Getting started with cover crops
Selection and establishment tips for on-farm research

By Dave Wilson
April 19, 2005

Cover crops offer effective tools for protecting your soil from erosion, building tilth and organic matter (and therefore increasing the microbial community), and delivering key nutrients to your planting beds. Specific cover crops are suitable for a range of applications and for different times of the year. Some are planted in spring, some in the fall, some will overwinter, and others are winter-killed. Some are very easy to work with, while others can be very tenacious, especially if you let them go to seed.

To determine which cover crop species work best overall on your farm and in your crop rotation, establish two or more 10 feet by 50 feet long (or longer) test strips with different cover crop species or mixtures for one season. (A 10 foot by foot strip equals 500 square feet; an acre is 43,560 square feet).

Next, you need to determine the seeding rate for the small plot, based on your per-acre seeding rate required for the cover crop. For example, if the seeding rate of cereal rye is 3 bushels per acre, and a bushel of rye weighs 56 pounds, 3 bushels totals 168 pounds per acre. Take the square-foot measurement of your plot (500ft²) and divide that by the number of square feet in an acre (43,560 ft²). In this example, 500 / 43560 = 0.011468. Use this decimal amount to multiply the number of pounds of cover crop seed required on a per-acre basis. In this example, 0.011468 x 168 pounds rye = 1.926624, which you round off to 1.93 pounds of rye seed for your 500 ft² plot. Therefore, 1.93 lbs of rye seed per your 500ft² plot will be equivalent to the 168 pounds per acre seeding rate.

Use a resource such Managing Cover Crops Profitably (Sustainable Agriculture Network, 1998) to help determine the optimal seeding time and rate for each cover crop. Refer to local extension for bulletins on cover crops commonly grown in your area. Look at other farming systems in your region and refer to local farmers. Many of the crops grown as cover crops are also grown in other systems for other uses, such as the small grains as a cash grain or the legumes and grasses in forage mixtures. When planting cover crops in fall, look at the seven day or longer weather forecasts for your area. Does the seed have enough time to germinate and establish itself before winter in your particular USDA Cold Hardiness Zone? If rain is in the forecast, try to plant beforehand.

For no-till rolling of cover crops, consider the amount of biomass that the cover crop can potentially produce and its typical flowering date in the spring to determine the kill date that will be most effective for rolling mechanically. If you roll too soon— before at least 50 percent of flowering has occurred—not only may you get less biomass, but you won’t get an effective kill because the crop is still in the vegetative state.

Try new cover crop species. Some seed companies’ suppliers will often give a grower a 1- to 5-pound trial sample bag of seed. Rye will produce more biomass than wheat or barley. Sorghum-Sudan grass hybrids, being warm season annuals, produce the most biomass when they are utilized as a summer cover crop. Sorghum-Sudan has also been used in mixes with sesbania, sunn hemp, cow pea, and buckwheat. Hairy vetch, a winter annual legume, produces a natural heavy mat of biomass with a nitrogen content of 3.5 to 4 percent.

Rye: The gold standard of cold-climate cover crops

Of all the small grains, cereal rye has the lowest seed germination temperature and thus can be seeded later in the fall. For this reason, cereal rye offers the most flexibility in rotation systems. Rye is the most winter hardy of all the small cereal grains; its cold tolerance exceeding even that of the hardiest winter wheats. Rye grows taller and, as a result, produces more biomass than wheat or barley.

Popular cultivated varieties of rye include ‘Merced’, ‘Aroostook’ (very cold tolerant), ‘Albion’ (which repels nematodes), ‘Abruzzi’ (the earliest southern variety), ‘Elbon’ (slightly later to bloom) and ‘Rymor’ (a late bloomer). ‘Wheeler’ is known to have allelopathic properties and is typically more expensive. Preliminary research from North Carolina State University suggested that ‘Vita Graze’ and ‘Athens Abruzzi’ may be the varieties of choice for weed suppression in no-till corn.

