Herbicide resistance found on 48% of Canterbury cereal farms - Stuff.co.nz

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A combine harvester offloads grain in mid-Canterbury. Herbicide resistance is higher than expected in the region.

Weeds with resistance to herbicides are much more common than expected, a newly published study shows.

Resistant weeds were found on 48 per cent of Canterbury wheat and barley farms, the study led by Dr Chris Buddenhagen​ of AgResearch​ found.

Buddenhagen and nine colleagues had expected that five per cent to 10 per cent of these farms would contain resistant weeds when they applied for research funding.

"We were surprised it was that high," Buddenhagen said.

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Weeds grow quickly and spread vigorously, either by seed or rhizomes and underground roots.

Herbicides were not causing the resistance per se, he said. Rather, the "genetic mutations that confer resistance occur rarely in some individual plants in a population. For example, resistance mutations have been found in herbarium specimens from the 1700s, prior to the advent of herbicides".

When farmers sprayed a field for a specific weed, they were "selecting the resistant individuals in the population and killing everything else. All you're left with is the resistant individuals", he explained.

The surviving individuals bred and their offspring carried the mutation on. Those resistant weeds can be killed by alternative weed control measures employed by farmers – including the use of another class of herbicide.

Some farms in the study had severe problems, while others had none of the weeds present at all. Resistance on a farm was a like lottery.

"This study is the first random survey carried out in New Zealand to detect herbicide resistance for a range of arable weeds and estimate its prevalence on wheat and barley farms," wrote the authors in the open-access journal Plos One.​

Buddenhagen and colleagues collected seed samples from common weeds from 87 Canterbury wheat and barley farms just before harvest. They raised more than 40,000 seedlings in glasshouse conditions.

With appropriate controls, the seedlings were sprayed with three common herbicide classes – known in simplified terms as ACCase,​ ALS​ and EPSPS,​ which is basically glyphosate.

Those plants that still lived two or three weeks later were determined to have herbicide resistance.

"In the random survey, some form of resistance … was detected on 42 farms (48 per cent of those surveyed, with 95 per cent confidence," the authors found. "Resistance was found for ALS-inhibitors on 35 farms (40 per cent) and to ACCase-inhibitors on 20 (23 per cent) farms."

No cases of glyphosate-resistant weeds were detected in the random survey, but one was found in a weed supplied by seed industry representatives.

The 87 farms surveyed represented about 20 per cent of wheat and barley farms in Canterbury. The 48 per cent figure was low compared with some areas of Australia, where resistance was closer to 80 per cent, Buddenhagen said.

Kiwi farmers were increasingly aware of these problems and groups such as Foundation for Arable Research​ were encouraging farmers to discuss problems, ideas and solutions. "They're now able to talk about these issues a little bit more openly," said Buddenhagen.

Farmers need to rotate between herbicide classes (not just herbicide brands), while also rotating crops, resting fields, tilling in weeds and the like. "There's always strategies … to reduce the amount of herbicide-resistant seedlings in your paddock," he said.

Farmers needed to be alert to what was happening in their fields.

The study suggested that weed resistance was historically under-reported by farmers and the chemical industry and under-investigated by scientists, the authors wrote.

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