THE DETECTION of glyphosate resistance in ryegrass on some SA farms during 2001 has prompted a warning about the risks of over-reliance on this popular herbicide.
According to John Matthews of RESITECH Herbicide Resistance Testing Service, there is potential for rapid development of widespread glyphosate resistance if preseeding ryegrass control, or suppression of ryegrass seed set, is completely dependent on glyphosate.
He recently tested 40 ryegrass populations from selected SA farms and found that 50 per cent of these had a low level of plants that survived the recommended field application rate for glyphosate. Until this analysis, glyphosate resistance in ryegrass had been confirmed only on a handful of properties in Australia, mainly horticultural enterprises.
"This new find reinforces previous surveys in SA by Chris Preston of the CRC for Australian Weed Management and it shows widespread potential for resistance in annual ryegrass populations, all from annual cropping systems," Dr Matthews said.
The seed was germinated and plants at the two- to three-leaf stage were sprayed at 0.75, 1.5 and 3 L of 540 GM/L glyphosate equivalent per hectare. Herbicides were applied in 120 L of water/ha. All survivors were resprayed six weeks after the initial treatment at 0.75 L/ha on the lowest initial rate and 1 L/ha on the two highest rates of 540 gm/L equivalent.
Dr Matthews provided the following summary of results.
- More than 75 per cent of the 40 populations submitted as being resistant to other herbicides had survivors to the 0.75 L/ha glyphosate treatment; 50 per cent had a low level of survivors at or above the recommended field application rate for glyphosate of 1.5 L/ha.
- The populations surviving glyphosate resistance all had some resistance to the 'fop' herbicides from Group A but only 10 per cent had resistance to Group A 'dims'.
- Most populations had resistance to the Group B herbicides, usually at low to moderate levels.
Dr Matthews said resistant ryegrass populations were mainly from the SA mid-north region but farms on Yorke Peninsula and in the South East were also represented.
In a subsequent survey, 80 per cent of the farmers owning properties from which the ryegrass seed had been collected said they had been using glyphosate for at least 15 years, while the rest had used it for between 10 and 15 years.
"In all cases application rates have risen from about 0.75 L/ha to 1.5 L/ha in recent years," Dr Matthews said. "Most of the paddocks were under continuous glyphosate use - at least once a year.
"The extent of resistance encountered in this survey is high and should serve as a warning to farmers that resistance may be more widespread than first thought and that there is potential for the rapid development of widespread glyphosate resistance.
"All of the populations that had survivors to glyphosate exhibited resistance to some other ryegrass herbicide. As other herbicides have failed or continue to fail, there may be an increased selection for glyphosate resistance as glyphosate survivors will not be killed by post-emergent products."
Farmers in the position to forestall resistance should ensure that water quality, or mixtures of other herbicides, or inadequate rates were not reducing glyphosate efficacy.
Non-chemical ryegrass control methods are also an important part of the armoury.
Program 3 Contact: Dr John Matthews 08 8303 7734 or 0419 865 824