3c: Cover Crops
Written by JennaWest
Introduction
In this chapter, cover crops will be introduced by first exploring their potential benefits in an organic farm system, then the factors to consider for single and multi-mix cover crop selection, followed by presenting an overview of the three main families of cover crops. This overview includes a figure from a review done by Chapagain et al. (2020) summarizes the main pros and cons of the popular cover crops in these families in Southern Ontario. Two key factors for the selection process involve selecting the objective for using the cover crop, and the time period available for growth within the crop rotation (Chapagain et al. 2020). Using numerous cover crops in a mix is a strategy to address multiple objectives at once, but requires more planning to find complementary species (Clark 2007). There are gaps in mixed cover crop research, according to Chapagain et al. (2020), in terms of the optimal number of species in a mix and regarding the short- and long-term effects of mixed cover cropping. Selecting the optimal cover crop on an individual farm takes more than one season, but with proper management, cover crops can still have a positive contribution to profits in the first year (Clark 2007). On-farm trials are a good strategy to learn which cover crop suits an individual farm (Clark 2007). Cover crops provide an array of benefits or ecosystem services that align with the goals of organic agriculture. Cover crops reduce required inputs like fertilizer and herbicides, improve soil structure and health, and benefit the environment (Clark 2007). There are three main groups of cover crops, namely legumes (Fabaceae), brassicas (Brassicaceae), and grasses (Poaceae), all of which play a role in the farm system.
Cover Crop Basics: Benefits
Cover crops are non-harvested crops grown in rotation or intercropped with main crops for the purpose of the agroecosystem services they provide (Bird et al. 2009; Chapagain et al. 2020). Cover crops deliver a variety of benefits depending on the species and the system they are used in. This chapter focuses mainly on benefits under the umbrella of input reduction and soil health improvement. Many of these benefits help the environment indirectly which is important as society moves towards a more sustainable farming model, since they provide a means for intensification in ecological systems (Wittwer et al. 2017). In a long-term study on the differences in the effects of cover crops in a variety of cropping systems in Switzerland, yield was found to increase in organic, reduced tillage systems (the most ecological system in the study) an average of 24% due to cover crops over a six-year crop rotation (Wittwer et al. 2017). At the other end of the farming spectrum, conventional systems with tillage saw only a 2% increase in yield due to cover cropping (Wittwer et al. 2017).
Cover crops also have a direct positive impact on the environment. An example is the reduction of nonpoint-source pollution from agriculture through reduced runoff and leaching (Clark 2007). Sediments, nutrients, and chemicals retained in agroecosystems because of cover crops helps prevent contamination and eutrophication in the natural ecosystem (Clark 2007).
Cover crops also increase plant diversity on the farm and promote biodiversity by supporting beneficial insects and microorganisms (Clark 2007; Chapagain et al. 2020). Supported insects can include pollinators when cover crops are allowed to flower (Chan 2012). For example, buckwheat (Fagopyrum esculentum) provides a high density of flowers which increases bee visitation, and sunflowers (Helianthus) attract solitary bees (Mallinger et al. 2019). This is especially important in Ontario where many of the 400 native species of are solitary bees (Chan 2012). Mixes of cover crops with diverse flower types and flowering times attract different species of pollinators and increase flower density throughout the season (Mallinger et al. 2019). Different flowering crops attract various species of bees, as was seen with sunflowers (Helianthus) being highly attractive to solitary bees, while phacelia (Phacelia), a plant in the borage family, is favourable for honeybees and bumblebees (Mallinger et al. 2019). Although it has been shown that multi-species cover crop mixes are positive for pollinators and other ecosystem services, there is a gap in the research regarding the optimal number of species in a mix that maximizes plant diversity (Chapagain et al. 2020).
