Herbicide resistance research

Trials funded by the Grains Research and Development Corporation have shed more light on how herbicide resistance can rapidly develop in weeds such as ryegrass when low herbicide rates are used repeatedly.

The field trials have been conducted by Western Australian Herbicide Resistance Initiative researchers, including post-doctoral fellow Roberto Busi, PhD student Sudheesh Manalil and Prof Stephen Powles.

The study demonstrates, for the first time in the field in a commercial wheat crop, the potential for reduced rates of herbicides to result in rapid resistance evolution, Dr Busi said.

When herbicides are used at low rates, it can rapidly lead to resistance and also cross-resistance, which may become a serious problem, depending on the herbicide used.

Dr Busi said the research had led to improved knowledge of the evolutionary process leading to herbicide resistance when sub-lethal rates were used.

This knowledge will be used to help prevent, or minimise, resistance and cross-resistance development in the future when new chemicals hit the market, he said.

The research is important, given herbicide rates used in Australia are very low by world standards, and there has been a culture of rate-cutting below the registered label rate.

WAHRI researchers believe the evolution of resistance when low herbicide rates are used is more likely to occur in cross-pollinated species, such as ryegrass, compared with self-pollinated weeds, such as wild oats.

Previous GRDC-funded research at WAHRI, using ryegrass in pots, had already shown how herbicide-susceptible ryegrass plants, subjected to consecutive selection cycles with the Group A chemical diclofop-methyl applied at low rates, led to very high resistance levels after only three generations.

The research was repeated in the field from 2006, with the aim of finding out if this phenomenon could also occur in commercial crop conditions.

The outcome of the field research was resistance evolving after two years, at a slower pace than with the pot trials.

The final resistance level was lower than that of ryegrass grown in pots, but it was still substantial, Dr Busi said.

While twice field-selected ryegrass progeny exhibited a four-fold increase in survival, twice-selected progeny in the pot trials exhibited a 17-fold increase in survival.

The twice-selected generations of ryegrass in the wheat field and pot studies also revealed the evolution of cross-resistance to different herbicide modes of action.

The weeds showed a substantial level of resistance to two other Group A herbicides, haloxyfop and fluazifop-butyl, and cross-resistance to Group B herbicides chlorsulfuron and imazethapyr.

"We want to maximise the life of each chemical we still have," Dr Busi said.

"The bottom line for growers is to use herbicides at the full label rate and then rotate different herbicide rates as much as possible."