Cover Crops: Increasing Diversity in Nebraska Crop Rotations

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By Sam Wortman and Charles Francis

Cover crops are planted for conservation purposes and typically provide soil coverage between cash crop cycles to reduce soil erosion. Depending on what species are planted, cover crops may provide additional benefits to crops and surrounding ecosystems. Some benefits help farmers to increase grain yields and profitability, while others are less tangible and build long-term soil quality that contributes value to farmers, our environment and society as a whole.

Are Cover Crops an Option in Corn-Soybean Rotations?

Cover crop plantings in the U.S. corn-soybean belt are limited because we don’t have the growing season typically needed for corn and soybean production plus a cover crop. As defined by the average last and first freeze, our Lincoln, Neb., growing season begins April 27 and ends Oct. 6.1 Corn and soybeans are typically planted prior to the second week of May and harvested in mid- to late October, which leaves only a narrow window, if any, for growing cover crops. In grain rotations, the best opportunity for cover crop growth is following a winter annual crop like wheat, which is harvested in July and provides a large window for establishment and growth of a productive cover crop. However, for many agronomic, social and especially economic reasons, the widespread adoption of a corn-soybean-winter wheat crop rotation throughout the Midwest is unlikely in the near future. Therefore, researchers and farmers are forced to create a window for cover crop growth where one does not currently exist in the current corn-soybean rotation of the Midwest.

The most common option for cover crop establishment within the corn-soybean rotation is to plant a winter annual cover crop immediately following soybean harvest. Soybeans are typically harvested sooner than corn, and soybean is a low-residue crop, thus the soil surface following soybean is far more susceptible to erosion. Therefore, planting cover crops following soybean in a two-year rotation is the highest priority. The most common challenge with this approach is that timing is critical. Soybean harvest may be delayed or corn harvest may need to be expedited, and typically activities to get crops out of the field take priority over any cover crop considerations. The result is often a late-planted cover crop, even in November, resulting in poor establishment and little growth prior to corn planting the following spring. Another possibility for cover crop establishment is to broadcast the seed via airplane prior to summer crop harvest. If successful, this option certainly creates a longer period for cover crop growth but may be a relatively expensive option that often results in nonuniform stands. A similar option attracting recent interest is attaching a broadcast spreader to a “high-boy” spray tractor and spreading cover crop seed after leaf drop in soybean but before harvest. If none of these fall seeding options is viable for a farmer, the only other possibility for cover crop establishment is in early spring. This option has promise but is less desirable than the potential benefits of achieving earlier soil coverage by planting winter annual cover crops in the fall.

All these options are possible, but each requires some amount of sacrifice and change by the farmer to ensure the full benefit of the cover crop. For example, traditional corn and soybean planting dates may need to be delayed or harvest may need to occur earlier to allow sufficient growth of cover crops, which may mean that shorter-season corn hybrids and soybean varieties need to be considered. However, this option may cause a reduction in corn and soybean yields. In short, diversification of the corn-soybean rotation with cover crops is possible, but it won’t be easy.

Traditionally, cover crop use and management have followed the conventional single-species paradigm. Monoculture systems were developed to facilitate ease of mechanical cultural practices, including planting, fertilization, weed control and harvest. However, with the exception of planting, farmers do not have to consider these management factors when growing a cover crop. Realizing this has prompted many farmers to consider using multiple-species cover crop mixtures. While many of the benefits are currently being researched, we have already demonstrated many potential short- and long-term biological and economic benefits of growing cover crops in multiple-species mixtures.

Biological Advantages of Cover Crop Mixtures

Cover crop species are generally chosen to meet specific farmer goals. Not surprisingly, the specific benefits associated with a cover crop vary by species and management methods. For example, legume species are typically chosen due to their capacity to utilize atmospheric nitrogen through a mutualistic relationship with nitrogen-fixing bacteria. Properly managed legume cover crops can reduce or eliminate the need for synthetic nitrogen additions for the subsequent cash crop. Another benefit often sought in a cover crop species is the capacity for reducing soil compaction. Species with long taproots, typical of cover crops in the mustard family, can often penetrate compacted soil layers up to 6 feet deep. The potential for specific cover crop species to suppress weeds is another area of increasing interest, especially in organic cropping systems where the use of synthetic herbicides is prohibited. While many species offer these specific benefits, many of the biological advantages associated with a healthy cover crop are not unique to individual species. The potential benefits of most cover crops include reduced topsoil erosion, increased nutrient cycling and reduced nitrate leaching, improved soil aggregation and water retention, increased organic matter content and soil carbon sequestration and a reduction in the incidence of disease and insect pests.

