US makes gains on herbicide resistance plans

AMERICAN farmers seem to have regained the upper hand on glyphosate resistant Palmer amaranth and waterhemp, which had overwhelmed many crops from 2008 to 2012, but there are still some longer term challenges.

Jason Norsworthy, professor of weed science at the University of Arkansas, said although many paddocks were cleaner than they had been for years, he did not believe the current practices were sustainable.

In the USA, herbicide tolerant varieties and hybrids have come to dominate in the sown areas of cotton (89%), soybean (94%) and corn (89%) across all regions.

Dr Norsworthy said many growers in the States were only one or two herbicides away from having no control options for Palmer amaranth in either conventional or herbicide tolerant soybean crops.

“Four modes of action are already lost on many populations of Palmer amaranth,” he said.

“The multiple-resistance to these four modes of action is a real game changer for growers.”

He said the practice of continuing to use a herbicide after resistance established was one of the key reasons why many growers were in a serious predicament.

ALS (Group B) resistance inevitably led to glyphosate (Group M) resistance, which in turn led to PPO (Group G) resistance.

It was only a matter of time before PPO resistance was also widespread in weeds like Palmer amaranth.

“PPOs are currently being applied both at planting and in-crop in soybean crops where there are few other herbicide options,” he said.

“This high selection pressure will inevitably lead to failure of the herbicide, leaving growers with dicamba [Group I] products as their last resort.”

Researchers demonstrated full dose dicamba survivors could evolve in just three generations in Amaranthus spp.

In Australia, there were almost as many species resistant to glyphosate as were present in the United States; however, there were not as many herbicide tolerant crop options.

Dr Norsworthy suggested Australian growers continue to adopt diverse weed management practices, including harvest weed seed control, competitive crops, strategic tillage and pasture phases rather than over-relying on herbicide tolerant hybrids as they come available.