Chapter 8

CONTENTSPage

Intercropping. $ . . . . . . . . . . . . 00 . . . . . . . . . . . . . . . . . . ....................195Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .....................195Various Types of Intercropping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ....196Benefits of Intercropping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..........196H i g h A d o p t i o n P o t e n t i a l o f I m p r o v e m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9 8Research Needs and Constra in ts for In tercropping . . . . . . . . . . . . . . . . . . . . 198

Agroforestry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .....................200Summary . . . . . . .. .. .. .. .. .. . . $ . . . . . . . . . . . . . . . . ...................200The Role and Nature of Agroforestry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .201Potential for Adoption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............204Problems and Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..........205

Chapter 8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................208

BoxBox Page

8-1. Community Agroforestry in Somalia . . . . . . . . . . . . . . . . . . . . . .........207

FigureFigure Page

8-1. Growing Thirteen Crop Species on and Between Raised Mounds inNigeria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............197

TablesTable Page

8-1. Examples of Common Intercrops in the Agroecological Zones inSub-Saharan Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..............196

8-2. Intercropping of cereals in Africa . . . . . . . . . . . . . . . . . . . . . ...........1978-3. Possible Effects of Intercropping on Insect Pest Populations. . ........1998-4. Grain Yields Under the Crown of Acacia albida Compared to Grain

Yields Outside the Crown. . . . . . . . . . . . . . . . . . . . . . ..................202

Chapter 8

Improved Cropping Practices

●

●

●

●

●

195

196

Various Types of Intercropping

Intercropping, as discussed in this chapter,is the growing of two or more crop species orvarieties simultaneously in the same field. (Thiscan include agroforestry—the use of trees inintercrops, which is discussed in the follow-ing section). Various types of intercropping sys-tems exist:

. Mixed intercropping: a variety of crops areplanted with no distinct row arrangement.

● Row intercropping: crops are planted inrows, either adjacent rows of differentcrops, or mixed within the row.

● Strip cropping: several rows of a crop aregrown together forming a strip. Strips, eachhaving a different crop or variety, are wideenough to permit independent cultivation,but close enough to interact agronomically.

● Relay intercropping: growing two or morecrops simultaneously during part of the lifecycle of each. A second crop is planted af-ter the first crop has reached its reproduc-tive stage of growth but before it is readyfor harvest. If crops are planted succes-sively in the same year, but with no signif-icant overlap in time, the system is calledsequential cropping (7)0

Food and export crops are grown as inter-crops throughout all agroecological zones ofAfrica (table 8-l). National statistics rarely iden-tify the production system, but studies clearlyindicate that intercropping accounts for themajority of Africa’s agricultural production (ta-ble 8-2).

In arid areas, two or three species commonlyare mixed, but in wetter zones the systems be-come increasingly more diverse (47). The diver-sity of crops produced in the humid lowlandsis illustrated in Zaire, where farmers sometimesgrow 80 varieties of 30 different species. In onestudy, this included 27 varieties of banana andplantain and 22 varieties of yams and other rootcrops (13). In another example, Nigerian farm-ers designed a system of mounds which allowedthem to plant crops with differing soil mois-

Table 8-1 .—Examples of Common Intercrops in theAgroecological Zones in Sub-Saharan Africa

Zone Crop mix

Arid and semiarid millet/sorghummillet or sorghum/cowpea

Subhumid uplands maize or sorghum/beans orcowpea

rice/cassava

Humid lowlands root crops/maize/foodIegumes/perennial crops

Tropical and subtropical maize or sorghum/beans orhighlands other food legumes

bananas/coffeeSOURCES: David J. Andrews, “lntercropping in Low-Resource Agriculture in Afri-

ca,” contractor report prepared for the Office of Technology Assess-ment (Springfield, VA: National Technical Information Service,December 1987);B.N. Okigbo and D.J. Greenland, “Intercropping Systems in TropicalAfrica,” Multiple Cropping, R.I. Papendick, P.A. Sanchez, and G.B.Triplett (ads.), American Society of Agronomy PubIi. No 27 (Madison,Wl: American Society of Agronomy, 1976), pp. 63-101;D.R. Harris, “Traditional Systems of Plant Food Production and theOrigins of Agriculture in West Africa,” Origins of African Plant Domes-tication, J.R. Harlan, J.M.J. De Wet, and A.B.L. Stemler (eds.) (Mou-ton, The Hague, 1976);Charles Francis (cd.), Multiple Cropping Systems (New York, NY: Mac-millan Publishing Co., 1966);K.G. Steiner, Intercropping in Tropical Smallholder Agriculture withSpecial Reference to West Africa, Deutsche Gesellschaft fur Tech-nische Zusammenarbeit (GTZ), Bonn, West Germany, 1982.

ture requirements and thus greatly diversifytheir production (figure 8-l).

Benefits of Intercropping

Reduces Risk and ImprovesProduction Stability

A principal reason why farmers have adoptedintercropping is that it reduces risk, i.e., it in-creases the reliability or stability of production(1,16,30,36,41). Millet, for example, is less sus-ceptible to drought than sorghum, with whichit is often intercropped. The two crops also dif-fer in their susceptibility to diseases, pests, andweeds. Thus, growing both increases the likeli-hood that there will be some harvest regard-less of the damage of that season’s pests orweather. If one crop dies, the remaining cropcan help compensate for the loss by using someof the water and other resources that becomeavailable. Moreover, since different speciesusually are not planted at the same time, thefarmer can compensate for the failure of thefirst crop by increasing the density of subse-quent crops.