Due to the fact that many biological benefits of cover crops are species- or family-specific, there may be an advantage for farmers to grow multiple species in cover crop mixtures. Moreover, growing mixtures of cover crops should increase the efficiency of resource use. Species with a variety of canopy and root structures, along with variable demands for water and nutrients, will ensure that the entire plant community maximizes productivity given the available resources. The positive relationship between plant community diversity and productivity has been demonstrated by ecologists.2 However, certain species are extremely competitive or antagonistic toward other species when grown in mixed species communities, so cover crop mixtures should not be chosen carelessly.

In addition to the specific benefits of individual species in a mixture and the potential for increased resource-use efficiency, a multispecies mixture will provide an explosion of biodiversity in an otherwise biologically simple corn-soybean rotation. The immediate increase in vegetative diversity during cover crop growth will likely lead to increased diversity of other species in associated trophic levels such as beneficial insects, birds and microorganisms that may utilize the cover crop community as a source of food, habitat or refuge. While the benefits of biodiversity are not always immediately realized by the farmer, most agree that conservation of biodiversity is inherently good for society. A mixture of cover crop species can offer many potential benefits packed into one growth cycle. However, if a farmer has one pressing problem, like soil compaction or low soil nitrogen, it may be more appropriate to develop a diverse rotation of cover crop species in sequence. Though the individual species will be separated in time, a diverse rotation of cover crops will still further many of the same biological goals of cover crop mixtures. The main difference is in the amount of time required to achieve these benefits and the potential economic advantage of a multispecies mixture.

Economic Advantages of Cover Crop Mixtures

There are both immediate and long-term economic incentives for using cover crops. In general, the immediate economic advantages of cover crop use include the cost savings associated with replacing off-farm inputs such as synthetic nitrogen, fuel, herbicides and labor, as well as any associated yield increases. Cover crop mixtures provide further economic advantage to farmers by reducing economic risk. Depending on annual weather patterns, certain cover crop species perform better than others in a given year, and this outcome is somewhat unpredictable. Therefore, a mixture of cover crop species will reduce the economic risk of choosing an unsuccessful single cover crop species and losing the investment of seed and labor associated with establishment. For example, seed of many legume cover crops can be expensive and also more difficult to establish compared to other cover crops. Therefore, combining legumes in mixture with other broadleaf and grass species will reduce the initial cost of seeding the cover crops and also ensure the farmer gains some biological and economic benefit, even if growth of the legume is unsuccessful in a given year. In some cases competition among species may increase the productivity of each individual species compared to growth in monoculture. For example, if nitrogen fixation per legume plant can be maximized in a multiple-species mixture, the economic return per seed in the form of replaced nitrogen cost would increase.3 While not directly related to the biology of cover crops, another economic incentive is through USDA Natural Resource Conservation Service (NRCS) conservation programs. Several options provide payments to farmers for individual cover crop use, and there is currently one provision in the Conservation Stewardship Program that provides economic incentive for planting cover crop mixtures.4

In addition to short-term economic incentives associated with cover crop use, there are recognized long-term benefits. The long-term economic advantages are related to reductions in soil erosion and improved soil organic matter. Reducing soil erosion long-term is in the best interest of the farmer, local communities and society. It has been estimated that soil erosion costs farmers in the U.S. over $27 billion annually. Most of this cost is due to the nutrients lost in eroded topsoil, but this estimate also includes the cost of lost water and thickness of topsoil.5 Equally troubling is the cost of soil erosion to society, which is estimated at $17 billion per year in the U.S. The off-farm societal impacts of erosion include costs associated with the siltation of navigable waterways, sewers and roadways, and the associated clean-up costs.5 Including cover crop mixtures in the corn-soybean rotation will not eliminate the economic burden of soil erosion, but it would certainly be a step in the right direction. The second long-term economic advantage of cover crop use may be more easily observed by farmers. Cover crop use over time has been shown to increase organic matter content (stable carbon) in soils. For the farmer, increasing organic matter is generally a sign of improved soil quality and productivity and has also been shown to lead to greater yield stability. Increased organic matter in soils increases soil water retention, which improves the likelihood of yield stability in exceptionally dry years.6 This is an important economic consequence in a climate and society where water for agriculture is increasingly scarce and there is competition from other sectors of the economy.

