Ryegrass remains the most stubborn of Australia's $3 billion worth of weed threats.

Aware of this and very concerned about increasing resistance, the Grains Research and Development Corporation (GRDC) annually invests $750,000 in the CRC for Australian Weed Management.

Herbicide resistance is caused by herbicide resistance genes, which start out so rare that most plants don't have them. The distance, therefore, between individual resistant plants might be quite large and the distance between multi-resistant plants (resistant to more than one herbicide group) will be larger again.

This begs the question: if herbicide resistance could be stopped from moving in from elsewhere, could we stop herbicide resistance development altogether?

Art Diggle, of the Department of Agriculture, used computer modelling in his GRDC supported project on the development and control of herbicide resistance.

He suggested farmers should not be comfortable with continuous low numbers of annual ryegrass and should look at weed prevention and continuous reduction.

Where low weed levels can be maintained, continuous reduction in weed numbers is possible with only a small increase in control. If increased control is achieved without increased overuse of any one herbicide group, the probability of developing resistance steadily declines.

For herbicide types where frequency of resistance genes is low, aggressive weed management strategies, using a mixture of practices, can reduce the probability of resistance developing.

To achieve this, it is essential that resistance not be imported, hence effective crop hygiene is vital, according to Dr Diggle.