Lolium multiflorum. Photo: Rasbak. Used under Creative Commons licence.
Combating herbicide resistance in Australia
Last month, chemists from around the world visited Melbourne for the Royal Australian Chemical Institute’s National Convention and the 12th IUPAC (International Union of Pure and Applied Chemists) International Congress of Pesticide Chemistry (4–8 July 2010). Among many topics covered, the emerging problem of resistance to herbicides among weeds occupied several talks.
A big problem
Dr Harry Strek, leader of Bayer CropScience’s integrated weed management and weed resistance biology team in Germany, said that herbicide resistance across most of the major chemical groups was already a big problem that was only going to get bigger. He cited research showing that by 2018, half of all agricultural weed species will be resistant to glyphosate. Glyphosate resistance first appeared in Australia (SESL 2009). There are now significant problems with resistance to “fops and dims” (such as fluazifop and tralkoxydim) (Hashem and Dhammu 2001, O’Donnell undated), sulfonylureas and trifluralin, meaning that Australia has some of the highest instances of weed resistance in the world (Powles 2010).
Dr Strek’s presentation focused on the development of resistance to agricultural chemicals in weed populations around the world. “Weeds present a higher global threat to yield loss than even fungus infections and insect infestations. Inability to control weeds will certainly cause problems for agriculture in the future.”
Dr Strek said that the challenge in Australia will be to persuade farmers and agronomists to change their weed management systems before weed resistance becomes an even larger problem. “To simply respond to ecological threats as we are presented with them is not enough. We need to change our practices now to stop weed resistance from having a potentially devastating effect down the track.”
Dr Stephen Powles, from the University of WA, explained why Australia has the world’s greatest herbicide resistance problem. One reason is the importation of a wide range of ryegrass species and ecotypes from many origins for grazing, thus supplying a wide set of potentially resistant plants to select from across huge areas. More recent cropping in the same areas has subjected these populations to selection pressure from several herbicides applied typically at low rates, which allow escapes.
Herbicide resistance is accelerated by the continued use of the same herbicide in the absence of other tools of integrated weed management such as tillage and crop rotations.
As various speakers reported from different parts of the world, the over-reliance on one herbicide (or a single mode of action) has resulted in the appearance of herbicide-resistant weeds in many countries. This single-chemical practice has been encouraged by the introduction of glyphosate-resistant crops, such as Roundup-Ready maize, soybean and Canola, which can be sprayed with glyphosate but not other herbicides.
Diversify
The solution is to diversify:
- to rotate chemical use from year to year so that weeds that are resistant to one herbicide do not survive a different one
- to rotate crops, as cultural practices that favour a particular weed in one crop will disadvantage the same weed in a different crop
- to cultivate the soil from time to time (favouring minimum-tillage rather than zero-tillage agriculture)
- to use different crop management strategies, such as early or late sowing or fodder making.
“Integrated management practices, including crop rotation, maintaining herbicide diversity, non-chemical soil cultivation and equipment sanitation are key to preventing weed resistance in Australia,” said Dr Strek.
Sources and further reading
Bayer CropScience Australia. 2010. Bayer CropScience identifies future farming challenges at IUPAC. Press release.
Hashem A, Dhammu H. 2001. Resistance in wild oats to fop and dim herbicides in Western Australia. Crop Updates. Agriculture Western Australia.
Hemphill P. 2010. Glyphosate: victim of success. Weekly Times 20 July.
Heard G. 2010. Smart farming essential to manage resistance. Stock & Land 13 July.
O’Donnell C. (undated) Herbicide resistance in the northern grains region of Australia. University of Queensland.
Powles S. 2010. Explanation of why Australia has such a massive herbicide resistance problem. Abstract 82. International Congress of Pesticide Chemistry, Melbourne, 4–8 July.
RACI & IUPAC 2010. International Congress of Pesticide Chemistry, Melbourne, 4–8 July.
Strek HJ. 2010. Weed resistance diagnostics – anatomy of a problem. Abstract 81. International Congress of Pesticide Chemistry, Melbourne, 4–8 July.