Grains Research and Development

Date: 01.01.1998

Biting the bullet on herbicide resistance

Grower Ed Abe in front of a field of Stirling barley on land continually cropped for 27years: resistant ryegrass threatening sustainability.

Say 'integrated weed management' (IWM) to a grower and you may draw a blank. But talk about knockdowns, spraytopping, cultivation or burning to eradicate weeds, delayed sowing, slashing for silage or manipulation of pastures and you'll have a conversation.

We're talking about IWM 'bullets' for coping with the alarming problem of ryegrass resistance to herbicides, facing growers in the south and west. And research findings underline the need for farmers to use a shotgun approach when it comes to ryegrass.

In Western Australia, a four-year trials project, supported by growers through the GRDC, aims to identify the most effective and profitable combination of bullets — both for eradication of serious ryegrass infestations and as a preventive strategy.

The take-home message emerging from the trials so far is that continuous croppers with serious ryegrass problems may have to accept a downturn in gross margins for one year in order to come out ahead in the long term.

Project manager, Bill Roy of Agricultural Consulting and Research Services, says bluntly: "We've got to back away from short-term methods and look at a long-term solution."

Mr Roy said the four demonstration sites containing resistant annual ryegrass (three continuous cropping and one mixed crop and pasture) have so far confirmed the need for strong IWM programs which are not reliant on herbicides for grass control, with the exception of the judicious use of knockdowns.

In addition to expanded rotations, the trials are looking at various combinations of: cultivation for seed stimulation, delayed sowing, increased seeding rates, competitive crops, stopping seed set by cutting and/or the use of herbicides, use of narrow windrows to place seed and chaff in specific position for burning and/or .spraying out the following year, and the use of stubble burning.

Trials started in 1997 and trends in ryegrass data have not yet been established. The first year's gross margin data underscores Mr Roy's point: "In a continuous cropping situation with heavy ryegrass infestation some people may have to sacrifice one crop in order to get to a baseline where the other strategies will work effectively."

Not surprisingly, therefore, the lowest gross margins came with treatments where a poor crop and associated heavy ryegrass infestation were sprayed out in 1997, generating no income.

At the other end of the spectrum, a mixed-crop stock rotation yielded a healthy gross margin from a 3.22 t/ha crop of malting barley and ryegrass after the following strategy: early cut of the 1996 wheat crop (plus ryegrass) for silage (9.3 t/ha) followed by cleaning up with a knockdown herbicide.

"There would be many growers who fall somewhere between these outcomes, depending on the situation," said Mr Roy.

Grower experience

Corrigin area grower Ed Abe has volunteered an area as one of the demonstration sites. He has cropped continuously (and successfully) since 1980 and has adopted stubble retention and minimum tillage. He now has serious concerns about the sustainability of a herbicide-only strategy combined with early sowing

"In the last eight years resistance to 'fops' and 'dims' has grown exponentially and there are signs of resistance to others," he said.

He has expanded his crop rotations to also rotate related herbicides. "I'm thinking now, pulse to hard wheat, to canola, to soft wheat, to barley. Each calls for a different herbicide."

His overall goal is to reduce his chemical bill by one-third in five years. To do that, he and Bill Roy are investigating other strategies including: autumn cultivation 'tickle' to germinate weed seeds early; burning set paddocks to diminish seed burden on the ground; alternatively, creating a narrow windrow behind the header and burning only those sections. Bill Roy's advice: if you try this, ensure you cut low.

Mr Abe said he is changing his harvesting pattern into straight lines back and forth to accommodate windrowing. He said the trials will lead to a management strategy for his paddocks, depending on where the resistance is.

The majority of farmers in his district run a mixed operation. But Mr Abe believes the cost of stock as a management tool is "compaction like concrete". He'd rather cope with the soil effects of some cultivation and some targeted burning.

"Burning is a now and then strategy, but if you let the problem get too large, it takes drastic action to get the seedbank under control."

Program 3.3.3

Contact: Mr Bill Roy 08 9641 1080 ph/fax; email: royacars@avon.net.au