Herbicide resistance threat increases
Farmers in Western Australia are facing the threat of weed resistance to one of the most popular herbicides - glyphosate - say researchers.
The Western Australian Herbicide Resistance Initiative (WAHRI) at the University of Western Australia has found that glyphosate use by farmers in the northern wheatbelt is reaching the level at which resistance may develop.
Glyphosate is a herbicide commonly used to kill ryegrass by spraying fields before seeding occurs. In Western Australia, this is usually two weeks after the first autumnal rains - in July or August - when the ryegrass weed begins to sprout.
"One of the benefits of glyphosate is it is broken down immediately," explained Dr Paul Neve, a research associate with the WAHRI, who specialises in herbicide resistance. "Anything that is growing when it is sprayed will be killed, but it doesn't kill anything after that."
The problem, according to Dr Neve is there is a gradual buildup of resistance to the herbicide. "It's a similar situation to antibiotic resistance," he explained. "Changes in the ryegrass genetic makeup means they are evolving to become resistant."
Dr Neve is recommending that farmers try to avoid having resistant ryegrass in the first place. "If we stop using [glyphosate], the proportion of resistant individual ryegrasses will decline," he said. "But they won't disappear over five years."
Glyphosate is so widely used because it is highly effective, said Dr Neve, but there are alternatives. A chemical called paraquat has the same effect of killing weeds at the same time of year, he said. "But currently 90 per cent of herbicide used is glyphosate with the rest being paraquat."
He is not recommending that farmers abandon glyphosate use, but for them to be aware of the risks. "Instead of using glyphosate every year, at least alternate between the two," he suggests.
Glyphosate is the chemical name for Round-Up, which has recently come off its patent. "So now other companies are making it," said Dr Neve.
The first case of glyphosate resistant ryegrass in WA was confirmed late last year in the northern wheatbelt.
There are eleven identified resistant ryegrass populations Australia wide - in South Australia, Western Australia and New South Wales, where the first case was recorded in 1996. "It's really distributed across the southern agricultural region," observed Dr Neve.
A report that appeared in theSouth Australian Stock Journal in December showed detectable levels of glyphosate resistance in 50 per cent of the 40 ryegrass populations tested.
80 per cent of the paddocks the ryegrass had been collected from had been treated with glyphosate for at least 15 years. In recent years the amount of glyphosate used in the paddocks has doubled to 1.5L/ha.
The study found that resistance to the herbicide began some time between the 15th and the 20th application of the herbicide.
WAHRI researchers have found that farmers in the Western Australian wheatbelt have, on average, sprayed glyphosate 11 times to date. This means the next few years are crucial in the containment of ryegrass resistance.
Annual ryegrass is distinct from perennial ryegrass, used in cooler areas such as Victoria as pasture for cattle. There are two types of annual ryegrass that interfere with grain crops - Italian Ryegrass and Rigid Ryegrass. "But they are similar enough to be the same," explained Dr Neve. "They interbreed, so as far as the farmer is concerned, they are the same thing."