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COOL-SEASON COVER CROPPING STRATEGIES FOR ORGANIC GROWERS
David M. Butler*, Assistant Professor
Sarah Eichler Inwood and Heather Toler, Research Associates
Cody Fust, Research Assistant
Department of Plant Sciences,
The University of Tennessee
Cover crops are the primary soil fertility and soil management tool available to organic farmers. If properly managed, cover crops can provide a wealth of services to organic cropping systems, including nitrogen fixation, increased soil organic matter, increased soil biological activity, solubilization of less soluble nutrients such as phosphorus, scavenging of soluble nutrients remaining after cash crop harvest, prevention of soil erosion, improvement of soil structure and soil physical properties, protection of water quality, habitat and nectar for beneficial organisms, suppression of soilborne diseases, and weed suppression. With improper management, cover crops can interfere with cash crop establishment and growth, become weedy, or increase pest pressure. While cover crops are grown primarily for cropping system improvement rather than for market, it can be possible in some systems to utilize cover crops (especially for late fall or early spring grazing or forage harvest) without a substantial negative impact on cover crop utility. While there are numerous cool-season annual cover crops available to producers in Tennessee and neighboring states, proper selection of cover crop species and cultivar, whether in monoculture or mixture, as well as proper management, is critical to obtaining the desired services from cover crop use.
Choosing a cover crop.
Miles and Brown (2008) detailed three basic steps for choosing a cover crop which work well regardless of farming system. The first step is to identify the function that is needed from a cover crop. This is best answered by determining what is limiting crop production in a given system. For example, an organic grower may be limited by low nitrogen fertility, which would suggest use of certain cover crops (legumes) over others. The second step is to identify and classify the niche that is available within a given crop rotation to integrate the cover crop. Generally, we think of these niches as either warm-season (frost-free) or cool-season. Here, we focus on cool-season cover crops which fill a niche which spans mid-fall to mid-spring in Tennessee. It is also important to consider other factors within a given niche, for example, with cool-season cover crops it is important to consider species cold-hardiness in your region, if precipitation is adequate to support cover crop productivity, and if typical weather conditions will allow for cover crop maturity within the desired time period prior to cash crop planting. Likewise, consider previous and subsequent crops in rotation and how the conditions created by the previous crop and potential needs of the subsequent crop can inform cover crop choice. Finally, select a cover crop that meets the goals and requirements of the first two steps and then consider the benefits and drawbacks of those cover crops meeting those criteria, as well as the costs (seed, management, etc.) for those given cover crops.
Cool-season cover crops for Tennessee.
We can generally classify the well-adapted cool-season annual cover crops for Tennessee as either legumes, grasses, or brassicas, with the various cover crops in each of these classes generally providing similar services to cropping systems.
Legumes. The legume family of plants is important in organic agriculture due to the symbiotic relationship these plants form with certain soil bacteria (rhizobia) which allows for nitrogen fixation. This nitrogen, fixed from gaseous nitrogen in the atmosphere which is unavailable to plants directly, then potentially becomes available to crops within a given cropping system. In Tennessee, the most adapted and most available annual cool-season legume cover crops include crimson clover (Trifolium incarnatum), hairy vetch (Vicia villosa) and winter pea (Pisum sativum; Table 1). These cover crops are typically planted in mid-September to late October, work well mixed with small grain cover crops (see following section), and mature to flowering from mid-April to mid-May. Typically, crimson clover will mature earlier than vetch or winter pea, meaning that it can be terminated mechanically (e.g., by a roller-crimper or mowing) earlier in the season, helping to facilitate more timely planting of early summer crops. Within crimson clover cultivars, ‘AU Sunrise’ and ‘AU Robin’ are two earlier maturing cultivars (about 1 to 2 weeks), compared to the widely available cultivar ‘Dixie,’ but seed is typically less widely available. Hairy vetch cultivars likewise differ in flowering times, with two cultivars that typically flower earlier than most other hairy vetch varieties being ‘Purple Prosperity’ and ‘Purple Bounty.’ Two other vetch species also well-adapted to Tennessee cropping systems are common vetch (Vicia sativa) and woolypod vetch ‘Lana’ (Vicia dasycarpa), with woolypod vetch maturing earlier in the season (typically late April to early May) and common vetch generally later in the season (mid May). White lupin (Lupinus albus) is another well-adapted cool-season cover crop for the region, but seed availability is limited. In trials at the Organic Crops Unit, similar biomass productivity has been observed when these legume cover crops were grown with cereal rye (rye planted at 20 lbs/acre in mixture with legumes; Fig. 1).
