Resistance persistence - Repeat doses of herbicides are recognised as a chief cause of herbicide resistance, but it appears that withdrawing those applications may only stall, but not reverse, the trend.

As part of a GRDC-supported PhD with the WA Herbicide Resistance Initiative, David Ferris has been studying a resistant ryegrass population to determine if resistance naturally regresses after herbicide applications stop.

Mr Ferris compared the reported resistance of ryegrass plants grown in 1986 with that of their descendants grown in 2000 and found that, although not exposed to herbicides for 14 generations, 90 per cent of the plants in 2000 remained herbicide-resistant. While withdrawing herbicide applications did not reduce resistance rates, continuous use, without integrated management systems, would drive resistance rates higher.

Program 3 Contact: Mr David Ferris 08 9690 2000