Selecting the Right Cover Crop Gives Multiple Benefits
Chuck A. IngelsThe use of cover crops continues to increase as farmers seek to improve soil quality and reduce chemical inputs. Research and grower experience have shown that cover crops can provide multiple benefits related to soil protection, soil fertility, groundwater quality and pest management. Choosing the right cover crop is critical to successful cover cropping.
Mark Van Horn
Robert L. Bugg
P. Rick Miller
California Agriculture 48(5):43-48
Cover crops were frequently used on California farms in the first half of this century, but were rarely grown after the 1950s, when pesticides and fertilizers came into widespread use. In the last decade, however, farmers and researchers have taken a renewed interest in cover crops because of their potential role in reducing chemical inputs and improving soil quality. Cover crops are now recognized as an important component of "sustainable" production in most areas of California.
Cover cropping can have both advantages and disadvantages (see sidebar). Proper choice and management of cover crops are important in maximizing the benefits and reducing potential problems. To select a species or mix, the grower should first determine the primary benefits he or she desires. Then consider the specific conditions of the particular farming system, such as soil type, water availability and method of irrigation, cropping sequence and cultural practices. There may be only a few species or many that are appropriate to a given farming system (see table 1).
Adding and conserving nitrogen
A common reason for growing cover crops is to add nitrogen to the soil using nitrogen-fixing legumes. Winter annual legumes can fix 100 to 200 pounds of nitrogen per acre. Many growers use a vetch, such as "Lana" woollypod vetch, to add the greatest quantity of nitrogen. Summer annual legumes, such as cowpea, typically add 100 to 130 pounds of nitrogen. However, they can do so in 2 to 3 months, as compared to the 5- to 6-month growing season for most winter annual cover crops.
When a legume cover crop is incorporated into the soil, a substantial
amount of nitrogen is usually mineralized (converted from organic
to plant-available forms) within a few weeks. Nitrogen continues
to mineralize in ensuing weeks as the organic matter decomposes.
In nontillage systems, cover crops are mowed and the residues
left on the soil surface. In such systems decomposition and nitrogen
availability will usually be delayed. Nitrogen availability may
also be reduced due to volatilization of nitrogen into the atmosphere
from decomposing residues. Little information is available regarding
such volatilization losses under California conditions; research
is needed to determine the magnitude of such losses and the factors
which affect them.
Most nonlegumes do not fix nitrogen, but they can affect soil nitrogen availability. Grasses can take up large quantities of nitrogen from the soil, thereby reducing the potential for nitrate leaching. Because of the high carbon-to-nitrogen (C/N) ratio of grasses, their residues decompose slowly, and soil nitrogen availability may be substantially decreased following their incorporation into the soil. Several species in the mustard family and tansy phacelia also take up nitrogen readily. Due to lower C/N ratios, however, these species decompose and release their absorbed nitrogen into the soil more rapidly than grasses (see California Agriculture, September-October 1993).
Water use by cover crops
All cover crops require water for growth, but in most parts of
California winter cover crops have relatively little impact on
soil moisture. However, the earlier the cover crop is sown in
the fall or the longer it is allowed to grow in the spring, the
more water it will use. Summer annual cover crops use more water
than winter annuals and require irrigation. However, some species,
such as cowpea and hyacinth bean, are relatively drought tolerant
and can perform well with as little as two irrigations (J. Klein
and M. Van Horn, unpublished data).
In orchards and vineyards, summer-active cover crops (for example, perennial clovers) compete directly with the cash crops for water. A 4-year study of almonds grown in Merced and Stanislaus counties showed that both resident vegetation and strawberry clover used 20 to 25% more water than a bare floor in a mature orchard (see California Agriculture, July-August 1989). "Blando" brome (soft chess) used more water than the bare floor treatment during the spring. However, its clippings acted as a mulch, reducing evaporation from the orchard floor in the summer and offsetting earlier water use. Certain California native perennial grasses are summer dormant and therefore use little or no water during the summer, especially where drip irrigation is used. Some of these grasses are being tested in vineyards and have potential value in orchards as well. It is important to note that soil improvements by cover crops may lead to more efficient use of water, especially on sandy soils.
Pest management
Cover crops can also aid in pest management. For example, some
cover crops compete with and suppress weeds. Competitive cover
crop species include many perennials, such as strawberry clover,
and annuals with relatively large seeds and good seedling vigor,
such as cereal grasses, vetches and cowpea. Some other species,
such as annual clovers and small-seeded grasses, may not compete
well if left unmanaged, especially on very fertile soils. With
well-timed mowing or grazing, however, they regrow rapidly, often
more so than competing weeds.
Some cover crop species can become serious weeds if improperly selected or managed. For example, most vetches produce substantial viny growth, which can, if planted near tree or vine rows, overtake grapevines or entwine sprinklers. Also, certain species, such as cereal rye, produce tremendous amounts of biomass, which can hinder mowing and disking. Other species, such as perennial clovers, may invade tree or vine rows and resist herbicides; perennial clovers also have a reputation for increasing gopher populations.
Hardseededness is a characteristic of certain cover crop species that can help ensure self-regeneration of a cover crop stand. Hard seeds have seed coats that are impermeable to water, ensuring that a portion of the seed bank is held in reserve each year. These seeds are viable, but will not germinate in the first year after they mature. Thus, once a cover crop with hard seeds is planted, it can be expected to reestablish itself with fall rains each year, even in years when the previous cover crop did not produce seed. Rose clovers produce high proportions of hard seed, as do medics and some vetch species and subterranean clover varieties. Self-regenerated stands are usually desirable in orchards and vineyards, but these species may become problem weeds in annual systems.
