Winter 1996 (v8n1)
Cover crop biology: A mini-review. Part 2.
Robert L. Bugg
Article written for Sustainable Agriculture Technical Reviews. 1995
Introduction
Managing cover crops in orchards or vineyards depends in part on understanding their basic biology. This article, presented in two parts, reviews several aspects of cover crop biology. Part I (see Sustainable Agriculture, Vol. 7, No. 4, p. 15) dealt with seeds, seedlings, root-zone biology, nutrient uptake, and the fate of cover crop-derived nitrogen. Part II presented here concerns plant community dynamics and allelopathy. Most of the plant species discussed may be used as cover crops or as forage crops in rangeland settings. The issues raised have general applicability to a number of farming systems in California.
Yields and Competition in Multispecies Stands
Cover Crops and Trees
or Vines
Wick and Alleweldt (1983) showed that, when grown together
in containers, subterranean clover (Trifolium subterranean
cv 'Clare') caused a 20 percent reduction in growth of Riesling
grape vines. The mechanism for this inhibition was not known,
and it occurred regardless of the level of nitrogen fertilization.
No such inhibitory effect was seen for subterranean clover cv
'Daliak' or for white clover (Trifolium repens). The legumes
supplied nitrogen to the vines when only low amounts of nitrogen
were added, and there was higher use of water by vines with legumes.
When grown in a young vineyard, 'Daliak' showed more rapid early
growth than white clover, but was damaged by frost and did not
reseed.
In Arkansas, Stasiak (1990) planted peach trees of two scion and two rootstock types into either bare ground or preestablished stands of subterranean clover in the tree rows. With both bare and clover plots, permanent drive middles were maintained in a sod of mixed tall fescue and bermudagrass. Comparison indicated that tree rows with preexisting subterranean clover led to reduced peach tree vigor (shoot growth, trunk cross-sectional area, foliar nitrogen content) during the first year of growth. By the second year, this tendency was eliminated. Stasiak suggested planting subterranean clover in August following the first season of peach tree growth, rather than planting peach trees into preestablished subterranean clover.
Orchardgrass (Dactylis glomerata, cv 'Berber'), a vigorous
perennial bunchgrass, when grown as a cover crop reduced vine
growth by 58 percent and yield by 53 percent of Cabernet Sauvignon
grapes in Santa Barbara County of California, although the site
featured highly fertile soil (Wolpert et al. 1993). This effect
was due at least in part to increased water stress of the vines.
Multiple Legumes
Williams et al. (1968) showed that subterranean clover has,
on the average, larger seed than does crimson clover (Trifolium
incarnatum), and plot studies indicated that the former will
tend to dominate in mixtures due to more rapid early growth and
shading of the crimson clover. When larger seeds of crimson clover
were selected and interseeded with smaller seeds of subterranean
clover, the pattern was reversed. Crimson clover was not eliminated
from any of the mixtures evaluated.
Hill and Gleeson (1991) found that when paired with either 'Seaton Park' or 'Daliak' subterranean clovers (both Trifolium subterraneum ssp. subterranean) in a three-year field study, the cultivar 'Clare' (Trifolium subterranean ssp. brachycalycinum) dominated the mixed stands. Soil pH was about 5.5, mowing was once every four weeks. Petiole length of 'Clare' is much greater than the early-maturing 'Daliak,' and slightly greater than the mid-season maturing 'Seaton Park.' 'Clare' also showed better seedling vigor and survival and greater seed production per plant under stress. Dry matter production by 'Clare' also is less dependent on plant density. There was evidence of overyielding by mixtures of 'Clare' and 'Seaton Park.' (Overyielding refers to the situation where the yield of the polyculture exceeds the yield of its highest yielding component grown in monoculture.) 'Clare' seed reserves appeared to be more greatly reduced over the summer months than those of 'Seaton Park' or 'Daliak'. T. s. ssp. brachycalycinum is supposedly adapted to neutral to alkaline soils, and is believed less tolerant to close grazing and less able to bury its burs than is T. subterraneum.
