Re-Evaluating Cover Crops In Semi-Arid Cropping In Australia.

The Australian dry-land cropping zone is based largely on extensive dryland mixed livestock-cropping enterprises growing staple annual cereal crops, especially wheat (Triticum aestivum) and legume-based pastures supporting free-range sheep and cattle.  A trend towards intensification of cropping at the expense of pastures and declining sheep numbers has occurred over the last 30 years.  Despite the diversity of pulse and oilseed crops available in Australia and their demonstrated benefits to cereals in the crop sequence, cropping systems remain dominated by intensive cereals which comprise 80 to 84% of the cropped area.  Cereals are attractive due to wide adaptation, ease of management and marketing, and lower risk due to reduced set-up cost and more reliable performance in difficult seasons. Current economics in Australia appear to favour the tactical use of annual oilseed and legume break-crops in intensive cereal systems when required to manage intractable weed and disease issues, restore soil N levels or capture higher oilseed and pulse prices rather than in regular fixed rotations.  Until recently, a sacrificial cover crop terminated prior to seed harvest made little economic sense in annual cropping systems.  The potential income forgone in the cover crop year in a variable climate was rarely justified from the measured benefits of additional nitrogen or improved soil cover and water capture in sequences involving pasture phases or in continuous no-till cropping.  However the recent explosion of grass weeds with multiple herbicide resistance, especially ryegrass (Lolium rigidum) in cereals has forced a re-evaluation of cover crops cut for hay, grazed or brown/green manured.  On crop-only farms, low-input legume cover crops such as vetch or field pea terminated early (spring) can provide an economic alternative to high-input break crops by simultaneously providing control of intractable weeds, breaking disease cycles, fixing high levels of N and storing water for subsequent crops.    In northern Australia, summer and winter crops are grown on deep clay soils in sequences with long or short fallows based on capture and storage of summer rainfall.  Here, millet cover crops, grown for a short periods and terminated to provide cover to bare soil between wide summer-crop rows have increased water available to following wheat crops.  The water used by the crop is more than compensated by the improved capture and storage of summer storm rain as a result of the surface cover provided.  We will provide an overview of these recent examples of cover crop use in Australian dryland systems.