Prospects for summer cover crops in southern Australian semi-arid cropping systems

Abstract

CONTEXTCover crops are widely promoted in temperate regions of the northern hemisphere to replace or reduce fallow periods between cash crops. The reported benefits (improved ground cover, reduced nitrate leaching, increased soil organic matter among others) have prompted widespread interest in cover cropping in other agro-climatic zones, including Mediterranean environments such as southern Australia, where the wet, mild winters and hot dry summers are a stark contrast to temperate environments. OBJECTIVEThis review considers the reported benefits of cover crops in temperate environments in the context of cropping systems in the water-limited, Mediterranean climate in the grain-growing regions of southern Australia. METHODSPublished studies on cover cropping were critically assessed to evaluate whether the reported impacts may be relevant to Mediterranean-type climates. RESULTS AND CONCLUSIONSNitrate leaching is not generally problematic in southern Australian cropping systems due to low summer rainfall and high temperatures, and complete retention of crop stubbles can adequately protect the soil from erosion. Further, summer rainfall is low and sporadic which not only limits cover crop planting opportunities but means that water conserved over summer fallows contributes significantly to winter cash crop yield. Consequently, cover crop duration would need to be short (1–2 months) to balance the benefits of increased cover for water infiltration with transpiration loss during cover crop growth. Restricted cover crop biomass production due to water constraints and short fallow length will limit the potential value of the cover crop to increase soil organic matter, microbial activity, and biologically-fixed nitrogen contributions – all reported benefits for higher biomass cover crops in temperate environments. Appropriate tactical use of summer cover crops may arise in situations where (i) soil cover is reduced by winter crop failure or prolonged drought creating significant erosion risk, (ii) recalcitrant herbicide resistant weeds in winter crops require a chemical spring fallow and summer cover/forage crop, which could be grazed, (iii) full soil profiles at harvest and wet seasonal outlook provide opportunities for cover crop growth without significant pre-emptive competition, and (iv) sandy soil profiles with low water holding capacity combined with reliable winter rainfall minimise contributions of summer rainfall to winter crop yields. The value of more diverse cover crop species to soil health in cereal-brassica or cereal-brassica-legume cash crop rotations is not known. SIGNIFICANCEThe rationale for widespread summer cover crop use in southern Australia is currently not compelling without more evidence for the scale and mechanisms of the anticipated benefits.