Alarm over resistant wild oats

By Kellie Penfold

One of the most alarming trends discovered by the Farrer Centre Herbicide Resistance Testing Service during 2004 was the growing incidence of herbicide-resistant populations of wild oats in NSW.

Following the identification of the first wild oat population resistant to Mataven L® in 2002, a further two resistant populations were identified in NSW last year.

John Broster, a research agronomist specialising in herbicide resistance from the Farrer Centre, says two of the three populations of Mataven L® (flampop-methyl) resistant wild oats have never been sprayed with the herbicide.

"One of the samples resistant to Mataven L® in 2004 was also resistant to Topik®, Wildcat® and Achieve®, creating a worrying management dilemma," he says. "The wild oats are not necessarily just resistant to that herbicide, in many cases they are resistant to that mode of action."

Based at Wagga Wagga in southern NSW and part of Charles Sturt University, the Farrer Centre Herbicide Resistance Testing Service is a commercial enterprise. In 2004, it screened 444 samples supplied by farmers and resellers around Australia of annual ryegrass, wild oats, wild radish and brome grass that were suspected of having resistance.

From 2001 to 2003, the service assessed an additional 400 to 600 samples a year through the Dow WeedSense program, which offered free resistance testing to farmers.

"While we don"t make recommendations to farmers on how to attack resistant weeds, we provide the results which show what they are resistant to," Mr Broster says. "We suggest to farmers not to work on the theory that a new mode of action is going to be developed to beat resistance problems.

"They have to come up with programs for dealing with resistant weeds and then if a new treatment is available, consider it a bonus."

Possibly as a result of the drought, the number of wild oat samples received by the service in 2004 (28) was down compared with the high volumes of 2002 and 2003 (126 and 86). The level of "fop" resistance among the wild oat samples received in 2004 was 96 percent, slightly higher than the 88 percent observed the previous year.

Samples were also screened for Verdict®, Topik®, Pantera® and Wildcat® and all four had samples resistant to them.

Of the "dim" herbicides, only one sample was found to be resistant to Achieve® and none to Select®. No samples were found to be resistant to herbicides from groups B, C, D or E.

With 387 annual ryegrass samples received in 2004, 366 were tested to the standard cross-resistance test and 77 percent were classed as resistant or developing a resistance to a "fop" herbicide. This was slightly lower than the results of samples sent direct to the Farrer Centre in the previous year.

Ten percent of samples were classed as resistant or developing resistance to a "dim" herbicide. The majority of these were tested to Select® and 48 percent of samples were resistant to group B herbicides. This was higher than any year since 1997, when 58 percent of samples were resistant to group B herbicides. No samples were resistant to simazine (group C) and 13 percent were resistant to trifluralin (group D).

As in the past two years, Western Australia had the highest group B resistance overall with 73 percent of samples being resistant and South Australia was second with 44 percent.

"As was the case in 2003, last year a larger percentage of samples were susceptible to all tested herbicides. This may suggest farmers are using resistance testing as a pro-active measure rather than reactive," Mr Broster says.

Testing in the "fops" group was for resistance or developing resistance to Hoegrass®, Verdict® or Targa® and in the "dims" it was to Select®, Sertin®, Achieve® and Aramo®. Group B herbicides screened were Glean®, Logran® and OnDuty® with resistance detected to some degree to each.

Mr Broster says that while growers are becoming increasingly aware of the testing service available, "accurate results require accurate sampling". He recommends the collecting guide in the table above.

For more information:
John Broster, Farrer Centre, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2650; 02 6933 4001;

Region National, North, South, West