A cover crop can be any non-cash crop grown in addition to the primary cash crop. Cover crops can be annuals or perennials, and they can be planted as monocultures or in mixtures of multiple species. Cover crops are also known as green manure crops if one of their intended benefits is to provide nutrients to the cash crop. They can be either leguminous species, like vetches and clovers, which fix nitrogen from the air, or non-leguminous species, like rye and triticale.
Cover crops have many potential benefits, but there are also important tradeoffs and management implications to consider when deciding whether to use cover crops and which crop or mixture of crops to choose. The following sections briefly outline the common cover crops used in California, and the potential benefits, tradeoffs and management considerations of incorporating cover crops into farming operations. In each section, further resources are provided with more specific information.
A wide range of leguminous and non-leguminous cover crops are used in California. Table 1, adapted from Managing Cover Crops Profitably, shows some common cover crops of Coastal and Central Valley regions, categorized by common use. More detailed information for each crop is available in the SAREP cover crop database.
Potential Benefits and Tradeoffs
Use of cover crops can increase cash crop yields through several mechanisms. A recent national survey on cover crops concluded that, on average, corn and soybean yields planted after a cover crop were increased by 3 and 4.6 percent, respectively.1 In California, a study conducted in Yolo and Sutter counties found that processing tomato yield increased when a cover crop was grown and incorporated before planting of the cash crop. However, the impact on yield will depend on many factors, including water availability, initial soil quality, cover crop type or mixture and timing of cover crop operations. Supplemental fertilizer may also be needed at first to optimize yields, depending on the C:N ratio of the cover crop used and number of seasons that the land is under cover crop. 2
One common reason that growers say they use cover crops is for their ability to increase Soil Organic Matter (SOM), and the benefits associated with SOM. Many studies in CA have shown that cover crops increase SOM 3,4,and a recent survey showed that 73% of growers who use cover crops are interested in benefits associated with an increase in SOM.
Cover crops have been shown to impact nitrogen (N) cycling in agricultural systems through biological nitrogen fixation as well as temporary immobilization of N that may have otherwise been removed from the field. Leguminous cover crops convert nitrogen gas from the atmosphere to organic nitrogen with the help of symbiotic bacteria. This organic N can then become available for plants after microbial decomposition of cover crop residues, adding N for the subsequent crop and reducing the need for N from fertilizer. The amount of N that legumes fix is influenced by many factors, including time of stand establishment and termination5, as well as water availability and stress 6. Total N fixation is difficult to calculate, but obtaining some estimate of the additional N added to the system is important, as fertilizer applications to the subsequent crop (or interplanted in the case of vine and orchard crops) should take into account both the additional N added to the field by a leguminous cover crop and mineralization rates 7. Table 2 shows the range of N from fixation for some leguminous species commonly used by farmers in California.
The use of cover crops could also limit N losses, potentially improving nutrient use efficiency by taking up residual soil N that could otherwise have been lost from the system through leaching.3 Long-term research suggests that in comparison to conventional systems with winter fallows, cover crops reduce N loss8,9. It is important to note that incorporation of leguminous cover crops with a low carbon to nitrogen ratio could result in an excess of nitrate, and good water and nutrient management after residue incorporation is important to avoid leaching. 10
Cover crops can increase soil water retention by increasing infiltration capacity and reducing surface runoff11,12,13. For example, studies in hillside vineyards have shown runoff rates to be reduced from 23-77%, and associated reductions in soil erosion of 50-75% in permanent or cut annual cover crops when compared to bare soil managed with traditional tillage.3 Another study in Yolo County suggested that cover crops can successfully reduce runoff of water and nutrients, reducing total water discharge by 44% when compared to fallows14.
Cover crops have also been shown to lead to increased soil moisture in the surface soil layer15 although they may also compete with the cash crop for available soil moisture.
Consistent use of cover crops on the same fields over time can increase soil organic matter (SOM), which represents an increase in soil carbon sequestration4,16,17. A study modeling the use of cover crops in Central Valley cropping systems estimated that winter cover crops could increase soil organic carbon content up to 90%.18 However, these benefits remain only while the planting of cover crops, and the addition of plant biomass to the soil, continues.
