GRDC research into the matter has reinforced previous findings that weaker applications of herbicide allow weeds to build up a resistance to the particular chemical.

The trials were conducted by Western Australian Herbicide Resistance Initiative (WAHRI) researchers including post doctoral fellow Roberto Busi, PhD student Sudheesh Manalil and Professor Stephen Powles and found that weeds, such as ryegrass, bred up resistance when confronted with consistently low rates of herbicide.

“When herbicides are used at low rates, this can rapidly lead to resistance and also cross-resistance which may be a serious problem depending on the herbicide being used,” Dr Busi said.

With Australian producers having a heavy reliance on cheaper products such as glyphosate, resistance to some of the mainstays in the chemical rotation would place huge strain on crop production costs.

However, thus far, the major ryegrass resistance has been to group A and B chemical groups.

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

Previous work had already found herbicide susceptible ryegrass plants subjected to consecutive selection cycles of another Group A chemical, diclofop-methyl, applied at low rates, led to very high resistance levels after only three generations.

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.

GRDC officials pointed out that 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 in order to cut costs, in particular in drought impacted areas over recent years.

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.

“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.”