Managing Herbicide Resistance: Should you Conserve or Exploit your Herbicides?

Herbicide resistance has become a major problem in dryland agriculture. In Australia this particularly applies to annual ryegrass (Lolium rigidum) which has developed multiple resistance to a wide range of commonly used selective herbicides. Although herbicides are a very cost-effective means of reducing weed density, major changes to their use are required if sustainable weed management is to be achieved. In this study a model of ryegrass population dynamics was used to identify the best integrated weed management strategies and to evaluate changes in the economic payoff when choosing to conserve herbicides rather than exploit them rapidly. A situation of evolving herbicide resistance was simulated for a continuous wheat-lupin rotation and two cropping sequences including one and two pasture phases. Conservation of the last four shots of the selective herbicide Hoegrass was found to be less profitable than their exploitation. Nevertheless, conservation provided a better long-term weed control. Benefits from conservation were lower in a situation of reduced level of weed control, but appeared to increase with the inclusion of more pasture in the rotation.