Williams (1963) sowed crimson clover (strain S. Australian commercial),
rose clover (Trifolium hirtum, strain S.6), and subterranean
clover (cv 'Bacchus') in pure plantings and in three 1:1 mixtures
of two species each. Competition for light was assessed in relation
to leaf area and leaf position in the canopy. Leaf area in 4-cm
horizontal strata, leaf weight, shoot weight production, and light
penetration through canopies were measured at intervals during
the vegetative phase (i.e., through 99 days after sowing). Crimson
and rose clovers held apparent initial advantages over subterranean
clover, in terms of light-absorbing surface area of cotyledons
and first unifoliate leaves, and because these leaves were elevated
further from the soil surface. However, this situation changed
with time. In paired sowings, crimson and subterranean clover
became equally dominant over rose clover, while subterranean clover
overtopped crimson clover despite the greater total leaf area
of the latter. The most productive mixture (crimson clover + subterranean
clover) was no more productive than the best species in monoculture
(crimson clover). As noted by Williams (1963a), competition has
other dimensions than those reported here, including the advantage
conferred by hardseededness of rose clover, which enables it to
dominate multispecies stands following droughts that kill clover
seedlings.
Legumes and Non-Leguminous Forbs
Guerrero and Williams (1975) conducted several growth chamber
studies. In one study, Filaree (Erodium botrys) and subterranean
clover (T. subterraneum ssp. subterranean cv 'Woogenellup')
were grown in sole and mixed cultures in a phosphorus-deficient
range soil (from Butte County) and in sand with differing levels
of supplemental nitrogen, phosphorus, and sulfur. Filaree dominated
if phosphorus was limiting, whereas subterranean clover dominated
if nitrogen was left out of the fertilizer. Subterranean clover
has a higher requirement for phosphorus than does filaree, and
also appears less capable of exploiting insoluble phosphate sources.
For this reason, addition of superphosphate to rangeland soils
was suggested by the authors as a means of promoting subterranean
clover.
Moore et al. (1989) found in pot experiments in Australia that subterranean clover can suppress the seedlings of the perennial weed St. John's Wort (Hypericum perforatum) by overtopping the seedlings and shading them out. This study confirmed earlier findings that subterranean clover could suppress the weed if sown into native pastures, particularly if phosphate fertilizers had been applied. The importance of maintaining a closed canopy of subterranean clover during the early phase of weed seedling growth is emphasized.
Legumes and Grasses
Motazedian and Sharrow (1986) conducted a field study of stands
of subterranean clover and perennial ryegrass (Lolium perenne),
in which mowing height and frequency were varied. In stands dominated
by perennial ryegrass, greater stubble heights led to greater
productivity; the opposite was true for stands dominated by subterranean
clover. The greatest interval between defoliations (49 days) led
to the greatest productivity of the stands.
In the southwest portion of Western Australia, Cotterill (1990) used unirrigated 35 x 35 cm plots to evaluate competition between cool-season annual grasses and either 'Serena' bur medic or 'Seaton Park' subterranean clover in a ley farming system (wheat-pasture rotation). He found that dry matter production by the legumes was depressed linearly with increasing seeding rates for various cool-season annual grasses, including ripgut brome (Bromus diandrus), wild barley (Hordeum leporinum), a ryegrass (Lolium rigidum), and rattail fescue (Vulpia myuros). Additionally, second-year legume biomass production was not significantly depressed by grasses seeded the first year, as long as grass seeding rates were less than 40 percent of the full rates. Full seeding rates for grasses were over eight million seeds per hectare, while those for the legumes were about two million seeds per hectare. When seeded without grasses, the subterranean clover produced the equivalent of 3.1 metric tons of biomass per hectare and the bur medic about 1.5 metric tons per hectare. Pooled across all levels of grasses, the corresponding results for subterranean clover and bur medic were nearly identical at about 1 metric ton per hectare.
In Austria, Danso et al. (1991) conducted a two-year trial in
a triple-species mixed sward of white clover (Trifolium pratense
cv 'Zapican'), birdsfoot trefoil (Lotus corniculatus cv
'Gabriel') and fescue (Festuca arundinacea cv 'Tacuabe').