Input Reduction
Fertilizer
Cover crops allow input levels to be lowered for fertilizer, herbicides, and pesticides. This saves input costs for the farmer and helps protect the health of the farm workers, neighbours, and consumers (Clark 2007). Legume cover crops have the ability to fix atmospheric nitrogen (N) in the soil and make it available for subsequent crops, which can use up to 60% of the fixed N (Clark 2007). Hairy vetch (Vicia villosa) and red clover (Trifolium pratense) are both popular N-fixing legumes that provide an opportunity to reduce N fertilizer by 87–184 kg N ha-1 and 70-121 kg N ha-1 respectively (Clark et al. 1997). “Catch crops,” on the other hand, act as N scavengers (Martin and MacCrae 2014). These grass and cereal cover crops take N up into the plant biomass to prevent leaching (Martin and MacCrae 2014). However, this can have negative effects, as the nitrogen may be unavailable for the subsequent crop, depending on how fast the cover crop decomposes (Clark 2007). Additionally, if the grass and cereal cover crops are being grown to produce a large amount of biomass, they may require N fertilizer as an input (Chapagain et al. 2020).
Herbicides
As the threat of weed species developing herbicide resistance becomes prevalent, cover crops become increasingly important as an alternative method for weed suppression. Weeds cause reduced yields, and increased future weed pressure if allowed to mature, which makes weed suppression a high priority for farmers (Kadziene et al. 2020). Mustards, brassicas like fodder radish (Raphanus sativus var. oleiformis), and legumes like spring vetch (Vicia sativa) have been shown to produce compounds with allelopathic properties which inhibit weed growth (Kunz et al. 2016). According to a study by Kunz et al. (2016), these crops suppressed weeds by an average of 60%. Cover crop mixtures were shown to be even more effective, with weed suppression levels of 66% (Kunz et al. 2016). This may be due to synergistic interactions between brassica species and other cover crops (Kunz et al. 2016). Conservation tillage increases the weed suppression effects of cover crops since the residue can smother weeds by blocking light (Clark 2007). Grasses and grains also make good weed suppressors since they produce large amounts of biomass and residue that breaks down slowly (Clark 2007). Furthermore, cover crops can outcompete weeds for resources (Clark 2007), which can prevent weeds from taking over during a period between cash crops where the field might otherwise be empty.
Pesticides
Cover crops contribute to lessened pest pressure through various mechanisms including weed suppression, as it has been shown that weeds are often disease vectors (Kadziene et al. 2020). By way of illustration, several weed species act as hosts and vectors of Fusarium fungi, a pathogen that can be symptomless in the weeds but causes Fusarium head blight in cereals globally (Kadziene et al. 2020). In their 2020 study, Kadziene et al. found that white mustard (Sinapis alba) and white clover (Trifolium repens) effected a decrease in infection of grains by diseases caused by Fusarium fungi. Certain cover crops can also decrease pest pressure by supporting insect predators, parasitoids, and beneficial nematodes (Clark 2007). At the same time, crops like brassicas produce compounds that decrease nematode and insect pest numbers due to their biotoxic nature (Santos et al. 2021), while others still act as hosts to microbes that prevent disease (Clark 2007).
Soil Health Improvement
Soil Erosion
The FAO (2015) considers soil a non-renewable resource, making cover crops’ contributions to increasing soil fertility and tilth (“the physical condition of the soil as it relates to plant growth” (Van Es 2021)) integral to future food security. Improving soil health directly benefits the agroecosystem through improved yield, erosion protection, organic matter contributions, and improved soil moisture retention (Clark 2007). Topsoil erosion is decreased with cover crops because the roots stabilize the soil aggregates below ground, and the above-ground biomass slows water movement which prevents water from moving soil particles (Clark 2007). This is important in all agroecosystems, especially on sloped and terraced systems (Clark 2007). In these instances, cover crops with deep root systems, such as vetiver grass (Chrysopogon zizanioides), can prevent major soil losses (Dudai 2006). Vetiver grass is a perennial species that is often planted on the edges of terraces or in rows perpendicular to a slope due to its tolerance of many conditions, fast establishment, and its ability to grow roots up to 4 metres deep in the first year (Dudai 2006). Erosion control is increasingly important as the effects of climate change increase the frequency of heavy rainfall events associated with flooding and run-off (Government of Canada 2019).
