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Sowthistle showing signs of resistance

Sowthistle is the newest problem weed to show signs of glyphosate resistance in the north.

Trials currently underway to determine if the resistance exists and if so, at what level, have returned worrying results, according to NSW Department of Primary Industries technical specialist weeds officer Tony Cook.

“Individual plants within the suspect populations survived the glyphosate much better than they should have and within two or three weeks some plants were starting to regenerate again,” Mr Cook said.

The trials investigated five different populations, all treated with two robust rates of glyphosate and applied to two different sowthistle growth stages.

“More work is being done to determine whether it is in fact resistance, and if so what level, but it is a timely reminder to maintain diversity when undertaking weed control and to closely monitor weeds after sprays to detect and control survivors,” Mr Cook said.

Department of Agriculture, Fisheries and Forestry Queensland (DAFFQ) research scientist Michael Widderick recently conducted trials into the best ways of tackling the weed and found the double knock treatment to be the most effective.

“The double knock achieved 100 per cent control of sowthistle regardless of the weed’s size at the time of spraying, while many stand-alone treatments had a limited impact,” Dr Widderick said.

The trials, conducted on the eastern Darling Downs near Cecil Plains in April this year, consisted of 27 different applications of either herbicides applied alone, in glyphosate mixture or as part of a double knock applied seven days after the first treatment.

Dr Widderick said stand-alone treatments had limited impact, with the exception of glyphosate alone on small weeds which was effective, however could give rise to a whole new raft of problems.

“An over-reliance on stand-alone glyphosate treatment may result in widespread cases of glyphosate-resistance in this weed, especially in light of the current testing taking place,” Dr Widderick said.

The Grains Research and Development Corporation (GRDC) is supporting the adoption of integrated weed management strategies to combat herbicide resistance and is encouraging farmers to test suspect weed populations.

Dr Widderick said sowthistle had always been considered relatively easy to control, yet it was still widespread because survivors could set large numbers of viable seed.

“Our weed control efforts are just not effective enough. If weeds, even at low density, are allowed to set seed, the efforts and expense in controlling the majority of the population are undone,” Dr Widderick said.

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