Grasses. The grass family of plants contains many common cool-season cover crops used in Tennessee, including the small grain crops such as cereal rye (Secale cereale), common wheat (Triticum aestivum), barley (Hordeum vulgare), common oats (Avena sativa) and triticale (a hybrid of cereal rye and wheat). Another commonly used grass cover crop is annual or Italian ryegrass (Lolium multiflorum), although it is less used in organic agriculture than conventional due to its tendency to become weedy without herbicides for management. The small grains are especially useful as cover crops due to their adaptability to cool season growing conditions, rapid establishment to prevent erosion and take up soil nutrients, an extensive network of fine roots, and high biomass productivity in early spring. Residues of mature small grains also generally resist decay more so than other cover crops, making them appropriate for mulch residue in reduced-tillage systems. Cereal rye, wheat, triticale, and in general, barley, are winter hardy in Tennessee and are typically planted as cover crops from late September to early November. Varieties of common oat and black oat (Avena strigosa) are in general less winter hardy (although more winter-hardy cultivars of common oat are available), and typically do not overwinter in Tennessee. This can be a useful management tool for early spring cash crop planting, as oats can be planted in late summer or early fall to produce substantial biomass and then winter kill, leaving a field covered with residue to prevent erosion, but ready for early spring planting. In general, try to choose small grain varieties for cover crops that were developed for forage use rather than grain use, as these cultivars typically produce far more aboveground biomass, are taller, and have greater rooting volume and thus are typically more suited as cover crops (Table 1).
Brassicas. Several broadleaf plants in the brassica (mustard) family are promoted for use as cover crops in Tennessee, and work well if used appropriately. These cover crops include arugula (Eruca sativa), rape or canola (Brassica napus), mustards (especially Indian mustard, Brassica juncea), turnip (Brassica rapa), and radish (Raphanus sativus). Cold-hardiness varies significantly in these cover crops, both by species and cultivar. In general, the species above are listed from most cold-hardy to least. Generally, arugula, rape and mustard should be planted in the mid-fall to be in the rosette stage with ~ 6 leaves when entering sub-freezing temperatures, which improves winter-hardiness (Table 1). Radishes (oilseed, forage and daikon types; often branded with names such as ‘tillage,’ ‘gopher’ or ‘soilbuster’) are typically planted in late summer to mature prior to winter in the hopes that they will winterkill to reduce management costs, provide aeration to compacted soils, and not interfere with spring planting. Mustards are well-known for their glucosinolate content, a natural compound that when incorporated into the soil creates biofumigant properties that can help to suppress certain soilborne pests. Arugula is known to suppress certain plant-pathogenic nematodes in the soil. As with small grains, select cultivars developed for forage or cover crop use as they will typically produce greater biomass than vegetable-type cultivars.
Cover crop species mixtures.In many cases, producers will be best served through the use of simple, complimentary mixtures of cover crop species. For example, a simple mixture of crimson clover and cereal rye works well because both species have similar planting and maturity dates, the crimson clover provides nitrogen fixation, and the cereal rye grows quickly to suppress weeds and take up soil nutrients. When terminated in the spring, the crimson clover residue decomposes rapidly to provide nitrogen, whereas the high biomass cereal rye breaks down more slowly to suppress weeds if at the soil surface or increase soil organic matter if incorporated into the soil. Research has also shown that grasses can help to increase legume nitrogen fixation by continually removing soluble nitrogen from the soil. Research at the Organic Crops Unit has also shown the cover crop biomass benefits of simple mixtures of cereal rye and legumes compared to a cereal rye monoculture (Fig. 1). Another popular mixture is that of cereal rye, crimson clover and hairy vetch. In this case, the vining growth habit of hairy vetch can further help suppress weeds and potentially increase total cover crop biomass. When creating mixtures, try to combine species and cultivars that have similar planting dates and maturity dates in order to simplify management and provide the optimal benefits from each species in the mixture. For example, if using a roller-crimper, combine late flowering legume species with later flowering small grains (such as wheat) so that one species is not setting mature seed while you wait for the other to flower. However, in some cases it may be best to plant a single species cover crop, such as a rye cover crop prior to reduced-tillage soybean; the rye cover crop will lower soil nitrogen and allow the soybean (a legume) to better compete with weeds in the low nitrogen environment. Likewise, a hairy vetch monoculture may work best for reduced-tillage corn when a large amount of nitrogen is required and you want to ensure consistent flowering time for successful killing with a roller-crimper or other mechanical method. In general, it is harder to establish and manage very diverse mixtures of cover crops (more than 3 species), but it is feasible with some management systems.