Cover crops may also impact emissions of nitrous oxide, a potent greenhouse gas. Nitrous oxide is produced by the microbial processes of nitrification and denitrification, which are controlled by the availability of an N source, the organic matter content of the soil, soil moisture, temperature and oxygen. However, many factors determine whether the use of cover crops will increase or decrease overall nitrous oxide emissions. These include the type of cover crop (leguminous vs. non-leguminous)19 and C:N, rainfall and irrigation practice, available soil mineral N and cultivation techniques, 20 as well as additional N applied, soil carbon content, pH and texture. 3 A recent meta-analysis found that in 40% of included observations, cover crops decreased N20 emissions, while in 60%, N20 emissions were increased following the addition of a cover crop.19 In California, one study in a tomato system found that winter legume cover crops resulted in higher N20 emissions under furrow irrigation, but did not have any effect on N20 emissions in fields under sub-surface drip in the dry season, while in the rainy season N20 emissions with cover crops were higher than treatments where no cover crop was used, regardless of the irrigation system that had been in place in the preceding summer.21 Another study in a vineyard system found that while the use of cover crops did increase N20 emissions, they also increased soil carbon content by 40-50% after five years of cover cropping when compared to soils that were continuously cultivated17,22, highlighting a need for holistic evaluations of management practices with respect to the multiple potential sources and sinks of greenhouse gas emissions in agriculture.
Although it appears that cover crops can have an impact carbon sequestration and N20 emissions, few studies have assessed the influence of cover crops on the net global warming potential (add to glossary) of cover crops, and more long-term research on this issue is needed.
Although cover crops have been shown to hinder the growth of weeds in some cases (more citations needed) the timing of cover crop establishment, termination and planting of the cash crop7 as well as type of cover crop or cover crop mixture19 could be important. For example, one study conducted in Salinas, CA suggested that burning nettle growth was decreased by a mustard cover crop, but not by cover crops composed of oats or oats and legumes19. Cover crops have also been suggested as habitat for pest predators, or trap crops for pests.
The timing of establishment, termination and incorporation of a cover crop can affect the potential benefits provided by the cover crop and the performance of the cash crop. In regions such as northern California, where winter temperatures significantly slow the growth of winter cover crops, early fall planting is important to obtain a good stand establishment in the 1-2 months when temperatures are still relatively warm, for maximum biomass production and, in the case of legumes, nitrogen fixation. For summer cover crops, temperature but also availability of irrigation water may affect choice of planting date, with earlier spring dates likely allowing for better establishment with residual soil moisture if capacity to irrigate is limited. The best time for cover crop termination will vary depending on the cover crop species or mixture used, the weather, and the target cash crop. Generally, winter cover crops decrease flexibility in planting dates, which could impact the production and profitability of spring crops.4 This impact could be positive or negative, as cool season cover crops could delay spring planting, or, conversely, they can improve early access to the fields in the spring by managing soil moisture.
In conventional systems, starter fertilizer applications may be used for cover crop establishment, with herbicides applications used for cover crop termination. In organic systems, cover crops may be terminated by rolling with a roller-crimper or mowing with a flail-mower.20 In both conventional and organic systems, cover crops are often flail-mowed to aid in incorporation. The NRCS publishes cover crop termination guidelines for non-irrigated cropland (including fallow periods) with lots of useful information.
Cover crop seeds can be expensive, ranging from $30 to $200 or more per acre (Table 3). Survey results from the Northern SARE 2013-2014 Cover Crop Survey indicate that farmers in the West spend a median of $22.5 per acre on planting and establishment, and $40 per acre on seed. A cost study conducted in 2003 by U.C Cooperative extension suggests much higher costs, estimating that total materials (seed, water, fuel, lube, repairs) for establishing an oat cover crop could reach $95 per acre with an additional $147 per acre in operations costs. However, for some systems, the costs can be minor compared to costs associated with conventional management of winter fallows. 21
The Solution Center for Nutrient Management profiles farmers who are employing innovative solutions to nutrient management issues:
- Report of the 2013-14 Cover Crop Survey. (Joint publication of the Conservation Technology Information Center and the North Central Region Sustainable Agriculture Research and Education Program, 2014).
- Muramoto, J. Nitrogen Contribution of Legume/Cereal Mixed Cover Crops and Organic Fertilizers to an Organic Broccoli Crop. at <http://hortsci.ashspublications.org/content/46/8/1154.short>
- Brennan, E. B. & Boyd, N. S. Winter Cover Crop Seeding Rate and Variety Affects during Eight Years of Organic Vegetables: II. Cover Crop Nitrogen Accumulation. Agron. J. 104, 799 (2012).
- Jackson, L. E. et al. On-farm assessment of organic matter and tillage management on vegetable yield, soil, weeds, pests, and economics in California. Agric. Ecosyst. Environ. 103, 443–463 (2004).
- Christopher, S. F. & Lal, R. Nitrogen Management Affects Carbon Sequestration in North American Cropland Soils. Crit. Rev. Plant Sci. 26, 45–64 (2007).