797

Table 8-2.—intercropping of Cereals in Africa (percent intercropped)

Cereal Ghana Ivory Coast Nigeria Sierra Leone Uganda

Maize . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 80 76 NA 84Millet . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 81 90 NA NARice . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 75 58 91 NASorghum . . . . . . . . . . . . . . . . . . . . . . . . 95 72 80 NA 46NA=not availableSOURCES: David J. Andrews, ”lntercropping in Low-Resource Agriculture in Africa:’ contractor report prepared for the Office of Technology Assessment (Springfield,

VA: National Technical Information Service, December 1967).M. Rae, “Cereals in Multiple Cropping, ” Mu/tip/e Cropping Systems, Charles Francis, (cd.) (New York, NY: Collier Macmillan, 1966).D.S.C. Spencer, “Rice Production and Marketing in Sierre Leone,” I.M. Ofori (cd,) Factors of Agricultural Growth in West Africa, Procedure of Internal Confer.ence, Accra, Ghana, 1973.

Figure 8-1 .—Growing Thirteen Crop Species onand Between Raised Mounds in Nigeria

Ca—Cassava Pk—PumpkinG —Groundnut R—RiceL—Lagenaria D1—Dioscorea rotundataPp—Pigeon pea D2—D. alataV—Voandzeia D3—D. bulbiferaCu—Melon D4—D. cayenensisM—Maize

SOURCE B. Okigbo and C. Greenland, “Intercropping Systems in TropicalAgriculture,” Multiple Cropping, R. Papendick, P. Sanchez, and G.Triplett (ads.), American Society of Agronomy Special Publication Num-ber 27 (Madison, Wisconsin: American Society of Agronomy, 1976) pp.63-101

Increases Yields Per Unit of Land byMore Efficiently Using NaturalResources

Intercropping also provides yield benefitsover monocropping, usually measured as nu-tritional or economic gains, that can average15 to 20 percent or in some cases more (51).One explanation for increased yields is that spe-cies and varieties differ to some extent in theresources that they need and how they obtainthem (23). Differences among crops in theirshoot and root geometry can allow mixturesof crops to exploit more of their environmentthan is possible in monocropping (48).

Competition between crops in the same fieldcan have a negative impact on production butthis problem is reduced when the selectedplants differ in their life cycles and criticalgrowth periods. For example, pearl millet anda traditional cowpea variety are often inter-cropped in the West African Sahel. Millet isplanted with the first rains, and cowpeasplanted only when the millet is well established.As a result, cowpeas offer little competition tothe millet. The cowpeas are at first suppressedby the millet but this is of little consequencesince cowpeas can only begin to flower afterthe rains end. By then, the millet is ready toharvest. The cowpeas continue to grow andflower after millet harvest so long as stored soilmoisture is available. If rainfall is below aver-age, cowpea pod yields will be low, but therewill be hay for animals. In good rainfall years,the system has the flexibility to use the extramoisture efficiently with repeated harvests of

198

cowpeas. Thus, the total growing season, whethershort or long, is used more fully.

Intercrops containing legumes can help re-store nitrogen to the soil. unless the legumeis earlier maturing than the cereal, there is noimmediate transfer of fixed nitrogen from thelegume to the cereal, but there is the beneficialresidual effect of the legume on the next year’scereal crop. Legumes grown alone would alsobe able to add nitrogen to the soil, but the higherrisk of increased damage by pests and diseasecan prohibit resource-poor farmers from rais-ing them as monocrops.

Increases Returns to Labor andSpreads Labor Requirememts, TherebyReducing Labor Bottlenecks

Another important advantage of intercrop-ping is that it reduces labor bottlenecks andgives a higher return on the labor invested (35).Labor requirements are spread out becauseplanting, weeding, and harvesting schedulesare different for each crop.

Furthermore, intercropping can reduce weedproblems (6,13,20). In Nigeria, for example, anative legume has been intercropped withmaize to suppress weeds. Since farmers in thispart of Africa devote nearly half of their timeto weeding and the amount of land a familycan cultivate is normally controlled by howmuch family members can weed, intercroppingcan be very advantageous (3,21,35).

Improves Control of Diseases andInsects

Intercropped crops typically suffer less in-sect and disease loss than monocropped ones(5). Pest populations remain lower and they in-flict less damage in intercropped systems (9,43) (table 8-3). One reason for this is that thediverse crop environment provides shelter andnecessary food sources for predators and par-asites of the pest insects (42). In addition, pestsand diseases damaging one crop may not beable to survive on other crops and intercrop-ping decreases the number of plants on whichthey can live and makes those plants harder tofind.

Reduces Erosion and Runoff

Intercrops can reduce water runoff and soilerosion where they provide more continuouscoverage of the soil than occurs in monocrop-ping. Also, the deeper layers of vegetation canreduce the impact of heavy rains and allowmore water to infiltrate the soil (28,45). In onestudy, intercropping maize in cassava on a 15percent slope reduced runoff and soil erosionrelative to cassava alone by 38 percent (2). Wind-induced soil erosion and damage also can bereduced with intercrops. For example, on sandysoils in western Sudan sesame is planted withsorghum or millet when the cereals are largeenough to shield the sesame seedlings fromabrasion by windborne sand.

High Adoption PotentiaI oflmprovements

The long history and widespread acceptanceof intercropping by resource-poor farmersmakes it an excellent candidate for develop-ment assistance. Unlike many other technol-ogies, the potential of intercropping can be real-ized without many of the typical constraintsinvolved in transferring technology from theresearch station to the farmer. Perhaps thestrongest argument for improving existing in-tercropping practices rather than trying to sub-stitute monocrops is that all interventions—e.g., new varieties, fertilizer, pest and diseasemanagement, animal traction—have a goodchance of success when they build on an al-ready familiar base (17).

