Virginia Cover Crops Fact Sheet Series No. 2: Cover Crop Performance Evaluation in Field and Controlled Studies

Introduction

Cover crops increase soil organic matter, reduce erosion, suppress weeds, forage for nutrients, and reduce fertilizer costs (Clark, 2007). Cover crop species vary greatly and provide varied benefits. Performance evaluation of cover crop species and mixtures is needed in Virginia. The species of interest that we evaluated were:

Early cover hairy vetch (Vicia villosa) is a summer and winter annual legume. It is a N source, weed suppressor, topsoil conditioner, and erosion reducer. Vetch is described as a “semi-viney legume with tendrils; plants hairy; stems 3-5’ long; flowers bluish violet and white” with approximately 21,000 seeds per pound with test weights of 60 pounds per bushel (Abaye et al., 2000).

Austrian winter pea (Pisum arense) is a summer and winter annual legume (winter crop on the East coast and South region US) resembling garden pea with purple flowers. The abundant hollow-stemmed vining forage can be tilled and decompose easily, providing a quick source of N. It also acts as a weed suppressor and a forage. There are approximately 5,000 seeds per pound with test weights of 60 pounds per bushel (Abaye et al., 2000; Clark, 2007).

Common vetch (Vicia sativa L.) is a viney cool season annual legume with leaves composed of 4 to 10 paired leaflets and a taproot that can reach depths of 3 to 50 feet. Common vetch produces purple and white flowers and seeds develop in small brown pods. It accumulates N, suppresses weeds, and is moderately resistant to cold temperatures (Sattell et al., 1998).

Persian clover (Trifolium resupinatum L.) is a winter annual legume. It has non-hairy, oval leaflets with toothed margins, and small pink flowers that produce olive green to purple seeds. It is good to use for grazing, however, it is sensitive to weed competition (Hoover and Duiker, 2009; Mississippi State University, 2010).

Indian head lentil (Lens culinaris Medik) is a specialty lentil for cover crop use. Lentils are a cool season annual crop with compound leaves (4 to 7 pairs of leaflets) with a tendril at the tips. At flowering, “each flower produces a short pod containing one or two lens-shaped seeds. Flowers can be white, lilac or pale blue in color and are self-pollinated. At maturity plants tend to lodge because of their weak stems.” “The seeds (2 to 7 mm in diameter) come in colors of tan, brown, or black, and some varieties produce purple or black mottled seeds. Lentil seed number varies from 15,600 to 100,000 seeds/lb” (Oplinger, et al., 1990).

Yellow sweet blossom clover (Melilotus officinalis, Melilotus alba) is a biennial summer or winter annual legume. It can help build the soil, act as a fertility source, subsoil aerator, weed suppressor, and erosion preventer. This tall-growing, drought-tolerant plant has a taproot that can extract nutrients that have moved lower in the soil profile. It produces vegetative growth up to 24 inches in the establishment year. Plants can reach 8 feet tall and features bracts of tiny blooms through the second year. Abaye et al. (2000), describes yellow sweet blossom clover as “erect with many branches; deep taproot; stems grow from crown second year; yellow or white flowers; 2-5’ tall, leaflets notched on edges toward tips (unlike alfalfa with smooth edges). Plants and flowers have a sweetvanilla odor.” There are approximately 5000-8000 seeds per pound with test weights of 26-30 pounds per bushel(Abaye et al., 2000).

Arrowhead clover (Trifolium vesiculosum) is a drought tolerant annual winter legume with non-hairy, arrowhead-shaped leaves with pronounced veins and a white “V” marking. Blooms are white that can turn pink or purple. The stems are hollow, and fibrous at maturity. Arrowhead clover is often used for grazing (Ball et al., 2005).

Subterranean clovers (Trifolium subterraneum, T. yanninicum, T. brachycalcycinum) are reseeding cool season annual legumes. They aid in weed and erosion suppression, act as a N source, and provide a mulch and continuous orchard floor cover. They generally pile up their biomass in a compact layer close to the ground (Clark, 2007).

Crimson clover (Trifolium incarnatum) is a winter or summer annual legume. It grows rapidly and provides early season N for full-season crops, builds soil, and prevents erosion. Crimson clover has a “central taproot with many fibrous roots; 3 leaflets per leaf; stem and leaves hairy; pointed, conical flower at top of stem is bright crimson color; plants [stand] 1-3’ tall.” There are approximately 150,000 seeds per pound with test weights of 60 pounds per bushel (Abaye et al., 2000).

Sweet lupin – Lupins are cool-season annual legumes that provide N and have aggressive taproots. “Sweet” varieties, as opposed to “bitter” types, refer to lower concentrations of naturally occurring alkaloids. White lupin (Lupinus albus L.) and blue or narrow-leaf lupin (Lupinus angustifolius L.) are grown in the southeastern US (Clark, 2007).

Forage radish (Raphanus sativus) is a cool season annual. It is known for the taproot which can capture nutrients that have moved to deep depths in the soil profile. The plant can grow to a height of 2-3 ft (Clark, 2007).

