Evaluating Cover Crops for Benefits, Costs and Performance within Cropping System Niches

Abstract

The integration of cover crops into cropping systems brings costs and benefits, both internal and external to the farm. Benefits include promoting pest-suppression, soil and water quality, nutrient cycling efficiency, and cash crop productivity. Costs of adopting cover crops include increased direct costs, potentially reduced income if cover crops interfere with other attractive crops, slow soil warming, difficulties in predicting N mineralization, and production expenses. Cover crop benefits tend to be higher in irrigated systems. The literature is reviewed here along with Michigan farmer experience to evaluate promising cover crop species for four niches: Northern winter (USDA Hardiness Zones 5–6), Northern summer (Zones 5–6), Southern winter (Zones 7–8), and Southern summer (Zones 7–8). Warm season C₄ grasses are outstanding performers for summer niches (6–9 Mg ha⁻¹), and rye (Secale cereale L.) is the most promising for winter niches (0.8–6 Mg ha⁻¹) across all hardiness zones reviewed. Legume–cereal mixtures such as sudangrass (Sorghum sudanese L.)–cowpea (Vigna unguiculata L) and wheat (Triticum aestivum L.)–red clover (Trifolium pretense L.) are the most effective means to produce substantial amounts (28 Mg ha⁻¹) of mixed quality residues. Legume covers are slow growers and expensive to establish. At the same time, legumes fix N, produce high quality but limited amounts (0.5–4 Mg ha⁻¹) of residues, and enhance beneficial insect habitat. Brassica species produce glucosinolate-containing residues (2–6 Mg ha⁻¹) and suppress plant-parasitic nematodes and soil-borne disease. Legume cover crops are the most reliable means to enhance cash crop yields compared with fallows or other cover crop species. However, farmer goals and circumstances must be considered. If soil pests are a major yield limiting factor in cash crop production, then use of brassica cover crops should be considered. Cereal cover crops produce the largest amount of biomass and should be considered when the goal is to rapidly build soil organic matter. Legume–cereal or brassica–cereal mixtures show promise over a wide range of niches.