Research Needs and Constraintsfor Intercropping

Despite increased attention to intercroppingover the last 20 years, it remains inadequatelyresearched. Currently, only an estimated 10 per-cent of AID’s research and extension effortsin agriculture involve intercropping (12). Theknowledge base, research investment, and ex-tension efforts for intercropping are insufficientgiven its prevalence and importance to foodproduction as compared to monocropping.

The low level of attention and funding de-rives from the negative attitudes concerning in-

199

Table 8-3.—Possible Effects of Intercropping on Insect Pest Populations

Factor Explanation Example

Interference with host-seeking behavior:Camouflage

Crop background

Masking or dilutionattractant stimuli

Repellent chemical

of

stimuli

Interference with population

Mechanical barriers

Lack of arrestant stimuli

Microclimatic influences

Biotic influences

A host plant may be protected from insect Bean seedlings camouflaged bypests by the physical presence of other standing rice stubble helps limitoverlapping plants. damage from beanflies.

Certain pests prefer a crop background of a Aphids, flea beetle, and Pieris rapaeparticular color and/or texture. are more attracted to crops, (e. g.,

cabbage) with a background of baresoil than to plants with a weedybackground.

Presence of nonhost plants can mask or Phyllotreta cruciferae (flea beetle) candilute the attractant stimuli of host plants be diverted from collards toleading to a breakdown of orientation, intercropped wild mustard.feeding, and reproduction processes.

Aromatic odors of certain plants can disrupt Grass borders repel leafhoppers inhost-finding behavior. beans.

development and survival:

Companion crops can block the dispersal of pests across the intercrop. Restricteddispersal can also result from mixing resistant and susceptible cultivars of one crop.

The presence of different host and nonhost pIants in a field may affect colonization bypests. If a pest descends on a nonhost it may leave the plot more quickly than if itdescends on a host plant.

In an intercropping system favorable aspects of microclimate conditions are highlyfractioned, therefore insects may experience difficulty in locating and remaining insuitable microhabitats. Shade derived from denser canopies may affect feeding ofcertain insects and/or increase relative humidity which may favor entomophagousfungi which feed on pests.

Crop mixtures may enhance natural enemy complexes leading to a greater abundanceof natural enemies of pests in intercropping than in monocropping.

SOURCE Compiled by M. Altieri and M Liebmann, “Insect, Weed, and Plant Disease Management in Multiple Cropping Systems,” Multiple Cropping Systems, CharlesFrancis (cd.) (New York, NY: Macmillan Publishing Co., 1986), p. 186. Data from: V. Hasse and J.A. Litsinger, “The Influence of Vegetational Diversity on HostFinding and Larval Survivorship of the Asian Corn Borer, Ostrinia furnacalis,” IRRI Saturday Seminar, Entomology Department, International Rice ResearchInstitute (lRRI), Los Banes Philippines, 1981

tercropping and the difficulty in researchingit. Negative attitudes include the belief that in-tercropping is a primitive technology and thenotion that intercropping can only absorb tech-nical changes specifically researched for it.Such attitudes are inconsistent with researchresults however. A number of areas exist whereintercropping can benefit from research de-signed for monocrops. For example, the re-sponses of cereals to low levels of fertilizersare similar whether they are monocropped orintercropped. Row intercrops can also makeuse of advances in monocropping in suchrealms as new plant varieties, fertilizer and pes-ticide applications, and animal traction.

Research on intercropping can encounterunique difficulties. In some cases this is sim-ply a function of having been neglected. Inter-cropping is a relatively new research area and,

therefore, a smaller knowledge base exists. Inother cases it is more a function of understand-ing the complexity of intercropping and themultiple interactions of crop species. Address-ing this complexity is difficult because themajority of plant interactions probably takesplace below ground. The complexity is furtherincreased as specific types of intercrops areoften adjusted to meet social needs (e.g., laborconstraints), therefore, efforts to understand in-tercropping as an agricultural system must alsodraw on social science research.

As long as the majority of farms remain small,production per unit of land and labor will re-main important and will favor the retention andimprovement of intercropping. Specific inter-crop combinations (though not the practice ofintercropping itself) are relatively site-specific.Thus, improvements must necessarily come

200

from research at the farm level. Areas for site-specific research include: determining opti-mum plant densities, crop combinations, andrelative planting dates, and the best means toprovide plant nutrition through use of organicand inorganic fertilizer.

●

Notwithstanding the need to emphasize lo-cal research, the following general researchareas are also important to the improvementof intercropping.

Testing improved varieties for intercrop-ping: Although the best approach wouldbe to breed varieties specifically for an in-tercrop situation, this is a long-term solu-tion (12). For now, improvements can beachieved by testing and selecting for thebest combinations from the existing rangeof varieties (52).Incorporating animal traction with inter-cropping: This is important for cultivation,

weeding, transportation, and manure pro-duction. It will lead to an emphasis on rowcropping. Other problems associated withthe incorporation of animals into farmingsystems also will have to be resolved (seech. 11).Basic research: Apart from on-farm re-search designed to give results for quickuse by extension services, a need exists tounderstand more clearly how intercrop-ping works—what is the nature of compe-tition between species over the season, andwhat are the long-term environmental ef-fects. An important research need is to un-derstand competition for soil moisture andplant nutrients and resultant soil changes.Support could be given to institutions ca-pable of using advanced research technol-ogies such as neutron probes and isotope-labeled fertilizers needed to study below-ground interactions (7).

201

and hedgerows to mark field or garden boundaries, to control livestock movement, andto produce fuelwood and building material when they are pruned (53). Also, trees andshrubs can be planted on contour lines on sloping fields as a soil and water conservationpractice. Linear plantings commonly are sited along roads, trails, and waterways (18).