Perhaps the most important management consideration with cool-season cover crops is to plant at the appropriate time (Table 1) and ensure good seed to soil contact for quick establishment. Planting date is extremely important for most cool-season cover crops as planting too early in the fall can result in a cover crop that matures with reduced biomass prior to the winter or does not survive through the winter, and plantings occurring too late can result in complete stand failure due to lack of germination in cool soils, winter kill of seedlings, and reduced spring cover crop biomass. For more information on planting and cover crop management, see Clark (2008) and Bates et al. (2013).
The authors gratefully acknowledge support for projects with cover crop components, including USDA-NIFA, NSF, UT AgResearch Innovation Program, USDA-SARE, USDA-NRCS-CIG and the department of Plant Sciences, as well as field and lab support from UT’s Organic Crops Unit staff including Lee Ellis and Bill Lively, and Sarah Looper, Will Sublett, and Zach Fox, and seed donations from Blue River Hybrids Organic Seed, Kelley, IA; Welter Seed, Onslow, IA; Tennessee Farmer’s Cooperative, Lavergne, TN; Weaver Seed, Crabtree, OR; Dr. Edzard van Santen, Auburn University; Dr. Fred Allen, University of Tennessee; High Performance Seed (Dale Gies), Moses Lake, WA; Byron Seed (Tim Koeshell), Philadelphia, TN; and CT Smith Co., Pleasanton, TX.
REFERENCES AND RESOURCES
Bates, G.E., C. Harper, and F.L. Allen. 2013. Forage and field crop seeding guide for Tennessee. UT Extension PB 378. University of Tennessee Institute of Agriculture, Knoxville, TN.
Clark, A. (ed.). 2008. Managing cover crops profitably. USDA-Sustainable Agriculture Research and Education program handbook series. Available at www.sare.org.
Miles, A. and M. Brown (eds.). 2003. Teaching organic farming and gardening. CASFS, University of California-Santa Cruz, Santa Cruz, CA.
Snapp, S. et al. 2005. Evaluating cover crops for benefit, costs, and performance within cropping systems niches. Agron J. 97:322-332.
Table 1. Cool-season cover crop species, cultivars, planting rates and planting dates for Tennessee.
|Family||Common name||Scientific name||Some suggested cultivars*||Planting rate (monoculture)**||Cover crop planting dates|
|Crimson clover||Trifolium incarnatum||‘Dixie,’ ‘AU Sunrise,’ ‘AU Robin’||15 lbs./ac.||Mid-September to October|
|Hairy vetch||Vicia villosa||‘Purple Prosperity,’ ‘Purple Bounty’||25 lbs./ac.||Mid-September to October|
|Winter pea||Pisum sativum||‘Austrian Winter,’ ‘Frostmaster,’ ‘Whistler’||60 lbs./ac.||Mid-September to October|
|Cereal rye||Secale cereale||‘Wrens Abruzzi,’ ‘Wheeler,’ ‘Elbon’||100 lbs./ac.||Mid-September to November|
|Common wheat||Triticum aestivum||‘Forage Max’||100 lbs./ac.||Mid-September to October|
|Common oat, black oat||Avena sativa, A. strigosa||‘Jerry,’ ‘Everleaf,’ ‘Forage Plus’||100 lbs./ac.||Mid-August to September or mid-February to mid-March|
|Barley||Hordeum vulgare||‘Valor,’ ‘Nomini’||100 lbs./ac.||Mid-September to October|
|Triticale||X Triticosecale||‘Trical 815’||100 lbs./ac.||Mid-September to early November|
|Arugula||Eruca sativa||‘Nemat’||6 to 8 lbs./ac.||Mid-September to October|
|Rape, canola||Brassica napus||‘Athena’||6 to 8 lbs./ac.||Mid-September to mid-October|
|Mustard||Brassica juncea||‘Caliente 61,’ ‘Pacific Gold’||6 to 8 lbs./ac.||October|
|Turnip||Brassica rapa||‘Appin,’ ‘Pasja’||4 to 5 lbs./ac.||September to October|
|Radish||Raphanus sativus||variable||10 to 15 lbs./ac.||August to September|
**Suggested rate for drilling of monoculture stands; increase seeding rates by about 30 to 40% when broadcasting; reduce seeding rates of legumes by about 25 to 50% when mixing with small grains; reduce seeding rates of small grains by about 80% when mixing with legumes to enhance the proportion of legume biomass.
Figure 1. Cool season cover crop biomass from a trial on a site with low soil nitrogen and phosphorus at the Organic Crops Unit, spring 2014. Cereal rye was planted at 120 lbs./ac. in monoculture and 20 lbs./ac. in mixture with legumes. Bars indicated by the same letter are not significantly different in total biomass.
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