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

ID

CSES-121NP (SPES-240NP)

Authors as Published

Cathy Fleming and Wade Thomason, Dept. of Crop & Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 24061

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.

Table 1. Cover crop dry matter growth in response to GDD accumulation under controlled conditions.

 Cover Crop 200 GDD   400 GDD   600 GDD   800 GDD
Species --Aboveground dry matter, lb/ac--
Australian Winter Pea 42 402 2012 5705
Barley 24 617 2318 5580
Canola 41 884 3307 5006
Common Vetch 21 234 1289 3855
Crimson Clover 23 237 946 3106
Early Cover Hairy Vetch 17 226 1666 3673
Phacelia 19 825 2426 4521
Rye 35 425 2394 5654
Ryegrass 5 158 1524 5049
Spring Oats 14 381 1642 5424
Sweet Lupin 36 253 648 1460
Forage Radish 136 1304 4142 7175
Woolypod Vetch 27 417 1775 5322
Mean 34 489 2007 4733
LSD (0.05) 23 241 647 1159

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.

Table 2. Treatments and seeding rates for cover crop species.

  Cover Crop  Seeding Rate (lb/ac)
Early cover hairy vetch 20
Austrian winter pea 35
Common vetch 35
Persian clover 5
Indian head lentil 25
Yellow sweet blossom clover 5
Arrowhead clover 5
Subterranean clover 15
Crimson clover 15
Sweet lupin 50
Forage radish 8
Phacelia 8
Rye 113
Barley 96
Spring oat 64
Ryegrass 20
Wooly pod vetch (Lana) 20
Barley+crimson clover+forage radish 48+20+5
Rye+wooly pod vetch+Austrian winter pea+forage radish 56+13+13+5
Rye+ryegrass+forage radish 84+20+5
Spring oat+canola 32+4
Spring oat+forage radish 64+5
Barley+crimson clover+wooly pod vetch+Austrian winter pea+ forage radish+canola   20+8+8+8+3+3
Ryegrass+crimson clover+wooly pod vetch+Austrian winter pea +forage radish+canola   20+8+8+8+3+3  

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.

Table 3. Dry matter (DM) biomass and N uptake for 2011 Eastern sites (New Kent County and the VSU Randolph Farm in Petersburg).

  WINTER SPRING
SPECIES/MIXTURE DM BIOMASS N UPTAKE DM BIOMASS N UPTAKE
  ---------LB/AC--------- ---------LB/AC---------
Early cover hairy vetch 130 g† 4 f 2394 cd 63 bc
Austrian winter pea 942 bcdefg 18 def 707 e 19 d
Common vetch 846 bcdefg 16 def 536 e 16 d
Persian clover 299 efg 5 f 262 e 5 d
Indian head lentil 643 cdefg 16 def 0 e 0 d
Yellow sweet blossom clover 343 defg 8 ef 872 e 17 d
Arrowhead clover 144 fg 2 f 249 e 5 d
Subterranean clover 281 efg 4 f 258 e 5 d
Crimson clover 1126 abcde 19 cdef 995 e 27 d
Sweet lupin 693 cdefg 19 cdef 1133 e 23 d
Forage radish 1689 ab 53 a 206 e 4 d
Phacelia 1219 abc 31 bcd 236 e 5 d
Rye 1125 abcd 32 bcd 2823 c 62 c
Barley 1364 abc 40 ab 4378 ab 87 bc
Spring oat 1059 abcdef 29 bcd 1215 de 21 d
Wooly pod vetch (Lana) 1305 abc 37 abc 2454 c 81 bc
Barley+crimson clover+forage radish 1111 abcde 31 bcd 3274 bc 67 bc
Rye+wooly pod vetch+Austrian winter pea+forage radish 1335 abc 39 ab 4352 ab 92 ab
Spring oat+canola 931 bcdefg 25 bcde 3574 abc 67 bc
Spring oat+forage radish 1318 abc 38 ab 1071 e 22 d
Barley+crimson clover+wooly pod vetch+Austrian winter pea+forage radish+canola 1812 a 53 a 4576 a 121 a
† Means followed by different lower case letters within a column are significantly different ( alpha = 0.05).

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.

Table 4. Dry matter (DM) biomass and N uptake for 2011 Western sites (Blacksburg, Mauzy, and Edinburg).

