Cover Crop Biology: A Mini-Review
Robert L. Bugg
Sustainable Agriculture Research & Education Program, University
of California, Davis CA 95616
(Article written for SAREP's Sustainable Agriculture--Technical
Reviews, vol. 7, no. 4, Fall 1995.)
Introduction
Managing cover crops in orchards or vineyards depends in part
on understanding their basic biology. This article reviews several
aspects of cover crop biology: seeds, seedlings, root zone biology,
nutrient uptake, the fate of cover crop derived nitrogen, 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.
Seeds and Seedlings
Williams and Elliott (1960) studied dryland northern Californian populations of crimson clover (Trifolium incarnatum), rose clover (T. hirtum), and subterranean clover (T. subterraneum) with respect to: 1) amount of impermeable seed produced, 2) the rate at which seeds become permeable, and 3) factors causing breakdown of impermeability of rose clover. Crimson clover impermeability declined to a low level in the months after seed maturation. Subterranean clover followed a similar pattern, though slightly delayed. At one site under marine influence, subterranean clover retained a moderate proportion of impermeable seed into autumn. Rose clover maintained a large proportion of hard seed throughout the summer, autumn, and winter months. High temperatures at and slightly above the soil surface were demonstrated to cause breakdown of rose clover impermeability. Rose clover is able to persist better than crimson or subterranean clovers in most dryland northern Californian settings. This characteristic is probably in part due to its prolific production of impermeable seed.
According to Williams (1956), during establishment, the force produced by legume seedlings may be crucial in overcoming the weight of overlying soil and surface crusts. Using glass tubes containing vermiculite and glass rods of known mass, the force produced by seeds of crimson clover, rose clover, subterranean clover, and alfalfa was estimated. Mean forces exerted (in grams, +/ standard error of mean) were estimated as follows. Alfalfa (15.2 +/ 0.5), crimson clover (23.8 +/ 0.2), rose clover (24.1 +/ 0.5), subterranean clover (60.0 +/ 2.9). The force exerted by the seeds was highly correlated (R=0.999) with seed weight, but not so highly (R=0.837) with hydrolyzable carbohydrates, suggesting that other factors may be involved.
Williams (1963) later found that crimson clover varieties showed
great variability in the amount of force exerted by seedlings.
'Autauga' F.C. 32, 963 and Mississippi selection F.C. 32, 964
showed particularly great forces. These greatly exceeded those
of rose clover, 'Caliverde' and 'Ranger' alfalfa, and 'Kenland'
red clover. Forces exerted by crimson clover and alfalfa were
highly dependent on temperature, with the maximum force attained
for the former near 20 C, and for the latter between 25 and 30
C. These patterns suggest that plantings could be timed to correspond
with sufficiently warm weather to achieve good emergence of seedlings.
Root Zone Dynamics And Nutrient Uptake
Nitrogen
Schomberg and Weaver (1990) found that the addition of the equivalent of 31 kg per hectare of starter nitrogen did not reduce nitrogen fixation by arrowleaf clover, and substantially improved seedling vigor. Starter nitrogen can actually lead to improved nitrogen fixation by legumes.
In a three year field trial in Germany, Benkenstein et al. (1990) assessed uptake of nitrate (from sodium nitrate) and ammonium (from ammonium sulfate) by winter cereal rye (Secale cereale cv 'Janos') through the growing season, as a function of depth of placement in the soil (40, 60, and 80 cm beneath the surface). Sampling indicated that cereal rye plants absorbed 72 percent of the nitrate derived from the 40 cm placement, and 18 percent of that derived from the 80 cm placement.
In Germany, Raderschall and Gebhardt (1990) evaluated winter crops of rape (Brassica napus), barley (Hordeum vulgare), and Welsh ryegrass (Lolium multiflorum). These respectively accumulated in their above-ground structures 52.1, 36.2, and 22.9 kg of nitrogen per hectare following bell bean (cv 'Alfred').
Guiraud et al. (1990) conducted a lysimeter study in which a catch crop of annual ryegrass (Lolium multiflorum) reduced nitrate leaching from 124 kg of nitrogen per hectare to 40 kg of nitrogen per hectare. The percentages of fertilizer nitrogen (as a percentage of total nitrogen) in the water were 19 and 7 percent in cover-cropped and bare fallow plots, respectively. To a depth of 30 cm, 23 percent of labeled nitrogen was retained in organic form where ryegrass had been incorporated, versus 15 percent under bare fallow.
