Thinking across and beyond disciplines to make cover crops pay

Abstract

As the name implies, a cover crop consists of plants grown primarily to keep the land covered, especially during the off-season or between cash crops. In temperate regions like most of Europe and North America, a cover crop sown immediately after the main crop harvest in fall is considered a winter cover crop. It will grow in the fall, either subjected to frostkill or go into relative dormancy during the dead of winter, and then, if winter-hardy, recommence growth in very early spring before soils are warm and dry enough for the next cash crop. If the climate is sufficiently mild, such cover crops may produce substantial above-ground dry matter (3000–6000 kg ha−1) and nearly complete ground cover before being terminated. For many decades, the use of cover crops has been promoted mainly to prevent the severe soil erosion that winter and spring rains can bring if soils are left bare. In addition, it is widely recognized that regular use of winter cover crops – as compared to bare fallow over winter – can provide enough carbon input to build – or at least slow the decline of – soil organic matter. For these reasons, many scientists view cover crops as an essential tool in managing farmland for long-term sustainability.1 A considerable amount of cover crop research has been conducted in the mid-Atlantic region of the USA during the past three decades. Most of this research focused on just a handful of cover crop species, mainly cereal rye and hairy vetch, which were found to be well adapted to the region’s climate and cropping systems. With the advent of programs to restore the health of the Chesapeake Bay, most cover crop research in Maryland has been directed towards using cover crops to capture residual mineral nitrogen (N) before it can leach away in the fall. Extensive research on coastal plain soils has demonstrated the ability of a rye cover crop to greatly reduce the loss of N to groundwater from maize grown in no-till production systems.2 However, relatively little has been done to demonstrate direct benefits to the farmer from the use of cover crops. One economic benefit that has been well quantified is the ability of legume cover crops, under some conditions, to replace by biological N2 fixation most or all of the fertilizer N needed for optimal production of nitrogen-demanding crops. However, under realistic conditions, research indicates that it costs about as much to grow and manage a hairy vetch cover crop as the value of the N fertilizer it saves.3,4 While biologically fixed N is likely to become more profitable as the cost of N fertilizer rises, legume cover crops grown alone are not very effective at capturing residual fall N. Although not often discussed by researchers, farmers recognize that growing a cover crop adds extra expense, complexity and uncertainty to the already risky business of farming. Under some circumstances, certain cover crops have interfered with crop production by using up water stored in the soil profile, by immobilizing N needed for the cash crop and by becoming weedy or producing excessive residues, hampering crop stand establishment or harvest. The most obvious direct costs associated with cover crops include those for cover crop seed, labor, fuel, fertilizer and herbicide or tillage to kill the cover crop. Given these considerations, the State of Maryland has for several years paid subsidies of $50–100 per hectare for timely planting of cover crops with a goal of keeping at least 75% of Maryland’s cropland acres under cover crops in winter. Despite this incentive, adoption rates remain relatively low, with only about 20–25% of cropland hectares receiving cover crops. We suspect that this is because most farmers in this region are not sufficiently aware of the direct benefits that cover crops potentially offer, possibly as a result of past research and extension work that emphasized cover cropping’s role in N fixation, environmental protection and long-term soil resource conservation. Although nearly all farmers desire to be good stewards of their land, most face tight (or negative) profit margins and cannot afford to engage in environmental altruism without first considering their operations’ bottom line and efficiency goals. Lacking credible information and examples that might convince them otherwise, many farmers have reached the conclusion that cover crops are simply not worth the cost and trouble.