Australia
March 19, 2012
Western Australian research has shown there is a strong link between intensive cropping, annual ryegrass ‘dormancy’ and herbicide resistance in the state’s grainbelt.
The research was conducted by the Australian Herbicide Resistance Initiative (AHRI), based at The University of WA (UWA), and funded by the Grains Research and Development Corporation (GRDC).
Ryegrass seeds in the petri-dish on the left are germinating normally, while seeds in the other petri-dish are dormant, with only a few seeds able to germinate. Ryegrass seeds outside the petri-dishes have not been exposed to moisture.
Dormancy refers to the situation where viable seed does not germinate under ideal germination conditions.
The research findings suggest growers should, where possible, delay seeding crop paddocks where there is dormant ryegrass, so they can kill the maximum number of weeds using knockdown herbicides.
This is because dormant ryegrass is more likely to be resistant to selective in-crop herbicides, and therefore more difficult to control when it emerges later in the crop.
AHRI researcher Mechelle Owen said that although other studies had already found evidence of a correlation between dormancy and resistance in annual ryegrass, the AHRI research was the first large scale study to demonstrate the link.
AHRI researcher Mechelle Owen inspects petri-dishes containing dormant ryegrass seeds, left, and ryegrass seeds which are germinating normally. Ryegrass seeds outside the petri-dishes have not been exposed to moisture.
She said the initial correlation was established by studying ryegrass seeds collected from 406 populations across the WA grainbelt.
“For each population, initial dormancy and change in dormancy over a six-month period were measured, and the resistance status of seedlings to four herbicides was assessed,” Ms Owen said.
“Herbicides used were Hoegrass®, Sertin®, Select® and Oust®, which are Group A and B herbicides.
“Greater seed dormancy correlated with higher levels of herbicide resistance for all four herbicides tested.”
Ms Owen stressed that ryegrass resistance did not itself cause dormancy, and that crop management factors may be selecting for both traits independently.
“Further research we conducted examined the causes of the relationship between resistance and dormancy, and found that higher dormancy levels were associated with more intensive cropping,” she said.
“This research used seed from populations collected in 2009 from fields across the WA grainbelt, including paddocks with long-term cropping programs and those which had never been cropped.
“The study considered a number of factors including cropping history, dormancy level and resistance data.
“Generally, ryegrass from paddocks with a longer and more frequent cropping history was slower to germinate and had a lower proportion of seeds that were able to germinate.”
Ms Owen said resistant, dormant ryegrass seeds posed a significant problem to growers.
“Intensive cropping programs are likely to select for increased dormancy when knockdown herbicides kill off the proportion of seed which is able to germinate with the opening rains, leaving the more dormant proportion of the population to germinate later in the season, usually in the crop,” she said.
“There are limited opportunities to achieve a knockdown of weeds before seeding, as many seeds have not yet germinated.
“And because later germinating seeds then have increased exposure to selective herbicides, this leads to greater resistance and a seedbank dominated by dormant, resistant ryegrass.”
Ms Owen said the research provided more evidence of the need to consider the implications of applying the same crop management practices year-after-year.
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