Aggregation & Aeration
Beyond improving soil fertility through prevention of physical removal from the system, cover crops have the ability to increase soil tilth through aggregation and by adding organic matter (Clark 2007). Soil is cemented together into aggregates or “crumbs” by polysaccharides or by a protein called glomalin (Clark 2007). Polysaccharides are complex sugars that are produced by soil microorganisms (decomposers) as they digest plant material (Clark 2007). The polysaccharides act as a glue that creates soil aggregates; however, this is a short-term effect that lasts approximately one season (Clark 2007). Glomalin, on the other hand, is a glycoprotein that acts as a longer-term glue for aggregation (Balota et al. 2014). Glomalin is only produced in significant quantities by beneficial fungi, called arbuscular mycorrhizae, which are largely associated with legume crops (Balota et al. 2014). Legumes work in symbiosis with mycorrhizal fungi by providing the mycorrhizae with carbohydrates in return for the secretion of glomalin in the soil surrounding roots, protection, and access to immobile nutrients like phosphorous (Balota et al. 2014). This exchange between the organisms occurs when root-like extensions from the mycorrhizae (called hyphae), enter the plant root by colonizing nodules (structures on the plant root) (Balota et al. 2014).
Well-aggregated soil prevents compaction, increasing yield, since compaction has negative effects on yield (Clark 2007). Planting cover crops over the winter has a lasting positive effect on aggregate size, even after spring tillage (Hermawan and Bomke 1997), a common practice in many farming systems despite the damage it causes to the soil structure. Furthermore, the increase in soil aggregation from cover crops has been related to higher levels of soil organic carbon (Hermawan and Bomke 1997), which is a good indicator for healthy and productive soil (Government of Canada 2021). Aggregation also promotes soil aeration, which positively affects crop growth, water infiltration, and water and nutrient storage in the soil (Clark 2007). Furthermore, cover crops like sweet clover (Melilotus) and forage radish help reduce compaction due to their extensive root systems and strong taproots, which break up compacted subsoil (Clark 2007).
Nutrients
In addition to soil tilth, cover crops contribute to soil fertility through their interactions with soil nutrients. As mentioned in the “Fertilizer” section, cereal and grain cover crops take up nutrients that would otherwise leach out of the system, conserving them for future food crops (Clark 2007). This is especially important for nitrate, a common form of N in the soil (Clark 2007). N is water-soluble in this form (Clark 2007), meaning it is vulnerable to leaching since percolating water can dissolve the nitrate and carry it away as run-off. In an experiment by Ramirez-Farcia et al. (2015), rye (Secale cereale), a cool season grain crop, performed best out of the cover crops tested for catch crop functions. Mustard (Brassica nigra) performed better than rye in terms of its N uptake in one season of the experiment, but performed poorly in the following season because of its intolerance to cold weather (Ramirez-Farcia et al. 2015). On the other hand, legume cover crops, like hairy vetch, are poor N scavengers, but add N to the system because of their ability to fix their own atmospheric N using rhizobia (Chapagain et al. 2020).
Cover Crop Selection
The first step to narrowing down a cover crop for an individual farming system is deciding on an objective to address (Clark 2007). The objective is an aspect of the benefits mentioned above under the umbrellas of input reduction and soil improvement. The second step is deciding the time window, or niche, available between the main crop plantings (Chapagain et al. 2020; Clark 2007). The winter fallow niche refers to planting the cover crop after the summer harvest, at least 6 weeks before the first hard frost (Clark 2007). This works well in Southwestern Ontario for winter wheat, which is harvested at the beginning of August, giving a large window for planting cover crops (Chapagain et al. 2020). Similarly, the summer fallow niche occurs in the summer season (Clark 2007). If there is an existing period between three and eight weeks where the field will be empty after the early spring planting and before the fall planting, the summer fallow niche works well as cover crops can be planting in that period without the loss of a harvestable crop (Clark 2007). The full-year niche is applicable in more permanent systems, like orchards, or where the focus in a current system is to rebuild the organic matter in the soil to improve fertility (Clark 2007). To further narrow down which varieties to select for a cover crop mix, it is a good idea to research which seeds are available for purchase commercially in one’s region, as well as which crops are popular in the region (Chapagain et al. 2020).
