Don't get hooked on herbicides

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Are Australian graingrowers hooked on herbicides? Professor Stephen Powles fears they might be heading that way. Director of the Western Australian Herbicide Resistance Initiative (WA HRI ) and an international authority on herbicide resistance, he says Australian farmers must not become "herbicide junkies" and must wean themselves away from over-reliance on chemical weed control, to preserve the effectiveness of herbicides.

"Herbicide-resistant ryegrass is now everywhere in the Western Australian wheatbelt and is increasing across the country," Professor Powles says. "Recently, the first ryegrass populations resistant to glyphosate became evident. Plants don't easily develop resistance to herbicides like glyphosate, which has worked so well for 20 years and is still invaluable.

"Growers can diversify and use alternatives - paraquat and Liberty® for example - but often there is overuse of what is simply the world's best herbicide. "If a new, magical herbicide that would kill ryegrass and not damage wheat was discovered tomorrow, and was used on every paddock across the country, it would still be a case of when, not if, resistance developed."

It is a warning that should be taken seriously. Professor Powles, a former director of the Cooperative Research Centre for Australian Weed Management, says failure to diversify herbicide applications with non-herbicide weed management strategies threatens the long-term effectiveness of invaluable products such as glyphosate.

The reality is that there are very few alternative herbicides on the horizon, although the industry remains optimistic something new will be developed: a few companies are looking for new herbicides, but the search is difficult and expensive.

Agriculture has to accept that there is not going to be a magical solution. The only way forward is to reduce reliance on herbicide controls and to incorporate any non-herbicide tools that can provide economic weed control.

This is a reality that farmers in general will not like much and, just as doctors could not imagine doing their jobs without access to antibiotics, many crop farmers will find it hard to imagine controlling weeds without herbicides. But in both situations, overuse threatens the long-term effectiveness of the chemicals.

Professor Powles says most Australian graingrowers now have experience and understanding of herbicide resistance in weeds. Many growers would remember when Hoegrass® and Glean® easily cleaned up ryegrass, but now in WA , these products are no longer effective against weeds in 88 per cent of that state's crop paddocks.

All professional growers in Australia are well aware of resistance, but the degree to which they are actively managing it depends on whether they have a major problem.

"In general, grower experience of the resistance problem goes from west to east across Australia, as more growers have problems in the west," Professor Powles says. "For the moment resistance problems lessen as you move east. There are lessons to be learned from that."

Professor Powles says the key to maximum sustainability in conservation farming - and the maximum continuation of essential systems like notill - is diversity of weed control methods, always with economic reality in mind. That means using a range of tools like high seeding rates, diverse cropping rotations, crop topping - which is used a great deal in the west - chaff carts, mechanical diversity and narrow windrows which could be burned. Growers could increase seeding rates and ensure strong, healthy crops to achieve maximum suppression of weeds.

In northern NSW and Queensland, alternating winter and summer cropping is an example of diversity that can be used to reduce the rate of resistance developing.

There is also the option of delayed seeding, which in many years - and in different parts of Australia - would allow weeds to be controlled by knockdown herbicides or by cultivation.

"I am a great believer in no-till, but I think even its best practitioners probably need tillage every now and then," Professor Powles says. "None of the alternative weed control methods are as good as herbicides but they will help reduce the threat of resistance.

"I am not saying don't use herbicides, but we must use them sustainably. We do have to change our approach to weed control but moving to less reliance on herbicides is not the end of the world.

"My view is that when farmers choose to use a herbicide, they should use it at the recommended rate - not cutting rates - to get high weed kill, and use that herbicide as infrequently as possible. Rotate and diversify as many things as possible to ensure the herbicide will work for a long time on your farm."

GRDC Research Code UWA399
More information: Professor Stephen Powles, 08 6488 7833, 0418 927 181, spowles@plants.uwa.edu.au

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