Evaluating combined land conservation benefits from perennial pasture: lucerne (Medicago sativa L.) for management of dryland salinity and herbicide resistance in Western Australia

The inclusion of perennial pasture phases in cropping rotations has been widelypromoted throughout Australia for reducing the incidence of dryland salinity. To alesser extent, they have also been promoted to enhance the management of herbicideresistantweeds. No previous economic analysis of perennial pasture has consideredboth of these benefits. This study combines a dynamic linear programming model toestimate the magnitude of salinity-related benefits and a complex simulation model toassess the economics of herbicide-resistance management. We present a case study ofthe perennial pasture lucerne (Medicago sativaL.) in the Wheatbelt of Western Australia,where the weed annual ryegrass (Lolium rigidumGaudin) is resistant to multiple herbicidegroups. Sequences incorporating lucerne are the most profitable land use at the standardset of parameter values if (i) annual ryegrass is resistant to all selective herbicides, (ii)the water table is so shallow (approximately <3.5 m deep) that frequent rotation withperennials is required to avert soil salinisation, (iii) sheep production is highly profitable,or (iv) there is a combination of less extreme cases. The value of perennial pasture issufficient under these circumstances to overcome its high establishment cost and thedisplacement of multiple years of crop. Consideration of dryland salinity and herbicideresistance are about equally important in evaluating the economics of lucerne; neithershould be neglected.