White clover showed good production for the first harvest of the
first year; thereafter, birdsfoot trefoil dominated. In the first
year, both legumes contributed about equally to the approximately
130 kg of nitrogen per hectare fixed in the sward. In the second
year, white clover only contributed five percent of the 46 kg
of nitrogen per hectare fixed in the last two harvests. Mixtures
containing the two legumes have an advantage because the early
production by white clover is complemented by later production
and better persistence by birdsfoot trefoil. Stands with multiple
legumes often show better livestock weight gains.
Legumes, Non-Leguminous Forbs, and a Grass
Mohler and Liebman (1987) reported that high-density plantings
of barley were better at suppressing weeds than were intercropped
barley and field pea (Pisum arvense). Weed suppression
appeared to be a result of competition for soil moisture. Weed
populations were not reduced, but weed biomass was lower.
Liebman and Robichaux (1990) found that intercropped barley and
field pea were no better at suppressing weed mustards (Brassica
kaber) and white mustard (B. hirta) than was a dense
monoculture of barley. The main mechanisms of weed suppression
were shading (especially by the pea) and competition for nitrogen
(especially by the barley). A long-vined variety of field pea
('Century') was better than a short-vined variety ('Alaska') at
suppressing mustard growth by shading. 'Century' also showed a
greater yield.
Legume, Grass, and Various Herbs
A four-year study in Maryland by Teasdale et al. (1991) suggested
that, under no-till management, hairy vetch (Vicia villosa)
was particularly effective at reducing the densities of the following
weeds: goosegrass (Eleusine indica), stinkgrass (Eragrostis
cilianensis), and carpetweed (Mollugo verticillata).
Under conventional tillage, hairy vetch appeared during one year
to increase the densities of large crabgrass (Digitaria sanguinalis)
above those observed without a cover crop or with cereal rye (Secale
cereale cv 'Abruzzi'). In some years, cereal rye grown as
a no-till cover crop significantly reduced the densities of the
goosegrass and carpetweed. In one year, cereal rye managed with
tillage led to increased densities of common lambsquarters (Chenopodium
album).
Teasdale and Mohler (1993) in Maryland and New York State tested the effects of mulching on light transmittance, soil temperature, and soil moisture. The mulch in this study was the clipped residue of herbicide-killed hairy vetch or cereal rye (cv 'Aroostook'). Data for light transmittance and soil temperature suggest that cereal rye and hairy vetch residues have similar initial properties, but that there is more rapid and thorough decomposition of hairy vetch residue. Therefore, cereal rye provides a longer-lasting mulch that blocks light and reduces soil temperature longer.
In a study encompassing two growing seasons at Beltsville, Maryland,
Teasdale and Daughtry (1993) showed that living hairy vetch was
more effective than standing, paraquat-killed vetch at suppressing
weed germination and growth. During droughty periods in both growing
seasons, soil moisture was significantly greater in the surface
2.5 cm of soil under living or dead hairy vetch, as contrasted
with bare soil. In one year of this study (1990), living vetch
led to significantly lower soil moisture than did killed vetch.
Grass and a Non-Leguminous Forbs
Soft chess (Bromus mollis) suppresses broadleaf filaree
(Erodium botrys) by shading more effectively under conditions
of adequate sulfur (McCown and Williams 1968). When sulfur is
limited, broadleaf filaree accesses it sooner because of more
rapid extension of its young roots.
Allelopathy
Bialy et al. (1990) found that black mustard (Brassica nigra) and brown mustard (Brassica juncea) show allelopathic inhibition of other plants. Compounds involved probably include various isothiocyanates, which suppressed wheat germination and growth.
Cereal rye produces several compounds that inhibit crops and weeds. The most active compounds are two hydroxamic acids and their breakdown products (Chase et al. 1991). Wocjcik-Wojtkowiak et al. (1990) reported that residues of tillering plants and rye crop residues contain much lower amounts of allelopathic compounds (various phenolic acids) than do seedlings.
Various legumes in the tribe Vicieae (peas, lentils, and vetches) contain Beta-(3-isoxazolinonyl) alanine, which is released into soil as a root exudate, and apparently is an allelopathic compound (Schenk and Werner 1991). This chemical can cause reduced growth in seedlings of various grasses and of lettuce. Pea was only slightly affected.
References
Bialy, Z., W. Oleszek, J. Lewis, and G.R. Fenwick. 1990. Allelopathic potential of glucosinilates (mustard oil glycosides) and their degradation products against wheat. Plant and Soil 129:277-281.
