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Since 2001, there has been a dramatic increase in public and private interest in growing cover crops (Figure 1).1 Farmers are adopting cover crops because the use is linked with soil and water conservation and soil health, all important factors in sustainable productivity. Increased interest in cover crops has developed from information on enhanced crop performance, crop yield sustainability, and livestock feed value. Enhanced crop performance was demonstrated in the drought year of 2012, when corn and soybean crops that followed a cover crop had a reported average yield increase of 9.6 and 11.6%, respectively.1 The weather resiliency of these cash crops is attributed to better rooting and a reduction of moisture loss from evaporation.
Direct Benefits. Direct benefits to farmers may be realized by a reduction of inputs such as fertilizer or pesticides and increased crop productivity.2 Cash crop productivity may improve with additional years of cover crop usage. In 2012, growers using cover crops for three years or more reported greater corn yields than growers using cover crops for shorter periods of time.1
In addition to reduced inputs and increased productivity, cover crops can protect early stands of cash crop seedlings from wind damage. Growers may interseed cover crops with cash crops and kill the cover crop at a later date. Alternatively, early growth of a cover crop could be killed and followed with a cash crop seeded among rows of residue.
Indirect Benefits. Cover crops can help fields rebound from intensive farming and tillage practices that have led to erosion, compaction, and soil structure damage. Root growth of cash crops in soils with subsoil compaction can be improved by increasing the presence of active living roots. For example, soybean roots can grow into root channels created by deep-rooted annual ryegrass. 222 Many growers have tried to relieve compaction with tillage machinery. Deep soil compaction may come from the use of machinery with loads of 20 tons/axle or more. On Minnesota clay loam soils, consequences of deep soil compaction were long-term annual corn yield losses of 5 to 6%.3 Corn and soybean crops can be more sensitive to surface compaction in the top foot of soil caused by equipment weighing less than 10 tons/axle. In a two-year Minnesota study, corn yields on silt loams were reduced by an average of 7.5% from 4 and 6-ton axle loads.3 Yield loss from machinery used to relieve compaction is an important reason for some growers to consider cover crops as a biological relief for compaction. 2
Reduction of Erosion. Reducing erosion is one of the main goals of growing cover crops and is dependent on how much the cover crop reduces the forces of soil detachment and transport. Soils are susceptible to erosion when they lack cover from crop canopy and residues. Above-ground biomass from cash crops may only last four months, but cover crops can be used to supplement additional soil surface cover. Cover crop growth extends the period of biomass production and can build soil organic matter (SOM). Increasing SOM creates larger, more stable soil aggregates near the soil surface, which decreases soil detachment and the potential for erosion.
Soil Organic Matter. Soil organic matter is one of the best indicators of soil health and productivity. Soil structure, soil fertility and soil health are associated with SOM as are many physical and chemical characteristics of soils. Climate and vegetation are two of the most important factors influencing the amount of SOM in the soil surface. As mentioned earlier, cover crops that produce biomass that is retained in the soil can help build SOM levels. No-till practices help preserve SOM gained from cover crop growth. Conversely, tillage quickly breaks down SOM. An approximate 9% SOM increase in the top foot of soil was produced from rye and hairy vetch cover crops on a no-till corn and soybean field in Illinois.4 Agricultural top soils have SOM levels ranging from 1 to greater than 5% with the potential to release 10 pounds of nitrogen (N)/acre/year for each 1% of SOM.5
Additional soil fertility benefits related to cover crop use include increased rates of infiltration, nutrient cycling, and residue decomposition. Cover crops can reduce nutrient loss by improving soil water infiltration. By keeping precipitation in fields, sediment detachment and transport can be reduced, which is important for the retention of phosphorus (P). Phosphorus is attached to soil sediment, and P losses in runoff were reduced by 54 to 94% with various cover crops and different site years.4
Nutrients. Nitrate leaching can be reduced by extending the period of actively growing roots on cropped soils. Winter cover crops of rye or winter wheat reduced nitrate-N loss by 13 to 61% in Minnesota, Indiana, and Iowa.4 These results are associated with the extended period of active N and water uptake compared to the shorter growth period of cash crops. Cover crop selection can be important for N management goals. Non-legume cover crops are better suited than legumes to reduce N leaching; however, legume crop residues more readily decompose and release N to subsequent crops.