In addition to these benefits, which lead to higher and more stable crop yields becauseof improvements in soil and water use, trees in agroforestry systems supply several productsto resource-poor farmers and herders. An important product for livestock production is fod-der. The protein-rich prunings can improve animal nutrition, which is considered a majorconstraint to improving the health of African livestock (see ch. 11), Agroforestry can supplynumerous other products that may be consumed directly by a household or sold to generateincome (18,53).

Agroforestry’s contribution to the food security of resource-poor farmers and herders canbe improved substantially. Development assistance for efforts to integrate agriculture, for-estry, and livestock will be essential if this is to occur. Agroforestry programs have shownenough success to justify expanding such efforts, Key factors contributing to the potentialimportance of agroforestry systems are: 1) they fit well into existing African farming sys-tems, 2) they meet numerous needs of resource-poor farmers and herders, and 3) these tech-niques are less capital-intensive than many other technologies.

The Role and Nature ofAgroforestry

Agroforestry systems can help alleviate threeof the most important constraints in Africanagriculture—low-fertility soils; insufficient, er-ratic water availability; and lack of animal fod-der. Leaves from trees and shrubs, and to alesser extent branches and roots, increase soilorganic matter as they decompose. This organicmatter improves soil structure, soil fertility, andsoil water-holding capacity. The deep root sys-tems of trees enable them to use nutrients inthe deep soil layers. Some of these nutrientshave leached down from the topsoil, a prob-lem that is especially severe in degraded soils.The recycling mechanism of trees and shrubsbrings these nutrients back to the soil surfacewhere they again can become available to shal-low-rooted annual crops (34,5 o),

Trees and shrubs used in windbreaks can in-crease water availability y by reducing wind andthereby reducing evapotranspiration. Also,their vegetative canopies reduce the impact ofheavy rainfalls, cut run-off, and thus increaseinfiltration of water into the soil. Also, morewater remains available for plant growth be-cause the shade provided by trees lowers soiltemperature, which in turn acts to slow decom-position of organic matter (32).

By improving soils and increasing wateravailability, agroforestry systems contribute tohigher and more stable yields of crops and for-age. Tree and shrub prunings also contributeto livestock nutrition. Since poor nutrition isconsidered a major constraint to improving ani-mal health, the protein-rich browse possiblefrom agroforestry is an important considera-tion in promoting its use.

Agroforestry systems provide many of theproducts resource-poor farmers and herdersformerly obtained from forests: firewood andcharcoal; posts, poles, and construction wood;fruits, nuts and edible leaves; fiber for mats,baskets, and ropes; and plant materials formedicines, dyes, and cosmetics (18,53). Thesegoods may either be used by the household orsold. These benefits of agroforestry will con-tinue to grow in importance as remaining for-ested areas of Africa continue to succumb tohuman population pressures.

Dispersed Field-Tree Intercropping

Dispersed field-tree intercropping is the sec-ond most widely practiced general agrofores-try technique in Africa, (Traditional shiftingagriculture that relies on trees to restore soilfertility, is the most common,) Numerous vari-

202

ations exist on the mixture of species and thepatterns in which they are planted.

The practice is used extensively by resource-poor farmers in semiarid regions, particularlyin West Africa, “farm trees” are grown withinand adjacent to crop fields. When natural re-generation is relied on, the trees appear to havea random arrangement. When clearing the bushfor a new field, certain species are preserved.These most commonly are food-producing trees(fruits, nuts, leaves, etc.) such as the shea tree(Butyrospermum parkii) or the locust bean (Par-kia biglobosa). Such savanna species, however,commonly do not regenerate well under natu-ral field conditions (18).

Acacia albida is a particularly beneficial treeused widely in the semiarid areas of the Sahel.The most unusual feature of this nitrogen-fixingtree is that it loses its leaves during the rainyseason, making it possible to raise crops, suchas sorghum and millet, directly under the can-opy of the tree with little competition for light.Crop yields are much higher under the tree thanoutside the canopy (table 8-4).

Acacia albida also benefits livestock produc-tion. Its pods and leaves provide more fodderper unit weight than meadow hay, rice straw,or groundnut tops (11), and Acacia fodder isproduced during the dry season when annualgrasses have disappeared (32). In addition, live-stock concentrate near the trees and their ma-nure further enriches the soil (32).

Where the proper balance has existed be-tween Acacia albida, crops, and livestock, thesystem has been able to support several times

Table 8-4.—Grain Yields Under the Crown ofAcacia albida Compared to Grain YieldsOutside the Crowna (kilograms per hectare)

Yield without Yield withGrain Acacia albida Acacia albidab

Millet . . . . . . . . . . . 810 1,110Millet . . . . . . . . . . . 457 934Millet . . . . . . . . . . . 820 1,250Sorghum . . . . . . . . 457 934aData is from Senegal and Burkina Faso.bTwenty-five to forty mature trees per hectare.SOURCE: Michael McGahuey, Impact of Forestry Initiatives in the Sahel: Effect

of Acacia albida on Millet Production in Chad (Washington, DC:Chemonics International, January 1986).

the average human population for SahelianWest Africa (39). For instance, millet was con-tinuously cropped in Sudan for 15 to 20 yearsin association with Acacia albida, comparedto only 3 to 5 years without the tree (32).

Natural regeneration of Acacia is erratic andhas declined over the past 20 to 30 years be-cause of extended drought and grazing pres-sures. Few Acacia albida still exist in areas re-cently cleared for farming, but their numberis slowly increasing in existing farm fields be-cause some farmers are protecting the seed-lings. It may take about 10 years before the newtrees have much effect on crop yields, but thebenefits last the remaining 70 to 90 years of thetrees’ lifespan (25). Even on old fields wherethe tree is common, the tree cover is often farbelow that which would give optimum yields(18).