As farm systems are heterogeneous, even regionally, it is essential to try out different species of cover crops to find what works best. This is best done through trials where two to five species are sown in small plots, and then information regarding the field conditions and plant growth are recorded (Clark 2007). The process to find the optimal cover crops for a specific farming system often takes numerous trials over a period of time longer than a year, so it is recommended to buy small amounts of seed when starting out with cover crops (Clark 2007).
Mixing cover crops
A review by Chapagain et al. (2020) outlines that it is possible to mix two or more complementary cover crops to provide more agroecosystem services than any one cover crop alone. As advantages and disadvantages exist with mixed cover cropping, it is important to weigh both to ensure the advantages outweigh the disadvantages (Chapagain et al. 2020). There is a lack of research surrounding the ideal number of species to have in a mix, and what the empirical short- and long-term advantages are of the multi-mix systems versus single cover crops and no cover crops (Chapagain et al. 2020).
Some significant advantages can be had from using mixed cover crops. For example, using 2 or more complementary species of cover crops increases the chance of cover crop success and can lead to a higher production of biomass (Elhakeem et al. 2019). A multi-mix also provides the opportunity to maximize water, sunlight, and nutrient resource use (Elhakeem et al. 2019). It had been found that species with different growth traits are more commonly complementary (Chapagain et al. 2020). For example, a species with a shallow root system will take up water and nutrients from a different level in the soil than a cover crop species with deep roots, thereby minimizing water and nutrient competition (Elhakeem et al. 2019). The same goes for the canopy – species like cereals that have a straight canopy complement trailing species like winter peas and hairy vetch that tolerate some shading (Chapagain et al. 2020). Having different levels in the canopy ensures that sun energy usage is maximized and increases ecosystem services like weed reduction by blocking sunlight from seedlings (Chapagain et al. 2020).
The main issue with using a mix of cover crops is the cost of seed, since it requires purchasing numerous types (Chapagain et al. 2020). Brassica cover crops are the most expensive, while cereals and grasses are the cheapest (Chapagain et al. 2020). Seeding is also an issue with using numerous cover crops at once, since different plants have different size seeds and different planting requirements (eg. planting depth) (Chapagain et al. 2020). This can be overcome by using a drill to sow crops with large seeds, and then broadcasting the smaller seeds on a second pass (Chapagain et al. 2020). On the other hand, this leads to the issue with time and labour where making multiple passes means sowing will take more time than it would for a single cover crop (Chapagain et al. 2020).
To determine a good cover crop mix, one must take into account the factors mentioned above, including objective(s) and timing. There are also numerous factors to consider when deciding which plants are compatible and complementary. The objective factor becomes more complicated with multi-mixes, as the complementarity of benefits provided by the various species should be considered, as well as the possibility for neutralizing trade-offs. For example, non-legume cover crops take up a lot of N, which is helpful for reducing leaching, but may make it unavailable for the following main crop (Chapagain et al. 2020). This trade-off can be addressed by pairing the non-legume cover crop with an N-fixing legume where up to 60% of the N it fixes is available for the following crop (Clark 2007). Growth traits also help determine complementarity (Chapagain et al. 2020). Differing rooting and canopy patterns need to be considered as mentioned above to minimize competition for resources like moisture, nutrients, light, and space (Chapagain et al. 2020). The growth period is another important consideration. Certain cover crops, like oats and buckwheat, cannot over-winter, while others, like cereal rye and hairy vetch, can (Chapagain et al. 2020). This is an important consideration for determining a mix that has a coordinated termination time, as well as for the quantity and quality of biomass produced (Chapagain et al. 2020).