Jackson et al. (1993) evaluated several potential winter-annual
nitrogen catch crops for rotation with lettuce in the Salinas
Valley of California sown in November and harvested the following
March. Particularly promising in terms of nitrogen assimilation
were white senf mustard (Brassica hirta cv 'Martigena'
200 kg of nitrogen per hectare), cereal rye (Secale cereale
cv 'Merced' 129 kg of nitrogen per hectare), tansy phacelia (Phacelia
tanacetifolia cv 'Phaci' 182 kg of nitrogen per hectare),
white mustard (B. alba 205 kg of nitrogen per hectare),
and oilseed radish (Raphanus sativus cv 'Renova' 161 kg
of nitrogen per hectare). Annual ryegrass (Lolium multiflorum
85 kg of nitrogen per hectare) showed less capacity for nitrogen
absorption when used in this temporal niche.
Phosphorus
Hoffland et al. (1989) grew rape (B. napus cv 'Jetneuf') in agar plates and nutrient solution with or without phosphorus. Bromocresol purple was included as a pH indicator. Acidification, caused by exudation of citric and malic acids, occurred along a restricted zone about 1.5 cm in length, just behind root tips. Conditions were alkaline along the remainder of the root systems. Concentrations of citric and malic acids were generally lower for plants grown with sufficient phosphorus. Further work will be needed to test whether the acidification observed leads to solubilization of rock phosphate. Potassium, calcium, and nitrate were taken up about twice as rapidly by plants with sufficient phosphate than by plants grown in a phosphorus-deficient medium.
Gardner and Boundy (1983) found that wheat intercropped with white lupin (Lupinus albus) has access to a larger pool of phosphorus, manganese, and nitrogen than wheat grown in monoculture. The former two nutrients were probably mobilized by exudates from the lupin roots, then taken up by the closely-associated wheat roots.
Based on pot experiments and a literature review, Paynter (1990) concluded that burr medic (Medicago polymorpha) and barrel medic (Medicago truncatula) are not as efficient at absorbing soil phosphorus as is subterranean clover (Trifolium subterraneum).
Annan and Amberger (1989) investigated the ability of buckwheat
(Fagopyrum esculentum) to acquire phosphorus. The authors
investigated phosphorus uptake, morphological features, and chemical
changes in the rhizosphere. Root weight and length, and frequency
of root hairs were higher when plants were grown under phosphorus
deficiency. Phosphorus uptake rates were only moderate; concentrations
of phosphorus in the shoot were high (1.8% of dry weight). Release
of P from FePO4 and glucose-6-phosphate was not due
to a build-up of organic acids in the rhizosphere, but to high
activities of acid phosphatase, an enzyme produced by buckwheat
plants growing in low phosphorus conditions. The following parameters
were regarded as important for buckwheat's phosphorus efficiency:
1) a finely divided root system of considerable length, with a
high ratio of root surface to root or shoot length; 2) a high
storage capacity for inorganic phosphorus, 3) an increased release
of protons and FePO4 or MnO2 solubilizing
substances by phosphorus deficient plants; 4) a favorable ratio
of phosphorus uptake to root mass increase, especially at low
phosphorus supply; and 5) a high activity of acid phosphatase
in the rhizosphere and the capability to use phosphorus from organic
sources.
Fate of Nitrogen Derived from Cover Crops
Sawatsky and Soper (1991) grew field peas using a split-root procedure. The study suggested that at the time of harvest, 22 to 46 percent of the below-ground nitrogen had been shed into the rhizosphere (root zone). Because this nitrogen "rhizodeposition" has not previously been assessed for annual legumes, nitrogen fixation may be underestimated by about 10 percent.
Fox et al. (1990) reported that in order for nitrogen mineralization to occur, nitrogen concentration of incorporated residue must exceed 15 to 25 grams per kg. However, little mineralization will occur if there are high concentrations of lignin or polyphenols. These workers reported a study involving the decomposition of and nitrogen mineralization from residues of six kinds of legumes (alfalfa, round leaf cassia, leucaena, Fitzroy stylo, snail medic, and Vigna trilobata), in which the ratio of lignin+polyphenols to nitrogen content was a good predictor of nitrogen mineralization rate, whereas initial nitrogen concentration of residues was not. After 12 weeks, net nitrogen mineralized ranged from 11 percent from round leaf cassia to 47 percent for alfalfa.
