Australians take charge of herbicide resistance

Herbicide resistance is forcing farmers across the world to change their weed management strategies. According to the International Herbicide-Resistant Weed Database, there are 530 unique cases of herbicide-resistant weeds globally, involving 272 species. Resistance has been reported to 21 of the 31 known herbicide sites of action and to 168 different herbicides.

The bulk of these cases come from the US (132), Australia (89), Canada (56), Brazil (49) and China (48). In 1986, South Africa had one herbicide resistance case, involving Avena fatua (wild oats) in wheat. To date, 14 unique resistant weeds have been reported, involving Lolium spp. (annual ryegrass), Phalaris spp. (canary grass) and Raphanus raphanistrum L (wild radish), to five modes of action. Initially problems in South Africa were restricted to grainlands, but since 2001 it had also been reported on grape and fruit farms.

At a Weed Control Alternatives Workshop hosted by the Western Cape Department of Agriculture at Tygerhoek Research Farm, near Riviersonderend, Dr Michael Walsh, director of weed research at the University of Sydney, Australia, spoke about how weed management was evolving in Australian grain cropping to overcome herbicide resistance problems.

He said 97% of the annual ryegrass populations in Western Australia and roughly 70% of the populations in the eastern parts of the Australian wheat belt were resistant to one or more modes of action. In Western Australia specifically, about 95% of the annual ryegrass populations were multiple resistant.

The situation was more severe in the western parts because they focused on crop production, whereas the eastern parts also incorporated animal concerns.

While it might not have been the sole contributor, Walsh said herbicide resistance became the “final straw” for many growers, resulting in 10 000 of the then 13 000 growers leaving the wheat industry from 1994 to 2006.

Those who stayed had to be highly innovative to address herbicide resistance problems.

Walsh identified harvesting as a weed seed control system. With chaff tramlining and chaff lining for instance, the harvester throws the weed seed containing chaff on to tramlines or in narrow rows, which are then left undisturbed in the hope that the next year’s crop will outcompete the weeds. If not, only the tramlines need to be treated with a herbicide or nonherbicide tactics, such as grazing.

Farmers may also bale the chaff and seed directly if they have a suitable market for it, or narrow windrow burning to destroy weed seeds. Walsh said that burning was highly successful if done right, but presented high risks, so he would not recommend it.

More recently, farmers have also started using impact mills on their harvesters to destroy the weed seed as it moves through the harvester. –

GLOBALLY, THERE ARE 530 UNIQUE CASES OF HERBICIDERESISTANT WEEDS