The four comprehensive charts that follow can help orient you to the major cover crops most appropriate to your needs and region. Bear in mind that choice of cultivar, weather extremes and other factors may affect a cover crop’s performance in a given year.
Chart 1: Top Regional Cover Crop Species
This chart lists up to five cover crop recommendations per broad bioregion for six different major purposes: N Source, Soil Builder, Erosion Fighter, Subsoil Loosener, weed Fighter and Pest Fighter. If you know your main goal for a cover crop, Chart 1 can suggest which cover crop entries to examine in the charts that follow and help you determine which major cover narrative( s) to read first.
Disclaimer. The crops recommended here will not be the most successful in all cases within a bioregion, and others may work better in some locations and in some years. The listed cover crops are, however, thought by reviewers to have the best chance of success in most years under current management regimes.
Chart 2: Performance and Roles
This chart provides relative ratings (with the exception of two columns having quantitative ranges) of what the top covers do best, such as supply or scavenge nitrogen, build soil or fight erosion.
Seasonality has a bearing on some of these ratings. A cover that grows best in spring could suppress weeds better than in fall. Unless otherwise footnoted, however, the chart would rate a cover’s performance (relative to the other covers) for the entire time period it is likely to be in the field. Ratings are general for the species, based on measured results and observations over a range of conditions. The individual narratives provide more seasonal details. The added effect of a nurse crop is included in the “Weed Fighter” ratings for legumes usually planted with a grain or grass nurse crop.
Column headings
Legume N Source. Rates legume cover crops for their relative ability to supply fixed N. (Nonlegumes have not been rated for their biomass nitrogen content, so this column is left blank for nonlegumes.)
Total N. A quantitative estimate of the reasonably expected range of total N provided by a legume stand (from all biomass, above and below ground) in lb. N/A, based mostly on published research. This is total N, not the fertilizer replacement value. Grasses have not been rated for their biomass nitrogen content because mature grass residues tend to immobilize N. Brassicas are less likely to immobilize N than grasses.
Dry Matter. A quantitative estimate of the range of dry matter in lb./A /yr., based largely on published research. As some of this data is based on research plots, irrigated systems or multicut systems, your on-farm result probably would be in the low to midpoint of the dry matter range cited. This estimate is based on fully dry material. “Dry” alfalfa hay is often about 20 percent moisture, so a ton of hay would only be 1,600 lb. of “dry matter.”
N Scavenger. Rates a cover crop’s ability to take up and store excess nitrogen. Bear in mind that the sooner you plant a cover after main crop harvest— or overseed a cover into the standing crop—the more N it will be able to absorb.
Soil Builder. Rates a cover crop’s ability to produce organic matter and improve soil structure. The ratings assume that you plan to use cover crops regularly in your cropping system to provide ongoing additions to soil organic matter.
Erosion Fighter. Rates how extensive and how quickly a root system develops, how well it holds soil against sheet and wind erosion and the influence the growth habit may have on fighting wind erosion.
Weed Fighter. Rates how well the cover crop outcompetes weeds by any means through its life cycle, including killed residue. Note that ratings for the legumes assume they are established with a small-grain nurse crop.
Good Grazing. Rates relative production, nutritional quality and palatability of the cover as a forage.
Quick Growth. Rates the speed of establishment and growth.
Lasting Residue. Rates the effectiveness of the cover crop in providing a long-lasting mulch.
Duration. Rates how well the stand can provide long-season growth.
Harvest Value. Rates the cover crop’s economic value as a forage (F) or as a seed or grain crop (S), bearing in mind the relative market value and probable yields.
Cash Crop Interseed. Rates whether the cover crop would hinder or help while serving as a companion crop.
Chart 3 A: Cultural Traits
This chart shows a cover crop’s characteristics such as life cycle, drought tolerance, preferred soils and growth habits. The ratings are general for the species, based on measured results and observations over a range of conditions. Choice of cultivar, weather extremes and other factors may affect a cover crop’s performance in a given year.
Column headings
Aliases. Provides a few common names for the cover crop.
Type. Describes the general life cycle of the crop.
B = Biennial. Grows vegetatively during its first year and, if it successfully overwinters, sets seed during its second year.
CSA = Cool-Season Annual. Prefers cool temperatures and depending on which Hardiness Zone it is grown in, could serve as a fall, winter or spring cover crop.
SA = Summer Annual. Germinates and matures without a cold snap and usually tolerates warm temperatures.
WA = Winter Annual. Cold-tolerant, usually planted in fall and often requires freezing temperatures or a cold period to set seed.
LP = Long-lived Perennial. Can endure for many growing seasons.