Termination of cover crops should be timed so N release coincides with active periods of cash crop growth. Soils prone to erosion may require an inter-seeded or an additional cover crop species, which is many times a grass, mixed with legume seed (biculture) because of the relatively rapid decomposition of legume residue. Bicultures can help moderate both rapid release of N from legumes and slow release of N from grass cover crops. Cover crops that survive a winter freeze with regrowth in the spring can retain nutrients in the spring and can be especially important in areas where spring runoff is a concern.
The ability of cover crops to keep nutrients in the upper soil profile can be beneficial following drought-stricken growing seasons when unused nutrients remain in the soil profile. Deep tap-rooted cover crops such as radish (Figure 2), draw up nutrients from deep in the soil profile (up to 140 lb N/acre accumulated from Michigan soils).6 Consequently, the N is made more available near the soil surface as cover crops decompose. Fibrous root systems, such as those from cereal rye, typically retain 25 to 50 lbs N/acre until spring. Consider N soil test levels deep in the soil profile, and deep-rooted cover crop species to recover the nutrient.
Universities across the country are supporting the incorporation of cover crops into current rotation practices through research and demonstration projects. Studies at the University of Illinois predicted economic and environmental benefits from the adoption of at least 10% of possibly 27 million farmland acres to cover crops in Illinois.7 Despite the potential economic and environmental benefits, cover crop adoption may be limited by the perception of higher seed costs as well as management complications, such as timing of planting and termination, and moisture availability.
Growers in Illinois, Indiana, Iowa, and Minnesota were surveyed in 2006, and 40 percent of respondents from the survey reported that cover crops fulfilled requirements for conservation plans.8 Funding and technical assistance are available from the Environmental Quality Incentives Program (EQIP) carried out by the USDA Natural Resources Conservation Service (NRCS). Assistance is granted to producers with conservation plans that rank high enough for funding. Some states have held a special sign up for cover crop funding through EQIP when drought has led to the heavy loss of hay and forage.
Environmental. The environmental value of productive soils, or soils restored by cover crops, can be difficult to estimate; however, cover crops can reduce soil erosion and the direct losses of water and nutrients on agricultural soils. Reduction of direct losses may also mitigate off-site damages to recreation, human health, property, water storage, etc. In 1995, off-site damages were an estimated $44 billion per year in the United States.9
Off-site damages can mean widespread recovery efforts. Conservation practices, including cover crops, are being funded through the USDA NRCS Mississippi River Basin Healthy Watersheds Initiative in an effort to reduce nutrient loading associated with water quality problems in local waters and the Gulf of Mexico. A significant proportion of nitrate and P in local surface waters of the Upper Mississippi River Basin (Minnesota, Wisconsin, Illinois, Iowa, and Missouri) is reported to have originated from agricultural soils.10
Economic. Producers with livestock in Illinois, Indiana, Iowa, and Minnesota harvest crops for an animal feed about 27% of the time.8 Certain cover crops are tolerant of freezing temperatures and grow into the winter months for extended grazing and continued savings on feed costs. Farmers have also harvested successful cover crop species for seed and received some seed sale income.
Iowa State University studies suggested that for each ton of soil that a cover crop helps reduce erosion and nutrient loss, $6.06 can be credited to cover crop implementation.11 However, the studies also indicated that additional value could be captured through the reduction of gully erosion, use of released decomposed cover crop nutrients by the cash crop, and long-term soil improvement.11
Soil health and production goals will be different for each producer considering cover crop species. Benefits of cover crops accumulate over time. It is recommended that cover crops be incorporated to the production system after a testing and learning process. Adopting cover crops can be done with less risk on low productivity fields, by selecting a single species or a mix that matches the productivity concerns. Fields with marginal productivity may be the first place for an interested grower to observe the beneficial effects of cover crops. When overall productivity is increasing in a sustainable way, cover crops can become an important contributor to production agriculture.