Chase, W.R., M.G. Nair, and A.R. Putnam. 1991. Selective toxicity of rye (Secale cereale L.) allelochemicals to weed and crop species: 2,2'-oxo-1,1'-azobenzene. Journal of Chemical Ecology 17:9-19.
Cotterill, P.J. 1990. Influence of grass proportion in stands of burr medic and subterranean clover on legume re-establishment and productivity following wheat. Plant and Soil 123:113-116.
Danso, S.K.A., S. Curbelo, C. Labandera, and D. Pastorini. 1991. Herbage yield and nitrogen-fixation in a triple-species mixed sward of white clover. Journal of Range Management 28:275-278.
Hill, M.J., and A.C. Gleeson. 1991. Competition between Clare and Seaton Park, and Clare and Daliak subterranean clovers in replacement series mixtures in the field. Australian Journal of Agricultural Research 42:161-173.
Liebman, M., and R.H. Robichaux. 1990. Competition by barley and pea against mustard: effects on resource acquisition, photosynthesis and yield. Agriculture, Ecosystems and Environment 31:155-172.
McCown, R. L., and W.A. Williams. 1968. Competition for nutrients and light between the annual grassland species Bromus mollis and Erodium botrys. Ecology 49(5): 982-990.
Mohler, C.L., and M. Liebman. 1987. Weed productivity and composition in cole crops and intercrops of barley and field pea. Journal of Applied Ecology 24:685-699.
Moore, R.M., J.D. Williams, A.O. Nicholls. 1989. Competition between Trifolium subterraneum L. and established seedlings of Hypericum perforatum L.var. angustifolium DC. Australian Journal of Agricultural Research 40:1015-1025.
Motazedian, I. and S.H. Sharrow. 1986. Defoliation effects on forage dry matter production of a perennial ryegrass-subclover pasture. Agronomy Journal 78:581-584.
Schenk, S.U., and D. Werner. 1991. Beta-(3-isoxazolin-5-on-2yl)-alanine from Pisum: allelopathic properties and antimycotic bioassay. Phytochemistry 30:467-470.
Stasiak, M.J. 1990. The influence of subterranean clover (Trifolium subterraneum L.) on the growth and foliar nutrient status of young peach (Prunus persicae [L.] Batsch) trees. Master of Science Thesis, University of Arkansas, Fayetteville.
Teasdale, J.R., C.E. Beste, and W.E. Potts. 1991. Response of weeds to tillage and cover crop residue. Weed Science 39:195-199.
Teasdale, J.R., and C.S.T. Daughtry. 1993. Weed suppression by live and desiccated hairy vetch (Vicia villosa). Weed Science 41:207-212.
Teasdale, J.R., and C.L. Mohler. 1993. Light transmittance, soil temperature, and soil moisture under residue of hairy vetch and rye. Agronomy Journal 85:673-680.
Wick, U., and G. Alleweldt. 1983. Der Bodenfrüchtige Klee als Begrünungspflanze für den Weinbau im Vergleich zu WeiBklee (Cover crops in viticulture: comparison between subterranean clover and white clover). Die Weinwissenschaft 4:260-268.
Williams, W. A. 1963. Competition for light between annual species of Trifolium during the vegetative phase. Ecology 44(3):475-485.
Williams, W.A., J.N. Black, and C.M. Donald. 1968. Effect of seed weight on the vegetative growth of competing annual trifoliums. Crop Science 8:660-663.
Wocjcik-Wojtkowiak, D., B. Politycka, M. Schneider, and J. Perkowski. 1990. Phenolic substances as allelopathic agents arising during the degradation of rye (Secale cereale) tissues. Plant and Soil 124:143-147.
Wolpert, J.A., P.A. Philips, R.K. Striegler, M.V. McKenry, and
J.H. Foot. 1993. Berber orchardgrass tested as cover crop in commercial
vineyard. California Agriculture 47(5):23-25.
For more information write to: Robert L. Bugg, University of California Sustainable Agriculture Research and Education Program, University of California, Davis, CA 95616. rlbugg@ucdavis.edu.
(DEC.529a)
Contributed by Robert L. Bugg
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