Italian ryegrass resistance vexes scientists

Herbicide resistance of Italian ryegrass is glowing brightly on the Pacific Northwest research radar.

Nearly 90% of sites surveyed in the area were resistant to Discover, reports Washington State University weed scientist Ian Burke. A slightly smaller percentage was resistant to Hoelon.

Much of the research was conducted by Donn Thill, a University of Idaho weed scientist, and graduate student Seth Gersdorf. Oregon State University researchers are also investigating Italian ryegrass control.

Burke’s comments at a WSU field day were mostly based on UI findings. “We have kind of exhausted both of these technologies in terms of Italian ryegrass control,” he said.

Key Points

Italian ryegrass herbicide resistance is extensive.

An experimental product may bring some relief.

Growers are urged to watch their Italian ryegrass for resistant plants.


A major setback is the inclusion of Axiom in the Italian ryegrass resistance category, he notes. “We are the first region to discover resistance to this mode of action,” he says of the work by Gersdorf.

“The two herbicide modes of action we have been most dependent on for Italian ryegrass control are also the two prone to face the most resistance,” explains Burke.

“The recent registration of Select for pulses is, in my mind, the biggest step in the wrong direction we can take,” he said. “If you get target site resistance with Select, that’s it for ACC inhibitors” on this ryegrass.

His view is strongly echoed by Thill, who says he has been “preaching this for over three years.” Four to six applications of these inhibitors “is enough to result in Italian ryegrass resistance,” Burke says. “It happens very fast.”

More bad news

News from Australia says the use of ALS herbicides on Italian ryegrass has also resulted in resistance to herbicides with multiple modes of action.

“This is definitely a scary development,” he says. “Herbicide resistance is ominous.”

Growers need to be “diverse in their control of this weed species,” he says. “It is ready-made for adaptation to herbicides.”

Burke says weed scientists in the PNW have a “laundry list” of states and nations that have reported resistance to herbicides in Italian ryegrass. Idaho and Washington are on the list.

“This is a species which has developed resistance to about every chemical technology we have put into the field to control it. This is very definitely an ominous situation, and we need to pay close attention to Italian ryegrass.”

A grass and small-seeded broadleaf pyroxasulfone herbicide, being test by many formulators and in PNW field studies, works well to control Italian ryegrass in wheat, he says.

Using a preemergent with a postemergent herbicide “is a step in the right direction to delay resistance development,” Burke says of the experimental herbicide.

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MECHANICAL HOE: Equipment like this mechanical weed control rig may become more important as chemical herbicide resistances build, says scientist Ian Burke (right).

This article published in the August, 2011 edition of WESTERN FARMER-STOCKMAN.

All rights reserved. Copyright Farm Progress Cos. 2011.