Phacelia (Phacelia tanacetifolia) is an annual broadleaf that is native to the US and being reintroduced to be used as a cover crop. It is a heavy biomass producer and its flowers attract pollinating insects (Hoover and Duiker, 2009).

Rye (Secale cereale) is a cool season annual cereal grain. This hardy quick-growing cereal will scavenge for excess N, prevent erosion, add organic matter, and suppress weeds. “Seedings often have a reddish coloration; leaves have small auricles with short ligules; seeds are round with the germ-end distinctly pointed; seed color varies from greenish gray and tan to dark brown or black.” There are approximately 18,000 seeds per pound with test weights of 56 pounds per bushel (Abaye et al., 2000).

Barley (Hordeum vulgare) is a cool season annual cereal grain. It can be grown to prevent erosion, suppress weeds, scavenge excess nutrients, and add organic matter. “Leaves are green with long clasping auricles and a long ligule. Seed usually contains the husk (lemma and palea) that gives the seed a wrinkled appearance. Newer varieties maybe ‘hulless’ since the lemma and palea are removed at harvest.” There are approximately 13,000 seeds per pound with test weights of 48, 57.6, and 60 pounds per bushel for hulled, hulless for feed, and hulless for human consumption varieties, respectively (Abaye et al., 2000).

Spring oat (Avena sativa) is a cool season annual cereal. This quick-growing upright annual grass can suppress weeds, prevent erosion, scavenge excess nutrients, add biomass, and act as a nurse crop (Clark, 2007). “Panicle type head; long ligule, auricles absent; leaf margins are heavy; seed usually retains the husk (lemma and palea), which has a very smooth surface; seed color varies with variety from white, yellow, gray to somewhat red. Winter oats require a period of cold temperature to initiate heading. Spring oats have no temperature requirement.” There are approximately 14,000 seeds per pound with test weights of 32 pounds per bushel (Abaye et al., 2000).

Wooly pod vetch (Lana) (Vicia villosa ssp. dasycarpa) is a cool season annual that is a faster-growing alternative to hairy vetch in Hardiness Zone 7 and warmer. It is a good N source, weed suppressor, erosion preventer, adds organic matter to soil and attracts bees. “Wooly pod vetch has slightly smaller flowers than hairy vetch, and its seeds are more oval than the nearly round seeds of hairy vetch” (Clark, 2007). 

Canola (or rape), has two species commonly grown: Brassica napus and Brassica rapa. Brassica napus is a cool season annual in the mustard family with large dark green leaves. Brassicas help prevent erosion, suppress weeds and soil born pests, alleviate soil compaction via taproots, and scavenge nutrients. “At maturity it reaches a height of 3-6’ with brilliant yellow flowers and pods that produce 15-40 small black seeds.” There are approximately 160,000 seeds per pound with test weights of 50 pounds per bushel (Abaye et al., 2000).

Ryegrass (Lolium multiflorum), also known as Italian ryegrass, is a cool season annual grass. It is a quick growing, non-spreading bunch grass used to prevent erosion, improve soil structure and drainage, add organic matter, suppress weeds, and scavenge for nutrients. It has shiny, smooth leaves rolled in the bud, with long and narrow auricles, a short ligule, and spikelets edgewise on the stem with awns on the seed. There are approximately 227,000 seeds per pound with test weights of 24 pounds per bushel (Abaye et al., 2000; Clark, 2007).

Controlled environment chamber trials

In order to compare growth rate of various cover crop species under uniform conditions, seeds of 13 of the cover crop listed in Table 1 were planted into potting media in four-inch pots, watered to 85% field capacity and placed in a growth chamber in four replications. The chamber was set to deliver a 12 hour day/12 hour night light regime and day/night temperatures were set to mimic those experienced in eastern Virginia in late September with 75° days and 55° nights. Growing degree days (GDD, Celsius) with a base of 4°C were calculated and when accumulated GDD reached 200, 400, 600, and 800, pots were removed and all aboveground plant growth clipped at the soil level, dried, and weighed to determine biomass accumulation. Initial growth rate was greatest for forage radish, canola and phacelia. It should be noted, however that temperatures were maintained at the 75/55° level throughout the study and that growth rate of some of these species would slow dramatically if temperatures in the chambers would have been decreased over time. Among the cereal grains, barley and rye produced the greatest biomass by 400 GDD. Most of the legume cover crops accumulate little (less than 250 lb/ac) biomass prior to accumulating 400 GDD.

Field trials

Field trials were conducted at five locations in Virginia; three western sites (Blacksburg, Mauzy, and Edinburg) and two eastern sites [New Kent County and the Virginia State University (VSU) Randolph Farm in Petersburg, VA] during Winter/Spring of 2010-2011 and 2011-2012. Plots were planted no-till into corn grain or silage stubble at seeding rates indicated in Table 2.

Cover crop species and mixtures were evaluated under rain-fed conditions and no fertilizer applications. In 2010/2011, 25 treatments were evaluated in the western trials. The treatments containing ryegrass were not included in the eastern trials, resulting in 22 treatments. In 2011/2012, some treatments were eliminated, while a spring oat+barley treatment was added, resulting in 20 and 17 treatments in the western and eastern sites, respectively. The experiment was a randomized complete block design with three replications at each location.

