From burning to blunt force, weeds hit with grower ingenuity
GroundCover™ Issue: 137 November - December 2018 | Author: Rebecca Jennings
The harvest weed seed control story is one of Australian grower innovation, with the next chapter tipped to be increased adoption of this approach underpinned by research to better understand the effectiveness of different practices
The adage “necessity is the mother of invention” rings true when it comes to harvest weed seed control (HWSC), with Australian growers at the forefront of strategies and technologies developed to combat herbicide resistance.
Weeds carry a hefty price tag – an average $146 per hectare in expenditure and yield losses – according to a GRDC-invested survey. With rising herbicide resistance, it is no surprise more growers are turning to non-chemical controls to target problematic weeds that survive through to harvest.
Australian growers now have a toolbox of techniques to capture weed seed survivors, from collecting chaff for burning or grazing to mechanical impact mills that crush weed seeds.
In 2014, a GRDC-invested survey conducted by CSIRO looked at how growers across the country used HWSC. Some of the technologies were still in their infancy, yet 43 per cent were already using HWSC and 82 per cent indicated an intention to use it within five years.
This was heartening news to Associate Professor Michael Walsh, the director of weed research at the University of Sydney, because it showed the speed with which HWSC was becoming an established weed control practice by Australian growers.
This has proved to be the case with a more recent snapshot of growers gathered from a Twitter survey run by the GRDC-invested WeedSmart project in spring 2017. It revealed similar results and increasing adoption of chaff lining and impact mills.
Peter Newman, of the Australian Herbicide Resistance Initiative (AHRI) and WeedSmart, says 78 per cent of growers surveyed were using HWSC tools in 2016 and 86 per cent reported an intention to use them for the 2017 harvest. Further, only two per cent of growers indicated they did not intend to do any HWSC in the coming three years.
Mr Newman says the results indicate a significant change is underway because HWSC techniques are a very important part of an integrated weed management approach to reduce weed seedbanks. “The survey results reinforce that Australian grain growers are world leaders for managing weeds at harvest. Growers in other parts of the world have been closely watching what has occurred in Australia and are now also investigating how to use HWSC practices,” he says.
Evolution and adoption
The ability to target weed seeds at harvest is possible with chaff dispersal systems on modern headers, which redistribute the major portion of weed seed-bearing material across the paddock.
The arsenal of tools to collect this chaff and its weedy contents was mainly developed by WA growers to fight herbicide-resistant ryegrass. Growers in that state continue to lead the charge, with 63 per cent now practising HWSC.
Outgoing AHRI director Professor Stephen Powles recalls that successful pasture-topping experience meant Australian growers already knew the value of stopping weed seed-set. “In the 1980s, two WA growers imported tow-behind chaff carts from Canada, where they were being used to capture chaff for stockfeed,” he says.
“Chaff cart adoption was limited due to practical limitations with the transfer of chaff to the cart, but this has now been overcome.”
HWSC accelerated when narrow windrow burning began in the mid-1990s as it was low cost and easy to implement with a chute channelling chaff and straw residues into a narrow windrow for burning in autumn.
This relatively simple and inexpensive practice is still effective (AHRI trials have achieved weed seed kill levels of 99 per cent for annual ryegrass and wild radish) and the GRDC survey showed it had been adopted by 30 per cent of growers nationally and half of WA growers.
However, the more recent WeedSmart survey showed narrow windrow burning is declining as growers shift to chaff lining and seed-destroying mills.
Baling harvest residue began in the early 2000s but a lack of suitable markets in Australia has seen limited uptake of this practice with just three per cent of growers using the strategy.
Chaff tramlining and chaff lining allow growers to channel chaff into narrow rows either on the wheel tracks of controlled traffic systems (chaff tramlining) or behind the middle of the harvester (chaff lining).
“These approaches rely on placing weed seed-bearing chaff material into an inhospitable environment that prevents weed seed germination and emergence,” Dr Walsh says.
“The concentration of material produces a mulching effect on weed seeds, creating physical and chemical conditions which reduce emergence.”
Professor Powles tips chaff lining will become the most popular tool in the future as it is inexpensive, easy to implement, does not require burning and is suitable for continuous cropping and mixed farms.
This is supported by the WeedSmart survey, with 27 per cent of respondents planning to use chaff lining and 12 per cent planning to use chaff tramlining within three years.
Growers may be spoilt for choice when it comes to HWSC, but each tool has different economic and agronomic considerations. A guide to the best match for each farming system is available on the WeedSmart website.
Professor Powles has this advice when it comes to using HWSC and herbicides: “When you’re on to a good thing don’t stick with it – diversification is the most important word in farming.
“Change your strategy while it is still working to stay one step in front of resistance.”
He says crop competition tactics such as high seeding rates, narrow row spacing and east-west seeding should be overlaid with other strategies such as break crops, judicious herbicide use and HWSC.
GRDC manager weeds Dr Jason Emms says GRDC is targeting investment into cultural management of weeds, with a new project led by Associate Professor Gurjeet Gill of the University of Adelaide.
“Previous studies on cultural weed management have largely investigated single tactics and not their combinations, so this new project aims to quantify the effect of combinations of crop competition factors, such as sowing time, row spacing and herbicide, on weed seed-set and crop yield,” he says.
The aim is to give growers refined strategies to improve crop competitiveness across different rainfall environments in southern and western regions of Australia.
Dr Jason Emms
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