The list of useful trees, however, does not endwith Acacia albiba. For example, one investi-gation of trees and shrubs in the Sahel identi-fies some 114 multipurpose species. The useof Balanites aegyptica in agrosilvopastoral sys-tems (i.e., that combine crops, trees, and live-stock), or Acacia senegal in bush fallow sys-tems, provides two more examples oftraditional production systems that integratetrees (38). However, a combination of factorsis contributing to the decline of many species,including species that have historically pro-vided food during recurrent and critical food-shortage periods, or products for local use and

Photo credit: Mike McGahuey

The millet growing under this Acacia tree in Chad isdenser, taller, and greener than that in the foregroundbecause Acacia increases soil fertility and water avail-

ability for the intercropped cereal.

203

trade (44). Indigenous information on use ofthese resources is also being lost.

Alley Cropping

Alley cropping is a modern agroforestry tech-nique developed from well-established tradi-tional practices. Its precursor, the bush fallowsystem of shifting agriculture, is an indigenousform of agroforestry that has been practicedfor centuries. Fields were cropped for severalyears followed by an extended woody fallowwhen deep-rooted trees and shrubs played a keyrole in restoring soil fertility. In the past, withlow human population levels and land freelyavailable, this represented an ecologicallysound system of subsistence agriculture. Today,however, few areas remain where the popula-tion/land balance permits land to be left fallowfor the necessary 15 to 30 years to restore fer-tility.

Scientists at the International Institute ofTropical Agriculture (IITA) incorporated thedesirable features of bush fallow into a continu-ously productive farming system for the humidtropics. Rows of nitrogen-fixing trees or shrubs,such as Leucaena, Gliricidia, and Calliandra,are planted 2 to 4 meters apart, and the spacebetween is planted in an annual crop like maize.The shrubs are pruned frequently, with thetrimmings used as mulch, fodder, or fuelwood.Yields of maize stabilized at about 2 tons/ha af-ter 6 years of continuous alley cropping; with-out alley cropping, the yield was no more thanone-half ton/ha (24). An especially promisingshrub for use on waterlogged soils is Sesbaniarostrata, native to Africa. Rice yields were in-creased 55 percent with the addition of the Ses-bania prunings, comparable to the addition of120 kg nitrogen/ha (33).

Although experimental results such as theseindicate the technical feasibility of alley crop-ping, farmer acceptance and adoption is in theearly stages of evaluation. Alley cropping ismore labor-intensive than traditional methodsand requires a considerable change in farmingpractice. Farmer participation in farm trialsorganized by IITA and the International Live-stock Center for Africa has been enthusiastic,

however (19). Of particular interest is the evi-dence of farmer adaptation and experimenta-tion with introduced agroforestry systems, sug-gesting the ability to tailor systems to variablecircumstances and needs (37).

Alley cropping probably will find its great-est acceptance in areas where land scarcity isthe most acute, that is, where shifting agricul-ture is no longer possible. It will require adapt-ive research for the seasonally humid and high-land areas and major modifications before itcan be used in the semiarid zone where watercompetition between trees and crops would bea constraint. Furthermore, none of the speciesused for alley cropping in the humid zone seemsuitable for the non-irrigated semiarid zone, andlikely alternatives are not readily apparent. Thisis especially true for hardy, fast-growingnitrogen-fixers (18).

Windbreaks (or Shelterbelts)

Windbreaks are uniform rows of trees plantedin fields perpendicular to the prevailing windsto reduce evapotranspiration, soil erosion, andwind-induced crop damage. Windbreaks are avirtually unknown practice in traditional low-resource agriculture in Africa, but are receiv-ing some attention among the developmentagencies. The Majjia Valley Windbreak Projectin Niger is one of the most successful projectsin the Sahel. The project, begun in 1975 withthe assistance of the private voluntary organiza-tion CARE, has established about 350 kilome-ters of windbreaks to protect some 3,000 hec-tares of rainfed millet and sorghum fields (10).Early evaluations of this project indicate thatcrop yields had increased 23 percent, while amore recent estimate is that they increased 15percent. Both estimates take into account the6 percent of farmland “lost” to trees. The mostlikely explanation for the differing estimatesis that the trees are now larger, depressing cropyields by causing more shading and compet-ing for nutrients (53).

The small average field size and the need toorient the windbreaks perpendicular to prevail-ing winds makes it impractical for an individ-ual farmer to establish windbreaks. To be suc-

204

cessful, a group of farmers, ideally an entirevillage as was done in the Majjia, must cooper-ate in the effort so that windbreaks can extendacross adjoining fields. Another constraint inwindbreak establishment is that they need pro-tection from livestock. Livestock in semiaridregions usually are left to roam freely duringthe dry season. Windbreaks or other field plant-ings dispersed over large areas are difficult toprotect from grazing. The villagers in the Maj-jia agreed not to allow grazing during the ap-proximately 3 years required for the treebranches to grow out of the reach of livestock.This was enforced by guardians hired withproject funds. Also, rights to the trees can becontroversial; ownership of the windbreaks inthe Majjia Valley and the distribution of woodproducts harvested from them are still un-resolved 13 years after the project’s start (18).

The Majjia Valley project started out in re-sponse to a request for assistance from localvillages. It began on a small scale workingclosely with forest service agents and villagers.The project has developed enthusiastic supportfrom villagers who have seen the benefits first-hand. Now 60 farmer-owned, private nurser-ies exist in the valley and these help respondto requests for assistance from surrounding vil-lages (19). Periodic partial harvesting of thewindbreaks could make the participating vil-lages largely self-sufficient for their wood needs(18).