SP = Short-lived Perennial. Usually does not persist more than a few years, if that long.
Hardy Through Zone. Refers to the standard USDA Hardiness Zones. See map on inside front cover. Bear in mind that regional microclimate, weather variations, and other near-term management factors such as planting date and companion species can influence plant performance expectations.
Tolerances. How well a crop is likely to endure despite stress from heat, drought, shade, flooding or low fertility. The best rating would mean that the crop is expected to be fully tolerant.
Habit. How plants develop.
C = Climbing
U = Upright
P = Prostrate
SP = Semi-Prostrate
SU = Semi-Upright
pH Preferred. The pH range in which a species can be expected to perform reasonably well.
Best Established. The season in which a cover crop is best suited for planting and early growth. Note that this can vary by region and that it’s important to ascertain local planting date recommendations for specific cover crops. Season: F = Fall ; Sp = Spring; Su = Summer; W =Winter Time: E = Early; L = Late; M = Mid
Minimum Germination Temperature. The minimum soil temperature (F) generally required for successful germination and establishment.
Chart 3B: Planting
Depth. The recommended range of seeding depth (in inches), to avoid either overexposure or burying too deeply.
Rate. Recommended seeding rate for drilling and broadcasting a pure stand in lb./A, bu/A. and oz./100 sq. ft., assuming legal standards for germination percentage. Seeding rate will depend on the cover crop’s primary purpose and other factors. See the narratives for more detail about establishing a given cover crop. Pre-inoculated (“rhizo-coated”) legume seed weighs about one third more than raw seed. Increase seeding rate by one-third to plant the same amount of seed per area.
Cost. Material costs (seed cost only) in dollars per pound, based usually on a 50-lb. bag as of fall 2006. Individual species vary markedly with supply and demand. Always confirm seed price and availability before ordering, and before planning to use less common seed types.
Cost/A. Seed cost per acre based on the midpoint between the high and low of reported seed prices as of fall 1997 and the midpoint recommended seeding rate for drilling and broadcasting. Your cost will depend on actual seed cost and seeding rate. Estimate excludes associated costs such as labor, fuel and equipment.
Inoculant Type. The recommended inoculant for each legume. Your seed supplier may only carry one or two common inoculants. You may need to order inoculant in advance. See Seed Suppliers.
Reseeds. Rates the likelihood of a cover crop reestablishing through self-reseeding if it’s allowed to mature and set seed. Aggressive tillage will bury seed and reduce germination. Ratings assume the tillage system has minimal effect on reseeding. Dependable reseeding ability is valued in some orchard, dryland grain and cotton systems, but can cause weed problems in other systems. See the narratives for more detail.
Charts 4A and 4B
These charts provide relative ratings of other management considerations—benefits and possible drawbacks—that could affect your selection of cover crop species.
The till-kill rating assumes tillage at an appropriate stage. The mow-kill ratings assume mowing at flowering, but before seedheads start maturing. See sectional narratives for details.
Ratings are based largely on a combination of published research and observations of farmers who have grown specific covers. Your experience with a given cover could be influenced by site-specific factors, such as your soil condition, crop rotation, proximity to other farms, weather extremes, etc.
Chart 4A: Potential Advantages
Soil Impact. Assesses a cover’s relative ability to loosen subsoil, make soil P and K more readily available to crops, or improve topsoil.
Soil Ecology. Rates a cover’s ability to fight pests by suppressing or limiting damage from nematodes, soil disease from fungal or bacterial infection, or weeds by natural herbicidal (allelopathic) or competition/smothering action. Researchers report difficulty in conclusively documenting allelopathic activity distinct from other cover crop effects, and nematicidal impacts are variable, studies show. These are general, tentative ratings in these emerging aspects of cover crop influence.
Other. Indicates likelihood of attracting beneficial insects, of accommodating field traffic (foot or vehicle) and of fitting growing windows or short duration.
Chart 4B: Potential Disadvantages
Increase Pest Risks. Relative likelihood of a cover crop becoming a weed, or contributing to a likely pest risk. Overall, growing a cover crop rarely causes pest problems, but certain cover crops may contribute to particular pest, disease or nematode problems in localized areas, for example by serving as an alternate host to the pest. See the narratives for more detail.
Readers note the shift in meaning for symbols on this chart only.
Management Challenges. Relative ease or difficulty of establishing, killing or incorporating a stand. “Till-kill” refers to killing by plowing, disking or other tillage. “Mature incorporation” rates the difficulty of incorporating a relatively mature stand. Incorporation will be easier when a stand is killed before maturity or after some time elapses between killing and incorporating.