BARLEY grass often slips under the radar and pales in significance to annual ryegrass, however it is ranked in the top 10 weeds in Australian grain crops.
According to research from the University of Adelaide, South Australia, large differences in seed dormancy between barley grass populations has led to plants escaping control and causing seedbank replenishment late in the season.
In 2018, the university collected barley grass seed from 143 random populations from farms in the low rainfall zones of Western Australia, New South Wales, Victoria and South Australia.
Led by associate professor Gurjeet Gill, in 2019 the research team identified two barley grass populations with contrasting seed dormancy from the Upper Eyre Peninsula.
According to WeedSmart, these investigations into the role seed dormancy plays in weeds evading herbicide control have provided valuable insights into the management of barley grass.
Practices such as cropping intensity and sowing time can impose huge selection pressure on seed dormancy and shift the time of seedling emergence of weed populations.
Such high dormancy populations are more difficult to manage than those that emerge as a single flush after the opening rains.
With investment from the Grains Research and Development Corporation (GRDC), Dr Gill and weeds researcher Ben Fleet have shown effective barley grass control requires a two-year, multi-pronged strategy.
"Weed populations with high seed dormancy will have a slow and staggered weed establishment, which will reduce the effectiveness of pre-sowing knockdown herbicides, and dormant barley grass may also evade some pre-emergent herbicides," Dr Gill said.
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The effect of cropping intensity on weed seed dormancy has been studied extensively for more than ten years and it is now well-established that higher cropping intensities select for greater weed seed dormancy.
Dr Gill said the two barley grass populations with contrasting seed dormancy from Upper Eyre Peninsula collected in 2019 came from two paddocks - one with a low cropping frequency and one with a high cropping frequency.
"The difference in time required for 50 per cent seed germination between the two populations was more than three-fold - 13 days compared to 46 days," he said,
"In the random survey of barley grass across the low rainfall zones, seeds collected from the southern plains of NSW exhibited the lowest dormancy, emerging in an average of 8.9 days.
"The average time to emergence for Western Australian populations ranged from 13.8 to 17.7 days."
Barley grass seed which is still dormant at the time of crop planting will not be a target for pre-sowing knockdowns and depending on the pre-emergent herbicides applied, the seed may germinate after the herbicide has broken down.
When it comes to crop competition, growers should use effective pre-emergent herbicides such as Sakura in wheat, Clearfield crops using Group 2 (B) herbicides and stop seed set.
"Barley grass that germinates several weeks after the crop should be at a distinct disadvantage to the crop, resulting in lower weed biomass and seed production," Dr Gill said.
"However, if this seed finds itself in a poorly competitive crop there is an opportunity for significant seed bank replenishment.
"This is compounded if the dormant barley grass seed is also resistant to Group 1 (A) and 2 (B) herbicides, as detected in low levels in WA, SA and Victoria."
Higher crop seeding rate and altering the time of sowing have been consistently successful in combating weed seed dormancy and provide a good place to start.
Barley grass seed can remain viable for up to two years.
In WA, most barley grass seedlings emerged within a year, however some barley grass establishment was observed in years three and four.
"In paddocks with increasing numbers of late-germinating barley grass plants, implement a two-year Big 6 management program," Dr Gill said.
"That means incorporating competitive crops, alternative sowing dates, herbicide-tolerant break crops, effective herbicides and consider a winter clean with pasture or fallow, to target barley grass."
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