Few varieties of rye maintain distinct characteristics because they comprise a mixture of types produced by cross pollination. Cereal rye is largely cross pollinated; most plants are self-sterile. A variety grown in a different environment may quickly adapt itself to the new conditions. Changes in characteristics such as cold resistance, early maturation or development, and color are the results of segregation as well as outcrossing with local varieties. Winter rye varieties adapted to the northern states are usually wholly unsuited to conditions in the South, where mild temperatures fail to provide sufficient cold to force such true winter varieties into early heading (vernalization). The non-hardy varieties grown in the south and southwest actually have a partial or total spring habit of growth, flowering and maturing before the hot summer weather.

After planting the cover crops, take notes on emergence, overwintering capability and stand establishment. Now consider the “spatial niches” on your farm. Strip cropping provides a simple method for rotating a cover crop with a vegetable planting. Alternate field strips or beds of a fall- or spring-planted cover crop with strips of early-planted vegetables like potato, onion, cabbage, lettuce or peas. Adjust the width of your fields to accommodate easier cover crop seeding (using your seeding equipment as the standard of measure). Strip cropping is a low-cost, low-input way of getting the benefits of a cover crop.

For agronomic crops planted on a larger acreage, the strip-cropping concept can still apply, but the strips will be large, field-sized strips with the width adjusted for your particular farm planting and harvesting equipment. Rotating cover crops in field strips arranged parallel to the contour slope of a hill makes good agronomic sense; these rotated cover crop strips will prevent soil erosion over time. Even where the topography is largely flat, cover crop strips provide other added benefits—such as attracting beneficial insects, providing crop diversification, and buffering effects—which can all help break the rapid spread of disease and insect epidemics that often plague large monoculture cropping systems.

Cover crop systems do require time, money and management. If not properly managed, cover crops can interfere with vegetable or field crops. Find the least expensive cover that meets your goals. Low cost cover crops like cereal rye can be seeded for around $21 per acre while higher value cover crops such as vetch can cost upwards of $60 per acre. To save time and money, prepare the soil minimally prior to seeding. Conventional seedbeds that are plowed, disked and harrowed require time and labor. Consider no-till planting of the cover crop itself, as this practice will keep higher amounts of crop residue on the soil surface and save time, labor and fuel.

Know the characteristics and organic matter content of your soil. In some soils, cover crops allow for earlier field entry and planting in the spring. Cover crops such as alfalfa, barley and white clover require good soil drainage (additionally, barley requires higher nitrogen levels in the fall for tillering. (Tillers are branches or shoots of the main stem of grasses. When the small grains (Rye, Barley or Wheat) are grown as cover crops, more tillers means more potential biomass produced from the plant.)

For wet sites, use alsike clovers or winter rye. For low-fertility sites with low pH, use pearl millet, barley, alsike clovers or birdsfoot trefoil.

Think about the big picture. How will you get your cover crop established? What type and size of drill is available? What are the speed-ratio drive settings of the gearing for the seeders? Do you need to make adjustments? Check the drill manual to determine the proper fluted feed opening required for your particular cover crop seed. As you try new cover crops some drills may not have a “notch” setting for all the cover crop seed you’re using. For example, the John Deere grain drill on our farm does not have a setting for rapeseed. So we planted rapeseed using the grass seed box of the drill, calibrating it to the crimson clover or pearl millet setting (both have a seed size similar to rapeseed). With a little tinkering, we’ve found that there’s usually a solution close at hand.

The biodiversity you bring into your system with good crop rotation delivers many benefits, including improved yields, reduced and often prevented disease transmission, insect control, weed suppression, soil nitrogen management, improving soil tilth and structure, improved water utilization and reduced soil erosion. Cover crops are key in any rotation and, in the case of organic farming, are often the underlying drivers of the system.

Dave Wilson is a research agronomist with The Rodale Institute.