Two harvests were conducted to collect biomass; the first harvest (Winter) occurred in December and the second harvest occurred in March/April (Spring), depending on the site. The objective in every case was to manage the trials in the same manner as the farmer, so the spring harvest timing occurred just prior to terminating the cover crop. The clipped aboveground biomass was dried in a forced-air oven at 60°C for 48 hours, and ground to pass a 2mm screen with a Wiley (Thomas Scientific, Swedesboro, NJ) sample mill. Nitrogen uptake was measured from a ground subsample of the dried biomass using an automatic CN analyzer (Leco Corp, St. Joseph, MI).

Results

Field Trials

The Eastern sites in 2011 (Table 3) resulted in treatment mixtures of barley+crimson clover+wooly pod vetch+Austrian winter pea+tillage radish+canola and rye+wooly pod vetch+Austrian winter pea+tillage radish producing the greatest amounts of dry matter and N uptake in both the winter and spring. These treatments were similar to other treatments in biomass production and N uptake in the winter and spring; however, only these treatments resulted in consistently high numbers. Generally, treatments with smaller biomass production also resulted in lower N uptake. This occurred in the winter and spring for Austrian winter pea, common vetch, Persian clover, Indian head lentil, yellow sweet blossom clover, arrowhead clover, and subterranean clover. Smaller biomass production and N uptake was also observed in crimson clover, sweet lupins, tillage radish, and phacelia in the spring.

The Western sites in 2011 (Table 4) resulted in treatment mixtures of rye+wooly pod vetch+Austrian winter pea+tillage radish and rye+ryegrass+tillage radish producing the greatest amounts of dry matter and N uptake in both the winter and spring. These treatments were similar to other treatments in biomass production and N uptake in the winter and spring; however, only these treatments resulted in consistently high numbers. In the winter, all of the treatments with a mixture of cover crops resulted in the greater amounts of biomass and N uptake than other single-crop treatments in winter. Similar to results in the Eastern sites in 2011, common vetch, Persian clover, Indian head lentil, yellow sweet blossom clover, arrowhead clover, and subterranean clover resulted in the lowest biomass and N uptake over the winter and spring.

Treatments with mixed species resulted in greater biomass in the winter of the Eastern sites in 2012 (Table 5). Several single-species treatments were similar to multi-species treatments in biomass production including crimson clover, sweet lupins, tillage radish, rye, barely, and spring oats. No differences in N uptake were observed in the winter, 2012. In the spring of 2012, there were no differences in biomass production; however, differences in N uptake were observed. Early cover hairy vetch and wooly pod vetch resulted in the greatest N uptake, which were similar in N uptake to common vetch, crimson clover, rye, barley, barley+crimson clover+tillage radish, rye+wooly pod vetch+Austrian winter pea+tillage radish, and wooly pod vetch+Austrian winter pea+tillage radish+canola.

On the Western sites in 2012 (Table 6), early cover hairy vetch, Austrian winter pea, and common vetch resulted in the lowest biomass production and N uptake in both the winter and spring. Multi-species treatments of barley+crimson clover+tillage radish, rye+ryegrass+tillage radish, spring oat+barley, and ryegrass+crimson clover+wooly pod vetch+Austrian winter pea+tillage radish+canola, along with the single-species barley treatment resulted in higher rates of biomass and N uptake than other treatments in both winter and spring.

References

Abaye, A.O., D. Whitt, S. Umberger, C.W. Swann, C.C. Stallings, et al. 2000. Agronomy Handbook. Publication 424-100. Virginia Coop. Ext., Virginia Tech, Virginia State University, Blacksburg, VA.

Clark A. 2007. Managing cover crops profitability. 3rd ed. Handbook series book 9. Sustainable Agriculture Research and Education, Beltsville, MD.

Ball, D.M., G.D. Lacefield, and C.S. Hoveland. 2005. Arrowleaf Clover. Circular 05-1. Oregon Clover Commission, Salem, Oregon.

Hoover, R., and S. Duiker. 2009. On-farm research investigated new cover crop options. Penn State Extension. http://extension.psu.edu/plants/sustainable/news/2009/10/5covercrop (accessed 13 Sept. 2013).

Mississippi State University. 2010. Mississippi forages: Persian clover (Trifolium resupinatum). Mississippi Agricultural and Forestry Experiment Station, Mississippi State University Ext. Serv. http://msucares.com/crops/forages/legumes/cool/persianclover.html (accessed 13 September 2013).

Opplinger, E.S., L.L. Hardman, A.R. Kaminski, K.A. Kelling, and J.D. Doll. 1990. Lentil. In: Alternative field crops manual. University of Wisconsin Cooperative Extension Service, the University of Minnesota Extension Service, and the Center for Alternative Plant and Animal Products. http://www.hort.purdue.edu/newcrop/afcm/lentil.html (accessed 13 Sept. 2013).

Sattell, R., R. Dick, J. Luna, D. McGrath, and E. Peachy. 1998. Common vetch (Vicia sativa L.). EM 8695. Oregon State University. 
http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/15229/em8695.pdf (accessed 13 Sept. 2013).