Abstract
Cover crops are grown primarily for the purpose of ‘protecting or improving’ between periods of regular crop production. There are four main types of use including; improving soil fertility, improving soil structure, managing weeds and pests and environmental management. The most appropriate cover crop species/management will depend on what the grower wants to achieve from the cover crop. Direct financial benefits can be assessed most easily though improvements in yield of following crops. However, sometimes yield improvements may be detected later in the rotation and repeated cycles of cover crop use may be required. Beyond yield improvement, cover crops can provide additional benefits for the farm and the environment (e.g. reduced soil erosion risk). However, there are also potential undesirable effects (e.g. rotational conflicts, increased weed issues and increased costs). This review investigates the scientific basis of the reported functions of cover crops to better understand the feasibility of these benefits including; nitrogen (N) fixation, uptake and release; weed suppression by allelopathic effects and physical competition; biofumigation against pests; soil erosion and runoff; soil health and fertility, including soil organic matter, soil physical properties and soil biology; cover crops as forage; biodiversity and habitat provision. Cover crop agronomy is reviewed including; common cover crop choices, establishment methods, starter fertiliser, pest management and cover crop destruction. Economic and decision making factors, including yield and economic responses are reviewed and methods for evaluating cover crops on farm given.
Some of the key conclusions include; the most important agronomic factor for achieving benefits for cover crops is to establish early (late summer/early autumn); N uptake during autumn/winter is typically 30-100 kg N ha-1, with 10-100 kg N ha-1 released to the following crop; N fixation is most effective between 7°C and 20°C which means little N is usually fixed over-winter; a canopy cover of 30% or more over winter decreases risk of soil erosion and run off; increases in soil organic matter following cover crops ranged from zero up to 42%, with no study reporting a decline; cover crops with allelopathic effects include several cereal and brassica species, buckwheat, clovers, sorghum, hairy vetch, sunflower and fescues.
Knowledge gaps include the characterisation and performance evaluation of different cover crop types, species and varieties, particularly under different conditions (e.g. soil type and weather). The effects of cover crops will be best understood using a network of long-term coordinated farm-scale experiments which feature common treatments and assessments. This will be particularly important for evaluating effects on soil organic matter which changes slowly over many years. A continually updated database is required to support decision making, calculate cost benefits and to focus research. Other priority knowledge gaps include characterisation of rooting, uptake and release of N and other nutrients, impacts on weeds, disease & pests in following crops, effects of livestock grazing and most appropriate techniques for cover destruction and establishment of the following crop.
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