting this tactic in cropping systems worldwide.
Significant weeds such as Palmer amaranth (Amaranthus palmeri), water hemp (Amaranthus tuberculatus), annual ragweed (Ambrosia artemisiifolia L.), Italian ryegrass (Lolium perenne subsp. multiflorum), charlock (Sinapis arvensis) and chickweed (Stellaria media) retain 50 to 99 per cent of the seed produced at the time of crop harvest. Several HWSC tools have been evaluated overseas, mainly in the USA and Canada, and shown to be effective against key weed species.
Further development and innovation may be required to overcome challenges presented due to different crop types, harvest environments, environmental regulations and machinery in cropping systems around the globe.
This review paper was written by Michael Walsh, Associate Professor and Director Weed Research at the University of Sydney and Stephen Powles, Emeritus Professor at the University of Western Australia. Both the University of Sydney and the University of Western Australia are WeedSmart research partners.
Most of the research reviewed in this manuscript was conducted with investment from the Grains Research and Development Corporation, a WeedSmart platinum sponsor.
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