Common Cover Crop Species in Ontario
Legume Cover Crops
Legume cover crops, the Fabaceae family, are largely used for their N-fixing capabilities (Chapagain et al. 2020). They can also be employed to prevent erosion, increase biomass, and to attract beneficial insects (Clark 2007). As climate and geography is largely heterogeneous, the best cover crop to be used for an N source depends on the region. For example, in Southern Ontario, popular legume cover crop species include red clover, crimson clover (Trifolium incarnatum), and berseem clover (Trifolium alexandrinum); hairy vetch (Vicia villosa); forage pea (Pisum sativum L. subsp. Arvense) (depicted in Figure 1c); and alfalfa (Medicago sativa L.) (depicted in Figure 1a) (Chapagain et al. 2020; Clark 2007). Legume crops acquire most of their N through a symbiotic relationship with rhizobia, which are N-fixing bacteria that form in nodules on legume roots and provide the plant with N in return for energy (Clark 2007). As the rhizobia fix N gas from the atmosphere, legume crops do not rely as much on soil N, so they are less efficient N scavengers (Clark 2007).
Brassicas and Mustards
Brassicas’ and Mustards’ bio-fumigation properties differentiate them from other cover crops. Part of the Brassicaceae family, brassica crops, and especially mustards, release allelochemicals that are toxic to certain soil pathogens during decomposition (Santos et al. 2021). These chemicals help in the management of certain soil pathogens (fungi, nematodes, and bacteria), insects, and even certain weed seeds, for the following crop (Santos et al. 2021). Brassica species like forage radish and purple top turnip have a large taproot that can extend as far as 6 feet into the soil (Clark 2007). This gives them the ability to penetrate subsoil to reduce soil compaction and bring up nutrients that are inaccessible to crops with shallower roots (Clark 2007). This makes these brassicas, as well as mustards, good nutrient scavengers (Clark 2007). Furthermore, brassicas produce biomass, prevent erosion (Clark 2007), and species like canola and sunflowers, depicted in Figures IV and V respectively, attract pollinators (Chapagain et al. 2020).
Grains and Grasses
Grasses and grains make up the Poaceae family and are useful for a variety of ecosystem services. As mentioned previously, buckwheat is a warm season grain crop that flowers at a high density and supports pollinators (Mallinger et al. 2019). Grains are efficient catch crops, or N scavengers, because of their high levels of biomass production (Ramirez-Farcia et al. 2015). In the experiment completed by Ramirez-Farcia et al. (2015), rye performed the best in terms of biomass production out of the cover crop species tested, in comparison with mustard, different species of triticale (x Triticosecale) (a type of grain crop), and numerous varieties of vetch and barley (Hordeum vulgare). Rye also took up a large amount of N in the experiment (Ramirez-Farcia et al. 2015). As a cold hardy, cool season grain as depicted in Figure 2c, rye is suitable for conserving N through the winter, giving it an advantage against mustards which are also good N scavengers but not cold hardy (Ramirez-Farcia et al. 2015; Chapagain et al. 2020). High biomass production from grain crops also contributes to a lot of residue which makes for good weed suppression while also adding soil organic matter (Clark 2007). With a high C:N ratio, meaning containing high amounts of carbon relative to nitrogen, grass and grain residues also break down slowly so weed suppression capabilities are relatively long-lasting compared to legume crops which have a lower C:N ratio (Clark 2007). The downside to slow decomposition is that the N taken up by grasses may be unavailable to the following cash crop (Clark 2007), but this issue can be avoided by pairing a grass crop with a legume crop in a cover crop mix, which fixes N that is more quickly available. Grasses and grains are also effective at preventing soil erosion, because their roots establish quickly to stabilize soil, and they produce a lot of biomass above ground which slows water movement (Chapagain et al. 2020).
Conclusion
This chapter on cover crops focused on a variety of the ecosystem services provided by cover crops that align with the values of organic agriculture concerning input reduction, soil health, and the environment. It went on to explain the main factors to consider for single and multi-mix cover crop selection in organic farming, namely the importance of determining an objective and a timeframe, but also details like seed availability and complementarity of species in a multi-mix. Finally, there was an overview of legumes, brassicas, and grasses – the three main families of cover crops. Understanding the different roles cover crops play in the organic agroecosystem, and how they can work together is important in any farming context, but especially for an ecological approach. Cover crops have been proven through research to be a positive addition to the agroecosystem but have an increased impact in organic systems. Although there is still a need for research regarding mixing cover crops, there is no doubt that they will be an integral tool as society attempts to adapt to and mitigate climate change in the agri-food system.
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