Live Fencing and Other LinearPlantings

Another agroforestry approach is to use treesor shrubs to form live fences or hedgerows tomark field or garden boundaries and controllivestock movement. These also can be prunedto produce fuelwood and building materials(53). Live fencing requires a large number ofclosely spaced plants and frequent pruning. Theuse of live fencing varies greatly between re-gions. In some places it is almost unknown, yetin the Fouta Djalon Highlands in Guinea thereis a social caste who make their living estab-lishing live fencing (18).

Live fencing, although labor-intensive toestablish, provides a low-cost alternative to

metal fencing (27). Fences in the semiarid re-gions formerly were made with readily avail-able thorn bushes chopped down and arrangedwhere needed. With desertification and in-creased demands on resources, thornbushes areincreasingly in scarce supply. Thus, live fenc-ing could be advantageous, especially arounddry-season gardens which must be protectedfrom free-ranging livestock. Unfamiliarity withlive fencing techniques seems to be a signifi-cant constraint in many areas.

Other linear plantings do not necessarily haveto be as densely planted or require as frequentpruning as live fencing. Encouraging the plant-ing of multipurpose trees and shrubs along fieldmargins often is easily achieved because manyfarmers want to define the limits of their prop-erty clearly. Field border plantings may be afirst step toward more integrated (e.g., of crops,trees, and livestock) agroforestry techniques.A second step can be planting trees and shrubson contour lines on sloping fields as a soil andwater conservation practice, Linear plantingsalso commonly are established along roads,trails, and waterways (18).

Potential for Adoption

Agroforestry offers strong potential becauseit tends to fit well into existing African farm-ing systems and meets numerous needs of re-source-poor farmers and herders. Agrofores-try can contribute to improved management ofsoil and water resources, leading to increased,more stable yields. The multiple benefits—food,fodder, fuelwood, building materials, and in-come—possible from agroforestry systems alsocan reduce pressure on natural forest and graz-ing lands.

Agroforestry techniques are rarely capital-intensive compared to many other technologies,thus encouraging farmer and herder experi-mentation and adoption. If seedlings are pro-vided by a service or project, the main inputsfrom the farmer or herder is labor. An impor-tant fringe benefit of agroforestry developmentis that by increasing soil organic matter it en-hances effectiveness and reduces potentialwaste in commercial fertilizer applications.Most tropical soils are characterized by highly

205

oxidized, low activity clays that are unable tobind nutrients in a form usable to the plants.The addition of organic matter to the soil im-proves its ability to retain fertilizers until cropscan make use of them.

Newly developed, synthetic, water-absorbingpolymers applied to form a water-absorbinglayer at the root zone may prove to be in-strumental in afforestation efforts, particularlyin the arid/semi-arid zone. In experiments con-ducted in Sudan, the survival period of tree see-dlings was increased fivefold when polymers,able to hold 400 times their weight in water,were used in the soil mixture. The present sur-vival rate for tree seedlings in Sudan is no morethan 50 percent and in Ethiopia the perform-ance is even worse, with only 15 percent sur-vival recorded among 500 million seedlings. Ata cost of 14 to 22 cents per tree, the new tech-nique could be a cost-effective way of im-proving afforestation efforts (8).

Despite its promise, development assistanceagencies have become interested in agrofores-try only recently. PVOs, and CARE in particu-lar, have been innovators in agroforestry. CAREhas 13 agroforestry projects in 11 African coun-tries (26). Few projects are as much as 10 yearsold, but these have already made substantialprogress toward developing stable, sustainablefarming systems. It appears that developmentagencies have only scratched the surface ofagroforestry’s potential for improving the lotof the resource-poor agriculturalist in Africa.

Problems and Approaches

lntegrating AgricuIture, Forestry,and Livestock

One of the most serious obstacles to promot-ing agroforestry as a sustainable land-use sys-tem is institutional, The fact that agriculturaleducation and administration typically are pur-

206

sued along narrow disciplinary lines createsfundamental problems for agroforestry–by def-inition an integrated production system requir-ing interdisciplinary research. In simplestterms, the dilemma this creates can be charac-terized the following way:

Agroforestry is institutionally considered asub-division of forestry. Forestry institutionsdeal with forestry and forest land. The majorpotential of agroforestry lies in the integrationof trees into agricultural and pastoral lands.The development of these lands is the mandateof agricultural institutions. Agricultural insti-tutions are not mandated to deal with agro-foresty [29).

Even forestry departments have until recentlyshown considerable reluctance in promotingagroforestry, Foresters now seem more willingto support agroforestry, realizing that farmersfaced with insufficient crop yields will not de-vote land and energies to tree plantations solelyfor firewood production. Those few projectssuch as the Majjia Valley Windbreak Projectthat have involved tree planting on farmers’fields to increase crop yields have enjoyedmuch greater success than projects that havejust emphasized maximizing wood volume/ha/year.

The agriculture, livestock, and forestry serv-ices of most African governments are asstrongly separated among disciplines as, andin part because they reflect, their Western coun-try counterparts (53). The need to improve in-tegration of these agricultural activities is par-ticular important in the case of Africanagriculture. Such institutional changes cannotoccur overnight, but increased integration andcooperation among disciplines could bestrongly encouraged, among agricultural aswell as social sciences. For example, develop-ment assistance could ensure that participationby all relevant government services be negoti-ated in the project planning stage, even thoughthis may make the project administrativelymore burdensome. Funding could be providedfor multidisciplinary agroforestry workshopsthat include foresters, agronomists, livestockspecialists, and social scientists.

The number of schools offering agroforestycourses in developed countries is increasing,but still is small. Probably no more than sixuniversities in the United States offer instruc-tion in agroforestry, usually a single, recentlycreated, course (46). This shortage is paralleledin Africa. Development assistance agenciescould support agroforestry courses as part ofdegree programs in tropical forestry. AID, forexample, could provide funding to selected U.S.universities to develop or bolster agroforestrycurricula. Support for regional agroforestryschools for the different agroecological zonescould also be promoted.

