Resistant ryegrass spreads to include Roundup

More than 500 people attended an 'It Won't Happen To Me' herbicide-resistance field day held by the Birchip Cropping Group on Roy Postlethwaite's property last spring. Many were stunned by the extent of the herbicide-resistant ryegrass.

Ryegrass populations resistant to Roundup (glyphosate) have been identified on at least two properties in Victoria as well as on one in New South Wales.

"If ryegrass develops widespread resistance to glyphosate, the problem of herbicide-resistant weeds will become extreme," predicted cropping consultant Harm van Rees.

Mr van Rees is project manager for ongoing herbicide-resistance trials being conducted by the Birchip Cropping Group (BCG) on a property in the Wimmera. Here, after 15 years of intensive chemical control, at recommended herbicide rates, a ryegrass population is developing cross-resistance to a wide range of herbicides in Groups A, B, D and M.

Past articles in Ground Cover have alerted growers to the dangers of relying on the grass-selective herbicides in Groups A and B, with widespread resistance in ryegrass occurring in Western Australia (Ground Cover issue 23); and GRDC research in that State identifying the importance of "using a shotgun approach when it comes to ryegrass" (Ground Cover issue 22).

BCG trial results raise the stakes as they indicate that glyphosate (Group M) is set to join the list of 'high risk' herbicides. The non-residual, knockdown herbicide has been a mainstay of the weed-control strategy selected by conservation croppers and no-till farmers.

Leader in minimum tillage

To the north of St Arnaud, the BCG's 960-hectare trial property is owned by Roy Postlethwaite, a leader in minimum tillage who has intensively cropped his whole property with canola, wheat, barley, lupin and chickpeas for 20 years. Over the last 15 years, he has retained the stubble, direct-drilled his crops and relied on grass-selective and knockdown herbicides to control ryegrass.

After two years of trials, the BCG has found herbicide-resistant ryegrass on all three of the trial sites on Mr Postlethwaite's property. Of ryegrass plants:

  • 88 per cent were resistant to fops in Group A
  • 35 per cent were resistant to Glean (Group B)
  • 20 per cent survived trifluralin (Group D), and
  • 23 per cent of plants were not affected by application of Roundup (Group M).

Mr Postlethwaite's experience is unlikely to be an isolated example. "He is simply in the forefront of minimum tillage, some five years ahead of thousands of other farmers Australia-wide," Mr van Rees said. "Unless farmers stop cropping out of a drum, herbicide-resistant weeds are set to become widespread."

No magic bullet

Unlike the 'high risk' Groups A and B herbicides and the 'medium risk' trifluralin, industry and government have categorised glyphosate as 'low risk' in developing herbicide resistance.

While Mr Postlethwaite said that farmers had hoped that Roundup would "never fall over", Mr van Rees argued there can never be a chemical 'magic bullet'. Some of the plants in a population under attack will survive. And in the bare paddock left post-spraying, their progeny will thrive and, quite literally, inherit the earth. It's a classic case of natural selection at work.

With other weed scientists, Mr van Rees advises that applying a wider range of control techniques, 'integrated weed management' is the only safe strategy.

The BCG has developed a list of 14 weed-control options that range from switching herbicides to making hay.

Try competitive cropping

Last year the BCG trialed reducing ryegrass numbers through the use of competitive cropping, in crop spraying (i.e. crop-topping) of early and late sown wheat crops with Gramaxone, as well as windrowing.

The results of the competitive cropping trials were the most surprising. "They run contrary to the conventional wisdom on the benefits of wide row spacing," explained Birchip Cropping Group's executive officer, Caroline Peters. Not only were weed numbers lowest when crops were most competitive, but the highest sowing rates (120 kg/ ha) and the narrowest row spacings (17 cm or 7 inches) also gave the highest yield in all wheat varieties tested.

Before crop topping of both early-sown (May) and late-sown (June) crops different rates of Roundup were applied to reduce ryegrass populations. However, there was no appreciable impact on ryegrass numbers.

Windrowing and crop-topping

In early-sown crops of Silverstar, a large proportion of ryegrass (304/m2) survived a spraying with 0.6 L/ha of Roundup CTXtra 10 days before sowing. Although the late-sown plots were sprayed with an additional 1.5 L/ha of Roundup CTXtra a few days prior to sowing, ryegrass resistance and regrowth led to an even higher population of 399/m2.

Wheat yields and ryegrass seed set were tested with windrowing and crop-topping with Gramoxone when the ryegrass was at the 'milky dough stage 7-10 days after flowering. Windrowing showed some promise and is to be the focus of BCG field trials this year. Crop-topping, however, resulted in a yield and quality penalty of up to 0.57 t/ha in early-sown plots of Silverstar and 0.06 t/ha in late-sown plots.

While crop-topping had proven successful in field peas, lupins and faba beans, it was only if done when the crop had almost matured, said Mr van Rees.

He said it's unlikely that the same will apply with wheat given the differences in maturity of the weed and the crop when it's time to spray the ryegrass.

Make changes but don't despair

"I'm in strife like everyone else," is how Mr Postiethwaite wryly described his predicament. "Still, it's not the end of the world."

He says his wheat yields have yet to deteriorate, and are far higher than they were in the early years when he let his cropped paddocks lie fallow for a year.

He intends to make some changes, such as reducing his row spacing from 12 to 9 inches, making vetch hay, growing summer crops and windrowing, but Mr Postiethwaite has no intention of abandoning minimum tillage.

"I'm not too keen in ripping up country and ruining my soil structure," he said. "I'd rather have a little yegrass."

Program 3.5.2 Contact: Mr Harm van Rees 03 5492 2787 Research results on

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