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FutureStarrBest Cover Crops
As harvest approaches, we also think about seeding cover crops, if we have not already. What type of cover crop seed you should use depends on your goals. If your goal is to graze cattle on the cover to extend your grazing days, the seed you choose will be different than if you want to increase the weed control in next year’s crop. Some cover crops are great at taking up excess nutrients that may be found in the field and others are great at breaking up heavily compacted areas of a field. These goals, as well as other factors, will help to determine the best species, or species mix, to use on your farm.Summer Annual – The cover crops in this category are heat-driven and typically are planted early-mid summer. They cannot handle a frost or freeze, so should not be planted in the fall. These are many times planted as a mix and used for grazing, harvested forage, reducing extreme compaction in areas or preventive planting acres. Common summer annuals include sorghum, sudangrass, sorghum-sudan, buckwheat, millet, and sun hemp.Winter rye is a great annual late-season cover crop to plant in the fall or early winter. It can even be planted after the first light frost and still grow tall enough to be a viable cover crop. With its deep root system, it's highly drought resistant and excels at loosening compacted soil. It's often planted in tandem with a legume like clover, which gives a structure for these climbing plants to ascend and provides the next season's crop with nitrogen in the soil.Often, a combination of a grass and a legume is used, since this enhances biomass production and therefore mulch thickness, weed suppression and organic matter inputs. The combination also offers a balanced carbon to nitrogen (C:N) ratio, which gives a gradual release of plant available N, in contrast to the N-immobilization (tie-up) by an all-grass cover, or the rapid N release and potential leaching losses from an all-legume cover. The higher diversity of a two-species cover crop can also enhance allelopathy (suppression of weeds by natural chemical substances from the mulch), diversity of beneficial soil microbes, and nutrient effects. For instance, legumes tend to enhance availability of phosphorus (P), while grasses, especially rye, enhance availability of potassium (K).
In recent years, growers and researchers have begun experimenting with a much wider range of annual cover crop species for no-till vegetables planted at other seasons. Other cool-season annuals like oats and fava beans can be planted in early spring, then killed in mid summer for late plantings of cucumber, bean or summer squash. Summer annual (frost-tender) cover crops like millets, cowpeas or soybeans can be planted after the spring frost date, then knocked down at the end of summer to plant fall brassicas or other fall crops. Finally, cover crops that are not winter-hardy in a given location can be planted in mid to late summer and allowed to winterkill, forming a mulch for no-till spring vegetables. One of the basic tenets of sustainable agriculture is that greater diversity yields greater agro-ecosystem stability, more beneficial organisms, fewer pests and diseases, more sustained crop yields, and more opportunities for farmer innovation. We feel that this is true also of cover crops, and one of the objectives of ongoing research is to develop a larger cover crop “toolbox” from which growers can select cover crops most suited to their regions and production systems. The following table gives some basic information on a number of cover crops, some tried-and-true, and some less-known experimental species. The table is organized into legumes and non-legumes, listed in order of cold-hardiness. The table is intended not as a cover crop prescription for organic no-till vegetables, but as an information resource for farmers and other experimenters to use in selecting cover crop combinations for their specific vegetable crop rotations and cropping systems.Cover crops provide multiple potential benefits to soil health and to the following crops, while also helping to maintain cleaner surface water and groundwater (Figure 10.1). They prevent erosion, improve soil physical and biological properties, supply nutrients to the following crop, suppress weeds, improve soil water availability, and break pest cycles. Some cover crops are able to break into compacted soil layers, making it easier for the following crop’s roots to more fully develop. The actual benefits from a cover crop depend on the species and productivity of the crop you grow and how long it’s left to grow before the soil is prepared for the next crop. In this chapter we focus on the principles of cover cropping, which are more comprehensively discussed in a companion book by the same publisher, SARE, titled Managing Cover Crops Profitably. A five-year experiment with clover in California showed that cover crops increased organic matter in the top 2 inches from 1.3%–2.6% and in the 2- to 6-inch layer from 1%–1.2%. Researchers found, when the results of many experiments were examined together, that including cover crops led to an organic matter increase of 8.5% over original levels and an increase of soil nitrogen by 12.8%. The longer the cover crop grows and the less tillage that is used, the greater the increase in soil organic matter. In other words, the beneficial effects of reduced tillage and cover cropping can be additive, and the combination of practices has greater benefits than using them individually. Low-growing cover crops that don’t produce much organic matter, for example, cereal rye that’s killed before it has much chance to grow in the spring, may not be able to counter the depleting effects of intensive tillage. But even if they don’t significantly increase organic matter levels, cover crops help prevent erosion and add at least some residues that are readily used by soil organisms. (Source: www.sare.org)