Perennial Cover Crop Mixtures

Faced with the staggering economic losses attributed to soil erosion, it seems worthwhile to consider alternative cropping systems that will increase year-round ground cover in a region dominated by annual crops. Excluding the possibility of abandoning the corn-soybean rotation in exchange for perennial forage crops, the best solution for reducing soil erosion in the Midwest is through the use of perennial cover crops. Perennial cover crops are most commonly used in vineyards and orchards for weed and erosion control, but there are creative possibilities emerging for the integration of perennial cover crops in annual cropping systems.

Typically referred to as living mulches, perennial cover crops growing concurrently with corn and soybean have received attention, but most research has been focused in organic production systems. However, recent cellulosic ethanol initiatives have many farmers considering the possibility of harvesting all grain and corn stover, leaving fields in a severe carbon deficit and quite susceptible to erosion. Therefore, many researchers are now looking at perennial cover crops, including Kentucky bluegrass and white clover, as potential solutions for the agronomic and environmental problems associated with cellulosic biofuel production.7

A perennial cover crop mixture would offer the same biological and economic advantages of annual cover crop mixtures, with several valuable additions. The most obvious advantage of a perennial cover crop mixture is the year-round soil coverage that is sure to limit erosion. In a cellulosic biofuel system, the cover crop, and the deep root systems typical of perennial species, will serve as a replacement for the carbon and soil cover harvested from the system. Also, a perennial cover crop would eliminate many of the practical problems associated with cover crop establishment in the U.S. corn-soybean belt. Instead, the benefits of the cover crop can be reaped annually with far less difficulty of establishment and recurring labor costs. However, a perennial cover crop mixture presents many challenges that have limited on-farm adoption up to this point. First, when most farmers hear the term living mulch they immediately think weeds. Perennial cover crops are selected for their low growing canopy structure to minimize competition for light, but it is difficult to avoid competition for water and nutrients. The challenge in the adoption of perennial cover crops is finding the right level of cover crop suppression throughout the cash crop growing season to minimize resource competition, without jeopardizing future growth and benefit of the cover crop. Depending on the farming system, this will result in either mechanical costs for mowing or chemical costs for several low-dose herbicide applications required to suppress the cover crop without killing it. If not managed properly, these living mulch systems can result in substantial grain yield loss, and this threat minimizes the adoption of an otherwise environmentally superior option.

Conclusion

When considering cover crop use in the Midwest, there is certainly a gradient of environmentally and economically sound options. Perennial cover crop mixtures or adding winter wheat to our current crop rotation seem to provide the most environmental benefit, but the threat of short-term economic risk associated with a living mulch or alternative cash crop will likely limit the immediate widespread adoption of these practices. While these options may be part of a long-term vision for our agricultural landscape, in the short-term researchers and policy makers should be developing evidence and incentives to encourage the use of at least one cover crop within the current corn-soybean rotation. Until farmers can observe the immediate biological and economic benefits of an individual cover crop within their cropping system, they will be hesitant to explore the possibilities of multispecies mixtures, perennials and alternative crop rotations. However, once a farmer enters the beneficial world of cover crops, they rarely return. If there were incentive for farmers to try cover crops even once, we could soon see an explosion of diversity and soil cover in the U.S. corn-soybean belt that would benefit farmers, the environment and society.

 

Endnotes

1. Lincoln, Nebraska Weather and Climate, 2011. University of Nebraska Applied Climate Science. (verified May 13, 2011). www.lincolnweather.org.

2. Tilman, David, et al. “Diversity and Productivity in a Long-Term Grassland Experiment.” “Science” 294 (2001): 843–845.

3. Brainard, Daniel. “Cover Crop Mixtures for Integrated Weed and Nitrogen Management.” Michigan State University, Department of Horticulture (verified May 13, 2011). www.mccc.msu.edu/documents/CovercropmixturesforintegratedweedandNmgmt.pdf.

4. USDA-NRCS. Conservation Stewardship Program (verified May 13, 2011). www.nrcs.usda.gov/programs/new_csp/csp.html.

5. Pimentel, David, et al. “Environmental and Economic Costs of Soil Erosion and Conservation Benefits.” Science 267 (1995): 1117–1123.

6. Lotter, Don W., Rita Seidel and William Liebhardt. “The Performance of Organic and Conventional Cropping Systems in an Extreme Climate Year.” American Journal of Alternative Agriculture 18 (2003): 1–9.

7. Comis, Don and Ann Perry. “In the Midwestern United States: Growing Biofuel Crops Sustainably.” USDA-ARS (2009). www.ars.usda.gov/is/AR/archive/sep09/crops0909.htm (verified May 13, 2011).

 

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