Obstacles of Land and Tree Tenure

Farmers rarely will plant trees, let alone pro-tect and care for them, if they have no assur-ance that they will reap the benefits. This makesagroforestry difficult for those farmers who lacksecure rights to their land. Few poor farmersactually hold title to the land they cultivate, ascentral governments generally claim most ofthe land. In practice, however, most of the farm-land is passed down from one generation toanother and remains under family control (18).

A large percentage of farmland in some areasis cultivated by families who borrow or leasefarmland. The landowner in such cases mayforbid tree planting by the tenant if local cus-tom associates tree planting with land tenurerights. Lack of land and tree tenure is especiallyproblematic for women, who could benefitgreatly from having an improved, more acces-sible supply of fuelwood and fodder. Evenwhere land tenure is well defined, land and treerights may be separate.

Communal farmland has also been the tar-get of a number of efforts to mobilize tree plant-ing efforts, but the track record of these effortsis not good. What belongs to the group is noone individual’s responsibility, and the care andprotection needed by young trees is too oftenlacking on communal lands (18). The problemcan be particularly acute in the case of com-munal grazing areas. Development assistanceefforts will be more successful when they takelocal land- and tree-tenure practices into ac-

207

count in the design of agroforestry projects. Therights to use the trees need to be defined as partof the project design. African governments mayneed to reassess their land tenure and forestrylegislation if agroforestry is to reach its po-tential.

Encouraging Investment inAgroforestry

The payback period varies considerably fordifferent agroforestry techniques. Alley crop-ping may start to improve yields during the firstyear or two. A live fence, if managed properly,may become effective in 1 or 2 years. Wind-breaks may begin to produce results in 3 or 4years. Some fruit trees used for intercroppingmay begin to bear fruit in 3 years. The shorterthe period before benefits are realized, the morelikely farmers are to invest scarce land, labor,and capital in agroforestry initiatives. Thres-holds of investment are obviously highly vari-able and will depend on such factors as levelof investments required, added risks that maybe created, how and to whom benefits are de-rived, or previous experience with innovation.A better understanding of these economictrade-offs from the farmers’ point of view is initself an important research area that could alsohelp “calibrate” research priorities in experi-ment stations to what is needed and adoptableby farmers.

Other agroforestry techniques may take muchlonger to produce a return on investment. Forexample, Acacia albida intercropping mayyield few benefits for the first 10 years, althoughthe long-term benefits may be very substantial,particularly in light of increasing demands be-ing placed on the resource base. Few resource-poor farmers have the luxury to approach in-

vestment decisions using such a long-term per-spective, however. Under such circumstances,supporting agencies may need to underwritecosts until farmers and herders begin to real-ize benefits. Expanding markets for agrofores-try goods also may provide incentives and sup-port the sustainability of such efforts. In othercases, however, continued support may dependon more permanent forms of government in-centives or restrictions, the costs and benefitsof which should be viewed within the contextof long-term national interests in sustaining thenatural resource base.

Support for Docentralizod, LocallyManaged Nurseries

Most seedlings for agroforestry plantings areproduced in central nurseries, usually in co-operation with national forest services. With-out development assistance, forest services ofmany African countries are incapable of pro-ducing and distributing the quantity of see-dlings necessary for large-scale plantings. Moreimportantly, many are not capable of helpinglarge numbers of widely scattered farmers, eachneeding small-scale plantings. Even if farmersaccept a particular agroforestry technique, itwill do little good if they have no source forthe required seedlings. Improving local capac-ity to produce seedlings would give farmers bet-ter control over access to desired tree species,and would greatly reduce the significant logisti-cal and transportation problems involved withcentralized nurseries. A few projects have be-gun to encourage and support the creation oflocal village, school, and private nurseries. TheCARE Koro Village Agroforestry Project inMali, the AID Community Forestry Project inGuinea, and the Somalia Community ForestryProject are examples (box 8-l).

Box 8-1.—Community Agroforestry in Somalia

For the first time, women have become an important force in a major agroforestry project in north-west Somalia—an area hit hard by desertification and a fuelwood crisis. At least 7,000 people, includ-ing members of the Somali Women’s Democratic Organization (SWDO), the National Range Agency(NRA), and local residents and refugees have learned a variety of skills that can be used in futuredevelopment work. In addition to planting some 300,000 trees, skills have been learned for establish-

.

208

ing and managing local nurseries, collecting and analyzing data, and coordinating large community-based reforestation and conservation activities. Some 60,000 persons have benefited in 2 years.

Associated enterprises include producing and marketing fuelwood and growing vegetables be-tween rows of newly planted trees. The trees fix nitrogen in the soil; protect the vegetables from windand soil erosion; and produce green manure, mulch, and firewood, The firewood provides fuel, andthe vegetables improve diets, and both will be marketed by the women. Feasibility studies are alsolooking at beekeeping/honey production as another income-generating enterprise because the trees’flowers attract large numbers of bees.

The communities involved want to expand their forestry and agriculture activities. Plans for waterreservoirs and irrigation systems are underway. The Overseas Education Fund (OEF) has providedextensive training to SWDO members and NRA extension agents in program design, implementationand management, technical agroforestry, and small business management and marketing to enablethem to carry out programs in other parts of the country. Training materials have been publishedfor use in similar efforts in the region.