Cover crops in vineyards and orchards may provide food or shelter
to beneficial insects, mites and spiders. In citrus, for example,
the predatory mite Euseius tularensis is a key predator
of citrus thrips and various other pests. As Ouyang, Grafton-Cardwell
and Bugg observed in 1993, in the absence of prey, this predatory
mite can also subsist for a time on windblown pollens of grasses
commonly used as cover crops. Field trials are still needed to
determine whether grass cover crops lead to improved biological
control of the pests mentioned.
Cover crops such as soft chess, "Salina" strawberry
clover and resident vegetation speed up the decomposition of unharvested
almonds remaining on the orchard floor after harvest. Left intact,
these nuts would provide overwintering habitat for navel orangeworm.
Decomposition is aided by the moisture in cover crops and by flail
mowing, greatly decreasing spring emergence of the moths from
almonds (see California Agriculture, July-August 1989).
Navel orangeworm emergence from walnuts remaining in a tall winter
cover crop may not differ significantly from those remaining on
herbicide-treated soil (see California Agriculture, September-October
1993).
Some pest arthropod species are also associated with cover crops
and may require management. For example, two ant species pavement
ant and southern fire ant damage almonds by feeding on the kernels
of nuts that have been shaken to the ground. The ants are more
damaging in orchards cover cropped with soft chess, strawberry
clover and resident vegetation than in orchards treated with residual
herbicide (see California Agriculture, July-August 1989).
In cover-cropped orchards, prompt pickup of fallen nuts is important
to avoid problems with ants.
Some leguminous cover crops harbor abundant lygus bugs, which are pests of some tree crops. However, studies in southern Georgia by Bugg and others in 1990 suggest that subterranean clovers harbor very few lygus bugs compared to vetches, medics and other clovers; our own observations in California confirm this.
Cover crops in annual rotations
In annual cropping systems, cover crops are often chosen to maximize
benefits such as biomass and nitrogen production. However, other
factors must also be considered. For example, fitting a cover
crop into the sequence of a crop rotation can be difficult. Therefore,
fast-growing, drought-tolerant cover crops that require minimal
management are preferred. Cover crops with fast germination and
good seedling vigor are usually chosen because of their ability
to compete with weeds. Also, species with the potential to reduce
pest populations should be chosen, while those that harbor diseases
or arthropod pests of the cash crops should be avoided.
Common cool-season legumes used as cover crops in annual rotations
include vetches, winter pea and bell bean. Because clovers and
medics grow slower and compete poorly with weeds and require more
management (e.g., mowing), they are used less commonly in annual
rotations. For similar reasons, cereal grains are usually preferred
over other grass species, such as bromes, in annual rotations.
In choosing warm- season cover crops, the ability to perform well with minimal irrigation is often of primary consideration. Legume species in this category include cowpea, hyacinth bean and sunnhemp. Typical grass cover crops for warm conditions include sudangrass and sorghum.
Self-reseeding cover crops
Many tree and vine crop growers prefer to use self-reseeding annual cover crops. Using these species can reduce or eliminate the costs associated with purchasing and sowing seed each year. Although many of these species compete effectively with weeds if properly managed, in some cases the reseeding cover may revert back to resident vegetation within a few years. It may be necessary to resow the cover crop in 3 to 5 years.
With some cover crops, self-reseeding can fail if insufficient moisture is available in the spring to mature the seed for the next yearís cover crop. This problem will most likely occur if the row middles cannot be irrigated, such as when using drip irrigation. In these situations, it may be wise to choose cover crops that produce seed early in the spring and/or produce a relatively high proportion of hard seed. Early-maturing varieties of crimson clover, subterranean clover and burr medic are commercially available. Early-maturing grasses include "Zorro" fescue and ëBlandoí brome.
Conclusion
Successful cover cropping requires the selection of a species or mix that will provide specific desired benefits and that will be compatible with the overall farming system. Most growers who use cover crops derive multiple benefits from them. In many situations, a successful cover crop stand and the greatest benefits can be obtained by using mixtures of species, such as grasses and legumes. Furthermore, as with cash crops, cover crops should be rotated periodically to avoid the buildup of plant-specific pests.
Cover cropping, like any new practice, should be approached slowly
and methodically. It may be valuable to test several species or
mixtures to find the most appropriate cover crop. It is usually
a good idea to begin on a small scale in order to learn from mistakes
without incurring unnecessary expenses. With persistence and creativity,
cover cropping can provide many benefits with little or no extra
cost.
C.A. Ingels and R.L. Bugg are Information Analysts, Sustainable
Agriculture Research and Education Program, UC Davis; M. Van Horn
is Director, Student Experimental Farm, UC Davis; P.R. Miller
is a visiting professor at the Federal University of Santa Catarina,
Florianopolis, Brazil.
We thank F. Thomas of Lohse Mill, Inc., and G. Wittenborn of
Germainís, Inc., for contributing some of the information
used in this article.
For more information, see Y. Ouyang, E.E. Grafton Cardwell
and R.L. Bugg. 1993. Effects of various tree and grass pollens
on development, survivorship and reproduction of Euseius tularensis
[Acari: Phytoseiidae]. Environmental Entomology 21:1371ñ1376).
Abstracts of research reported by the UC Cover Crops Workgroup
are available for $5 for a print copy or free through e-mail.
Contact Chuck Ingels, SAREP, UC Davis, Davis CA 95616. (916) 757-3276;
e-mail: caingels@ucdavis.edu.