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Resistance risk to eliminate herbicides on irrigated farms

Herbicide resistance is frequently identified first along fencelines, roadways and irrigation channels where herbicide use tends to be the same year in year out and often, less attention is paid to herbicide efficacy or survivor weeds.

This can be a high risk practice unless survivors are removed after every spray application as there is no crop competition to restrict weed growth, resulting in production of large volumes of seed.

Eric Koetz, NSW DPI weeds research agronomist, said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems.

Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems.

“A recent survey of cotton fields in Queensland and NSW had shown that cotton paddocks were generally relatively weed-free however the incidence of resistance to glyphosate is quite high in fleabane and windmill grass samples collected mainly from non-crop areas on cotton farms,” he said.

“Preliminary results from the samples collected across cotton farming systems in 2016–17 show 95 per cent of fleabane samples, 60 per cent of feathertop Rhodes grass samples, 80–90 per cent of windmill grass samples and 20 per cent of sowthistle samples tested as resistant to glyphosate.”

“Keep in mind that flaxleaf fleabane, feathertop Rhodes grass and windmill grass are not registered for control with Roundup Ready Herbicide with PLANTSHIELD as control of susceptible plants is generally poor, so further selection for resistance only exacerbates the problem,” he said. “Although still lower, the level of resistance emerging in sowthistle is very concerning, given that this species is listed on the herbicide label. According to a recent attitudinal study by Monsanto, growers also identify annual ryegrass and barnyard grass as showing signs of glyphosate resistance in the field.”

To date there have been no recorded cases of paraquat resistance in weeds on cotton farms, however with rising glyphosate resistance, and increased use of paraquat products, there is a high risk that paraquat resistance will also be found, leaving growers with few options to control these weeds. Mr Koetz now has additional funding available to test the seed collected in the survey for resistance to paraquat and diquat products – Gramoxone and Spray.Seed.

Mr Koetz said the lack of diversity in herbicide use in many cotton systems is likely to contribute to the increased incidence of herbicide resistance. “The attitudinal study by Monsanto indicates that less than 50 per cent of growers are applying a pre-emergent herbicide and only 25 per cent of growers apply a post-emergent herbicide in addition to their applications of glyphosate in cotton,” he said. “The label for Roundup Ready Herbicide with PLANTSHIELD states that this product must not be the only form of weed control used in Roundup Ready Flex cotton varieties.”

“In the next 5 to 10 years there will need to be a shift towards non-herbicide controls such robotic cultivation and microwave technologies, which are well suited to summer cropping on rows or beds,” he said. “Until then, optical spray technology is a good option for growers to keep weed numbers low in the fallow.”

Spray drift of Group I herbicides (e.g. 2,4-D products) late in the summer fallow to control large fleabane is causing considerable damage to cotton crops. Mr Koetz recommends growers change their fallow weed management program to target small plants earlier in the spring, before cotton emergence, using a double knock of glyphosate followed with cultivation or paraquat plus a residual herbicide as the second knock.

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