In Terman's (1979) review of ammonia volatilization, he noted that crop utilization of nitrogen from surface-applied materials ranges from 30 to 70 percent and may average about 50 percent. Losses of ammonia (NH3) increase with increase in the intensity of drying conditions (higher temperatures, more air movement, and lower humidity), with higher soil pH, with coarse-textured soils of low cation exchange capacity, and with lower initial soil moisture content. Losses are very low if various nitrogen sources are incorporated into the soil or are moved at once into the soil by rain or irrigation.
Janzen and McGinn (1991) reported three experiments evaluating ammonia volatilization from decomposing lentil residue (Lens culinaris Medik.). The first experiment showed that after 56 days, ammonia volatilization from residue left on the soil surface represented five percent of applied nitrogen. Incorporating the residue into the soil stopped nearly all NH3 loss.
In Georgia, on gravelly clay loam and sandy clay loam soils, McVay et al. (1989) conducted a trial of no-till corn and sorghum production under various cover-cropping regimes. Hairy vetch and crimson clover grown as winter cover crops, respectively, replaced on the average 123 and 99 kg per hectare of fertilizer nitrogen. The corresponding above-ground nitrogen contents of cover crop herbage were 128 and 108 kg of nitrogen per hectare. These results suggest efficient cycling of nitrogen in a no-till system, with minimal losses due to volatilization of ammonia. Harper et al. (1995) reported that in Georgia for crimson clover preceding sorghum under no-till management, volatilization of ammonia was minimal: 323 kg of nitrogen per hectare accumulated in the crimson clover, and an estimated 0.25 kg of nitrogen per hectare (less than 0.1%) was lost by volatilization of NH3.
By contrast, in the Northeast and the Midwest, cyclic wetting and drying may lead to the loss of from 1/3 to 1/2 of the nitrogen contained in surface-managed leguminous residues, regardless of the pH of the soil (Sarrantonio and Scott, 1988). Even if field conditions do not lead to substantial volatilization of ammonia, nitrogen availability to the target crop may be delayed in no-till systems, as suggested by Lemon et al. (1990) for berseem clover preceding grain sorghum in Burleson County, Texas.
Dr. William L. Hargrove of the University of Georgia has extensive
experience in managing no-till, cool-season annual legumes and
in assessing their nitrogen contributions to warm-season annual
field crops; he is also an authority on ammonia volatilization.
In discussions with the author, Hargrove (personal communication,
1994) stated that ammonia volatilization should be minimal and
nitrogen from cover crop residues should become available to trees
if understory conditions remain relatively moist during the principal
period that the clippings are decomposing. Shading and irrigation,
of course, would aid in maintaining the desired moist conditions.
Hargrove further indicated that staggered mowing patterns, as
often recommended in managing orchard cover crops, will further
reduce volatilization by reducing the concentration of ammonia
during any one period of decomposition. Rainfall in Georgia averages
about four inches per month throughout the year; this corresponds
well with the amounts of water applied to sprinkler-irrigated
almond orchards (91 cm [36 inches] over nine months), and is less
than the amounts applied to walnut orchards (122 cm [48 inches]
over seven to eight months).
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 pre-established 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 subter
ranean clover in August following the first season of peach tree growth, rather than planting peach trees into pre-established 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, 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' (T. 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. s. 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 (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 (B. 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.
Conclusion
In managing cover crops, many issues must be considered. Here,
I have presented a selection of research results on cover crop
ecology, emphasizing the dynamics of stand establishment and maintenance,
and nutrient cycling. Our knowledge of cover crop ecology is still
fragmentary, yet it is progressing rapidly. Thus, these and other
results may ultimately be used to develop comprehensive views
and management plans. For this to happen most efficiently, farmers,
advisors, and scientists should discuss not only the scientific
papers themselves, but also the applicability, adaptability, or
non-applicability of the findings within the context of particular
farms, or even individual fields. Scientific data like those presented
here take on their most important life when they enable farmers
to fulfill their aims with minimal expenditure and environmental
risk.
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Cover Crop Research and Education Summaries