Benefits

Reasons to Use Cover Crops

Cover crops have many potential benefits, but there are also important challenges 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 potential benefits and tradeoffs of incorporating cover crops into farming operations. 

Yield

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Use of cover crops can increase cash crop yields through several mechanisms. The 2019-20 Sustainable Agriculture Research and Education (SARE) National Cover Crop Survey concluded that, on average, corn and soybean yields planted after a cover crop were increased by 2 and 5 percent, respectively (CTIC & SARE, 2014). 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 (Muramoto et al., 2011).

Soil Organic Matter

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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 (Brennan & Boyd, 2012; Jackson et al., 2004), and a recent national survey showed that 94% of growers who use cover crops are interested in improving soil structure or soil health.

Nitrogen Management

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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 termination (Christopher & Lal, 2007), as well as water availability and stress (Zablotowicz, et al., 1981). 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 (Montemurro et al., 2013). Table 1 (below) 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 (Brennan & Boyd, 2012). Long-term research suggests that in comparison to conventional systems with winter fallows, cover crops reduce N loss (Poudel et al, 2001; Drinkwater et al., 1998). 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 (Jackson, 2000).

Table 1. Cover Crop Performance

Nitrogen fixation estimates for leguminous cover crops commonly used in California.

Water Conservation and Quality

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Cover crops can increase soil water retention by increasing infiltration capacity and reducing surface runoff (Aase & Siddoway, 1976 ; Battany & Grismer, 2000; Ruiz-Colmenero et al., 2011). 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 (Brennan & Boyd, 2012). 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 fallows (Smukler et al., 2012).

Cover crops have also been shown to lead to increased soil moisture in the surface soil layer (Jackson et al., 2003) although they may also compete with the cash crop for available soil moisture.

Greenhouse Gases

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Consistent use of cover crops on the same fields over time can increase soil organic matter (SOM), which represents an increase in soil carbon sequestration (Jackson et al., 2004; Andrews et al., 2002; Steenworth & Belina, 2008a). 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% (DeGryze et al., 2011). 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) (Basche et al., 2014) and C:N, rainfall and irrigation practice, available soil mineral N and cultivation techniques (Baggs et al., 2003), as well as additional N applied,  soil carbon content, pH and texture (Brennan & Boyd, 2012). 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 (Basche et al., 2014). 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 (Kallenbach et al., 2010). 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 cultivated (Steenworth & Belina, 2008a & 2008b) 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 of cover crops, and more long-term research on this issue is needed.

Weeds, Pests and Diseases

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Although cover crops have been shown to hinder the growth of weeds in some cases (Kruidhof et al., 2008; Teasdale et al., 2007; Haramoto and Gallant, 2004) the timing of cover crop establishment, termination and planting of the cash crop (Montemurro et al., 2013) as well as type of cover crop or cover crop mixture (Basche et al., 2014) 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 legumes (Basche et al., 2014). Cover crops have also been suggested as habitat for pest predators, or trap crops for pests.