  WINTER SPRING
SPECIES/MIXTURE DM BIOMASS N UPTAKE DM BIOMASS N UPTAKE
  ---------LB/AC--------- ---------LB/AC---------
Early cover hairy vetch 830 fgh† 27 defgh 1550 fghi 56 bcdefg
Austrian winter pea 964 efgh 35 cdefgh 1105 ghij 72 bcde
Common vetch 1091 defgh 38 cdefgh 832 ghij 32 defgh
Persian clover 585 h 20 gh 142 j 2 h
Indian head lentil 666 gh 25 efgh 215 j 8 fgh
Yellow sweet blossom clover 145 h 2 h  178 j 4 gh
Arrowhead clover 216 h 5 h 282 j 8 fgh
Subterranean clover 54 h 1 h 124 j 4 gh
Crimson clover 1039 defgh 30 defgh 1934 defg 60 bcdef
Sweet lupin 585 h 20 fgh 420 ij 13 fgh
Forage radish 2671 abc 96 abcde 120 j 5 gh
Phacelia 2662 abc 104 abc 158 j 4 gh
Rye 2831 abc 87 abcdefg 4485 a 134 a
Barley 3650 a 153 a 2622 cdef 86 abc
Spring oat 2241 abcdef 79 bcdefg 667 hij 13 fgh
Ryegrass 1495 bcdefgh 1495 bcdefgh
1698 efgh
43 cdefgh
Wooly pod vetch (Lana) 1321 cdefgh
50 bcdefgh
2010 defg
68 bcde
Barley+crimson+forage radish 3027 a
116 ab
3054 bcd
91 abc
Rye+wooly pod vetch+Austrian winter pea+forage radish 2890 ab
92 abcde
3797 abc 97 ab
Rye+ryegrass+forage radish 2650 abc 97 abcd
3938 ab
106 ab
Spring oat+canola 2560 abcd 91 abcdef 2651 cdef
81 abcd
Spring oat+forage radish 2792 abc 102 abc 1085 ghij 21 efgh
Barley+crimson clover+wooly pod vetch+Austrian winter pea+forage radish+canola 2482 abcde 92 abcde 2874 bcde 77 bcd
Ryegrass+crimson clover+wooly pod vetch+Austrian winter pea+forage radish+canola 2335 abcdef 87 abcdefg 2574 cdef 81 abcd
† Means followed by different lower case letters within a column are significantly different ( alpha = 0.05).

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.

Table 5. Dry matter (DM) biomass and N uptake for 2012 Eastern sites (New Kent County and the VSU Randolph Farm in Petersburg).

  WINTER SPRING
SPECIES/MIXTURE DM BIOMASS N UPTAKE DM BIOMASS N UPTAKE
  ---------LB/AC--------- ---------LB/AC---------
Early cover hairy vetch 250 bcde† 10 NS 750 NS 25 a
Austrian winter pea 215 cde 11 140 5 def
Common vetch 159 de 9 411 17 abcde
Crimson clover 376 abc 10 697 20 abcd
Sweet lupin 481 a 9 465 6 cdef
Forage radish 394 ab 12 7 0 f
Phacelia 104 e 4 10 0 ef
Rye 416 ab 9 647 14 abcdef
Barley 435 a 8 870 12 abcdef
Spring oat 411 ab 8 655 9 bcdef
Wooly pod vetch (Lana) 215 cde 8 699 25 a
Barley+crimson clover+forage radish 471 a 11 986 23 ab
Rye+wooly pod vetch+Austrian winter pea+forage radish 495 a 13 664 16 abcde
Spring oat+canola 483 a 8 469 8 bcdef
Spring oat+forage radish 434 a 10 531 8 bcdef
Spring oat+barley 334 abcd 6 718 9 bcdef
Barley+crimson clover+wooly pod vetch+Austrian winter pea+forage radish+canola 352 abc 11 799 21 abc
† Means followed by different lower case letters within a column are significantly different ( alpha = 0.05).

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.

Table 6. Dry matter (DM) biomass and N uptake for 2012 Western sites (Blacksburg, Mauzy, and Edinburg).

  WINTER SPRING
SPECIES/MIXTURE DM BIOMASS N UPTAKE DM BIOMASS N UPTAKE
  ---------LB/AC--------- ---------LB/AC---------
Early cover hairy vetch 63 g† 4 h 800 ghi 24 efghij
Austrian winter pea 144 g 5 gh 307 i 9 ij
Common vetch 209 defg 6 fgh 323 i 8 j
Crimson clover 199 fg 7 efgh 1915 bcdef 57 a
Sweet lupin 207 efg 7 efgh 903 fghi 14 ghij
Tillage radish 386 cdef 12 bcde 510 hi 10 hij
Phacelia 384 cdef 11 cdef 1619 cdefg 39 abcdef
Rye 632 ab 18 ab 1548 defgh 28 defgh
Barley 721 a 19 a 2950 ab 49 abc
Spring oat 498 bc 15 abcd 1910 bcdef 27 defghi
Ryegrass 414 cd 11 cdefg 1414 efgh 19 fghij
Wooly pod vetch (Lana) 71 g 2 h 1277 efghi 37 bcdef
Barley+crimson clover+forage radish 700 ab 18 ab 2740 ab 55 ab
Rye+wooly pod vetch+Austrian winter pea+tillage radish 651 ab 20 a 2005 bcde 42 abcde
Rye+ryegrass+tillage radish 670 ab 17 abc 2615 abc 45 abcd
Spring oat+canola 242 defg 8 defgh 2000 bcde 31 cdefg
Spring oat+tillage radish 414 cd 11 cdefg 2471 abcd 37 bcdef
Spring oat+barley 666 ab 16 abc 3457 a 53 ab
Barley+crimson clover+wooly pod vetch+Austrian winter pea+tillage radish+canola 410 cde 11 cdefg 2539 abcd 52 ab
Ryegrass+crimson clover+wooly pod vetch+Austrian winter pea+tillage radish+canola 518 abc 15 abc 2875 ab 53 ab
† Means followed by different lower case letters within a column are significantly different ( alpha = 0.05).

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).


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Publication Date

September 23, 2020