Diversity key to managing multiple resistance in weeds

Multiple herbicide resistance in weeds poses a bigger problem than glyphosate resistance in Australian and global cropping systems, and the only way forward is diversity in weed control practices.

Originally an Australian researcher, Dr Heap is now based in Oregon, United States, where he accumulates information on herbicide resistant weeds from scientists around the world.

“There are more than 60 countries where scientists are reporting herbicide resistant weeds and lately we have seen a big problem with weeds resistant to many different types of herbicides,” he said.

Dr Heap said a focus of discussion at the conference was resistance in weeds to glyphosate, the most important knockdown herbicide.
He said glyphosate resistance was a particularly big problem in countries such as the US where there had been rapid adoption of glyphosate tolerant crops since the 1990s.

“But I think the biggest issue going forward is: ‘What will we do with weeds that evolve resistance to many different herbicides?’” Dr Heap said.

“To manage this problem we have to use alternative means of weed control, and reduce our reliance on herbicides.”

Dr Heap said the problem of multiple herbicide resistance in weeds was compounded by a lack of discovery of new herbicides with alternative modes, or sites of action (MOA) in recent years.

He said paddocks containing more than one herbicide resistant weed posed effectively the same problem to growers as multiple resistance in one weed.

Dr Heap believed the issue of multiple resistance would force changes in cropping systems in the coming decade including the development of crops tolerant to more than one herbicide.

“Although a short-term solution, I think these crops with multiple stacked traits will play a big role in helping growers manage weeds which are resistant to more than one herbicide,” he said.

Dr Heap believed other medium-term changes would include re-education of farmers on weed control; diversification of cropping systems; the delivery of some new herbicides with alternative MOAs; adoption of resistance management strategies; increased tillage and cover crops; and zero tolerance for weed escapes in some crops.

“Beyond 2025 I think it is possible we could see the uptake of true integrated weed management systems, and that quite a few more herbicides with new MOAs could be released, possibly from China,” he said.

“Until these new herbicides come through, we need to maintain the effectiveness of existing chemicals by practicing integrated weed management.”

To avoid the development of herbicide resistant weeds, Dr Heap said farmers should:

•    Rotate herbicide MOAs;
•    Use full rates;
•    Tank mix herbicides with different MOAs (both herbicides must be active on the same target species);
•    Use pre-emergent herbicides in addition to post-emergent herbicides;
•    Use non-herbicide weed control where economical.

More information about Australian research programs targeting herbicide resistance and resistance management to encourage sustainable cropping systems can be found at the Australian Herbicide Resistance Initiative (AHRI) website www.ahri.uwa.edu.au

Australian growers can obtain regionally-specific information about weed control from the WeedSmart campaign hub at www.weedsmart.org.au

Launched at the Global Herbicide Resistance Challenge, the WeedSmart industry initiative provides a practical and innovative toolkit of resources for advisers and growers to keep weeds – and herbicide resistance – at bay.

WeedSmart involves the commitment of research and development organisations including the GRDC, advisers and agronomists, chemical companies, agribusiness and grower representative bodies.

The Global Herbicide Resistance Challenge held in Fremantle in January was hosted by AHRI and attended by 300 delegates, including 150 scientists, from 32 countries.

Major issues discussed at the multidisciplinary research conference included the threat of herbicide resistance and its impact on global grain production, alternatives to chemical weed control and the latest gene modification advances.