Like playing chess, outwitting annual ryegrass, a major weed in WA which can develop resistance to more than one herbicide at a time, requires constant vigilance and a strategic approach.
 
Farmers now have a greater understanding of the value of pasture phases for controlling herbicide-resistant annual ryegrass in low to medium rainfall areas, thanks to Graeme Doole from the School of Agricultural and Resource Economics, Institute of Agriculture, at the University of Western Australia (UWA).

Dr Doole recently completed a doctoral study on the value of different types of sown pasture for herbicide resistance and recharge management in the WA wheatbelt.

Supervised by Professor David Pannell of UWA and Dr Clinton Revell of the Department of Agriculture and Food WA, the research was funded by the Cooperative Research Centre (CRC) for Australian Weed Management and the Future Farm Industries CRC.

A variety of mathematical modelling techniques were used to identify profitable solutions for managing annual ryegrass.

Exploiting recent advances in computer science, these techniques allowed a substantial increase in the quality of recommendations arising from existing models used in WA agriculture.

According to Dr Doole, the value of pasture phases is sometimes overlooked, given the high relative value of cereal crops. His research indicates that under certain conditions their strategic inclusion is more profitable than maintaining a crop-only system.

The value of French Serradella, a legume used as a break pasture for nearly a decade in different land-use sequences, is difficult to assess given the range of factors affecting its profitability.

"Incorporating a single year of Serradella in a rotation is the most valuable option at high weed densities, or where Group A herbicides are ineffective against annual ryegrass.

"A three year phase is required if resistance to multiple herbicide groups is observed," he said.

In addition, cropping sequences incorporating lucerne can also outperform continuous cropping, but only if one of the following occurs:

1.      annual ryegrass is resistant to all selective herbicides;
2.      a water table is shallow so that dryland salinity is likely to occur; or
3.      prices for sheep products are high.
 
"The value of perennial pasture such as lucerne is then sufficient to overcome its high establishment cost," Dr Doole said.

He concluded that sowing annual pastures was an attractive option where grain production was threatened by ongoing herbicide resistance problems.

"The multiple benefits of such pastures for a mixed farming system are that legumes fix nitrogen, which benefits a subsequent cropping phase, they can be integrated with highly effective weed management options and they are also a valuable feed source for sheep.

"However, it's important that sown pastures are carefully managed if a farming system is to fully benefit from these features," Dr Doole said.