- Zablotowicz, R. M., Focht, D. D. & Cannell, G. H. Nodulation and N Fixation of Field-Grown California Cowpeas as Influenced by Well-Irrigated and Droughted Conditions1. Agron. J. 73, 9 (1981).
- Montemurro, F. et al. Organic Fertilization, Green Manure, and Vetch Mulch to Improve Organic Zucchini Yield and Quality. HortScience 48, 1027–1033 (2013).
- Poudel, D. D., Horwath, W. R., Mitchell, J. P. & Temple, S. R. Impacts of cropping systems on soil nitrogen storage and loss. Agric. Syst. 68, 253–268 (2001).
- Drinkwater, L. E., Wagoner, P. & Sarrantonio, M. Legume-based cropping systems have reduced carbon and nitrogen losses. Nature 396, 262–265 (1998).
- Jackson, L. E. Fates and Losses of Nitrogen from a Nitrogen-15-Labeled Cover Crop in an Intensively Managed Vegetable System. Soil Sci. Soc. Am. J. 64, 1404 (2000).
- Aase, J. K. & Siddoway, F. H. Influence of Tall Wheatgrass Wind Barriers on Soil Drying1. Agron. J. 68, 627 (1976).
- Battany, M. C. & Grismer, M. E. Rainfall runoff and erosion in Napa Valley vineyards: effects of slope, cover and surface roughness. Hydrol. Process. 14, 1289–1304 (2000).
- Ruiz-Colmenero, M., Bienes, R. & Marques, M. J. Soil and water conservation dilemmas associated with the use of green cover in steep vineyards. Soil Tillage Res. 117, 211–223 (2011).
- Smukler, S. M., O’Geen, A. T. & Jackson, L. E. Assessment of best management practices for nutrient cycling: A case study on an organic farm in a Mediterranean-type climate. J. Soil Water Conserv. 67, 16–31 (2012).
- Jackson, L. E. et al. Scientists, growers assess trade-offs in use of tillage, cover crops and compost. Calif. Agric. 57, (2003).
- Andrews, S., S. et al. On-Farm Assessment of Soil Quality in California’s Central Valley. Agron. J. 94, 12–23 (2002).
- Steenwerth, K. & Belina, K. M. Cover crops and cultivation: Impacts on soil N dynamics and microbiological function in a Mediterranean vineyard agroecosystem. Appl. Soil Ecol. 40, 370–380 (2008).
- De Gryze, S., Lee, J., Ogle, S., Paustian, K. & Six, J. Assessing the potential for greenhouse gas mitigation in intensively managed annual cropping systems at the regional scale. Agric. Ecosyst. Environ. 144, 150–158 (2011).
- Basche, A. D., Miguez, F. E., Kaspar, T. C. & Castellano, M. J. Do cover crops increase or decrease nitrous oxide emissions? A meta-analysis. J. Soil Water Conserv. 69, 471–482 (2014).
- Baggs, E. M. et al. Nitrous Oxide Emissions Following Applications of Residues and Fertiliser Under Zero and Conventional Tillage. Plant Soil 254, 361–370 (2003).
- Kallenbach, C. M., Rolston, D. E. & Horwath, W. R. Cover cropping affects soil N2O and CO2 emissions differently depending on type of irrigation. Agric. Ecosyst. Environ. 137, 251–260 (2010).
- Steenwerth, K. & Belina, K. M. Cover crops enhance soil organic matter, carbon dynamics and microbiological function in a vineyard agroecosystem. Appl. Soil Ecol. 40, 359–369 (2008).
- Brennan, E. B. & Smith, R. F. Winter cover crop growth and weed suppression on the central coast of California. Weed Technol. 19, 1017–1024 (2005).
- Wayman, S. et al. The influence of cover crop variety, termination timing and termination method on mulch, weed cover and soil nitrate in reduced-tillage organic systems. Renew. Agric. Food Syst. (2014).
- Wyland, L. J. et al. Winter cover crops in a vegetable cropping system: Impacts on nitrate leaching, soil water, crop yield, pests and management costs. Agric. Ecosyst. Environ. 59, 1–17 (1996).
- Nitrogen Fixation. Texas A&M Agrilife Research and Extension Center at Stephenville (2014). at <http://stephenville.tamu.edu/topics/forages/forage-species/nitrogen-fixation/>
- Jensen, E. S. et al. Legumes for mitigation of climate change and the provision of feedstock for biofuels and biorefineries. A review. Agron. Sustain. Dev. 32, 329–364 (2012).