GROWERS are urged to ensure grain retained for crop seed this harvest is properly cleaned, with research showing they tend to severely under-estimate weed seed contamination.
A study by the Australian Herbicide Resistance Initiative on 74 farms across the Western Australian grainbelt showed 73% of cleaned crop seed samples had some level of weed seed contamination.
"The research, supported by the Grains Research and Development Corporation, confirmed many farmers unknowingly introduce substantial levels of weed and volunteer crop seeds into their farming systems at seeding time, even when crop seed has been cleaned," AHRI senior researcher Mechelle Owen said.
"Furthermore, the 2010 results showed many of these weed seed populations are resistant to a range of commonly used post-emergent herbicides, indicating that farmers are unknowingly seeding herbicide-resistant weed seeds directly into their paddocks.
"Seeding crop seed contaminated with weed seeds, even at low levels, will have long-term impacts on their weed problems and farm business."
But Ms Owen said cleaned seed - particularly if cleaning was conducted by external contractors - had significantly less weed seed contamination than uncleaned seed.
"Uncleaned crop seed samples had almost 25 times more contamination than cleaned crop seed," she said.
Ms Owen said the type of cleaning method used strongly influenced the contamination level, with the use of a 'gravity table' being the most effective, followed by other methods such as rotary screens and sieves.
Crop type also had a significant effect on the amount of contamination, with wheat containing much higher annual ryegrass seed numbers than barley, possibly because barley was more likely to out-compete weeds during the growing season.
Ms Owen said growers should take heart from the fact that almost a quarter of all the cleaned crop seed samples were weed-free in the study.
"It is more likely that growers can achieve very clean crop seed if - in addition to the seed being properly cleaned - crop seed is obtained from weed-free paddocks and good farm hygiene has been employed," she said.
"Systems promoting farm hygiene such as meticulous seed cleaning, reducing weed burdens in paddocks, crop rotations and sanitising tillage and harvesting equipment between paddocks will help prevent the introduction of new weed species, noxious weeds and herbicide resistance.
"Harvest weed seed management techniques, including windrow burning and the use of chaff carts, will also help to reduce the weed seedbank."
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