Introducing a three-year, non-crop phase can drive down annual ryegrass (Lolium rigidum) seedbanks, giving farmers a handle on herbicide-resistant populations and allowing a return to consecutive cropping, new Weeds CRC research has revealed.

And rather than simply promoting a pasture phase, the research offers alternatives for both grazing and non-grazing enterprises.

The research comes as cases of herbicide resistance in Australia are increasing while soil fertility, grain protein levels and crop yields are declining.
As weeds develop resistance to more and more herbicides, it is getting harder to control weeds like annual ryegrass in crop.

Furthermore, increasingly intensive cropping systems in Australia have led to concerns about the sustainability of these systems.

According to Dr David Ferris (Weeds CRC/WA Department of Agriculture and Food), introducing a non-crop phase provides an array of weed control methods, many of them non-chemical, to help drive down weed seedbanks and reduce the number of weeds in the paddock before sowing.

‘Like most weeds, annual ryegrass is vulnerable to grazing. Introducing a three-year non-crop phase can successfully reduce weed seed-set and competitiveness, while encouraging growth of the desirable pasture species,’ said Dr Ferris.

‘For grazing systems, an effective treatment is to introduce a subclover phase with grazing and weed seedset control in spring (preferably paraquat).’

Grazing increased subclover coverage from approximately 20 per cent to 50 per cent to 95 per cent over three years.

As a result, the annual ryegrass seedbank was so depleted that when cropping began in year four there was only one annual ryegrass plant per square metre compared to over 1000 ryegrass plants in the first year of the non-crop phase.

The research also found that a grazed subclover phase supplied adequate nitrogen to subsequent crops, improving crop yield and quality.

Cropping specialists can also introduce a non-crop phase to get on top of their weeds - but they are advised to persist with the treatment for two to three years to adequately deplete the annual ryegrass seedbank.

‘For non-grazing systems, the best options are to introduce a Cadiz serradella pasture or a field pea or canola crop, brown-manuring them each year,’ said Dr Ferris.

An extended non-crop phase should benefit the crop too.

A brown-manured one-year break of Cadiz serradella and the field pea and canola crops all met the nitrogen demands of the subsequent crop, improving its yield and quality.