A factor in the success of this project is the government of Somalia’s recognition of the impor-tance of conservation issues, This region of Somalia has suffered for many years from severe droughtand desertification caused in part by mismanagement of natural resources. Because of the scarcityof trees, supplies of the region’s most important source of fuel—firewood—were very low and werefurther depleted by an influx of refugees from Ethiopia. In response to this crisis, the governmentdeveloped a 5-Year Plan (1982-86) which gives anti-desertification and forestry top priority. OEF, inturn, launched this 2-year pilot agroforestry project with funding from AID.

The refugees are mostly women and children who came from Ethiopia. While only 43 percentof the over 500 Ethiopian refugees and Somalis hired from the local communities are women, thisis considerably more than the usual number of women engaged in paid manual labor in rural Somalia.This is probably the first time that the men in the project have had so many female co-workers, orthat so many had access to training in technical and management skills.

SOURCES: Overseas Education Fund International, Washington, DC, Press Release, Dec 11, 1986; OEF International Annual Report, 1986; OEF Internatlonal FinalReport: Community Forestry for Refugee Related Areas, 1987

1.

2.

3.

4.

5.

CHAPTER 8 REFERENCES

Abalu, G. O., “A Note on Crop Mixtures UnderIndigenous Conditions in Northern Nigeria,”Iournal of Development Studies vol. 12, 1976,pp. 212-220.Aina, P. O., Lal, R., and Taylor, G. S,, “Soil andCrop Management in Relation to Soil Erosionin the Rainforest of Western Nigeria, ” Proceed-ings of National Soil Erosion Conference, Pur-due University, 1977, pp. 75-84.Akobundu, 1.0., “Live Mulch: A New Approachto Weed Control and Crop Production in theTropics, ” Proceedings of the 1980 British CropProduction Conference–Weeds, 1980, pp.377-382.Altieri, Miguel A., Agroecology; The ScientificBasis of Alternative Agriculture (Boulder, CO:westview Press, 1987).Altieri, M. A., and Liebman, M., “Insect, Weed,and Plant Disease Management in Multiple

Cropping Systems,” Charles A. Francis (cd.),Multiple Cropping Systems (New York, NY:Macmillan Publishing Co., 1986), pp. 183-218.

6. Altieri, M. A., Letourneau, D. K., and Davis, J. R.,“Developing Sustainable Agro-Ecosystems, ”Bioscience, vol. 33, 1983, pp. 45-49.

7. Andrews, David J., “Intercropping in Low-Resource Agriculture in Africa,” contractor re-

8

port prepa~ed for the Office of TechnologyAssessment (Springfield, VA: National Techni-cal Information Service, December 1987).Arthur, R., “New Weapon To Beat Drought,”South, No. 73, November 1986, pp. 127-128.

9. Bach, C. E., “Effects of Plant Density and Diver-sity on the Population Dynamics of a SpecialistHerbivore, the Striped Cucumber Beetle, Aca-]ymma Vittatta (Fab.),” Ecology, vol. 61, 1980,pp. 1515-1530.

10. CARE International, “Journees de Travail sur

209

11.

12,

13

14

15.

16,

17,

18,

19.

20.

21,

22

I’Etude des Brise-Vent de la Majjia,” Niamey,Niger, 1986.Charreau, C., “Soils of Tropical Dry and DryWet Climatic Areas of West Africa and TheirUse and Management, ” Lecture Series, CornellUniversity, Ithaca, NY, 1974.Francis, Charles, “Challenges of Intercropping:Policy Issues, ” contractor report prepared forthe Office of Technology Assessment (Spring-field, VA: National Technical Information Serv-ice, December 1987).Francis, Charles (cd,), Multiple Cropping Sys-tems (New York, NY: Macmillan Publishing Co.,1986).Francis, C., Harwood, R., and Parr, J., “The Po-tential for Regenerative Agriculture in the De-veloping world, ” American Journal ofAlterna-tive Agriculture, vol. 1, No. 2, Spring 1986, pp.65-75.Francis, C. A., and Harwood, R. R,, “EnoughFood, ” Special Publication, Rodale Press andRegenerative Agriculture Association, Emmaus,PA, 1985.Francis, C, A., and Saunders, J, H., “EconomicAnalysis of Bean and Maize Systems: Monocul-ture Versus Associated Cropping, Field CropsRes., vol. 1, 1978, pp. 319-335.Fussell, L. K., Serafini, P. G., Bationo, A., andKlaij, M. C., “Management Practices to IncreaseYield and Yield Stability of (Pearl) Millet inAfrica,” J.R. Witcombe and S.R. Seckerman(eds.), International Pearl Millet Workshop,ICRISAT/ INTSORMIL, Patancheru, India,1986.Hagen, Roy T., “Agroforestry in Africa,” con-tractor report prepared for the Office of Tech-nology Assessment (Springfield, VA: NationalTechnical Information Service, December1987).Harrison, P., The Greening of Africa: BreakingThrough the Battle for Land and Food, An In-ternational Institute for Environment and De-velopment-Earthscan Study (Toronto: PaladinGrafton Books, 1987).Hart, R. D., “The Design and Evaluation of aBean, Corn, and Manioc Polyculture System forthe Humid Tropics,” Ph.D. Dissertation, Univer-sity of Florida, Gainesville, 1974.Helm, L., “The Role of Weeds in Human Affairs,Weed Science, vol. 19, 1971, pp. 485-490.Horowitz, Michael M, The Sociology of Pastor-alism and African Livestock Projects, ProgramEvaluation Discussion Paper No. 6 (Washing-ton, DC: Agency for international Development,1984).

23.

24.

25.

26.

27,

28.

29.

30,

31

32,

Horwith, Bruce, “A Role for Intercropping inModern Agriculture,” Bioscience, vol. 35, No.5, 1985, pp. 2 8 6 - 2 9 1 .Kang, B, T., Wilson, G. F., and Lawson, T. L., “Al-ley Cropping: A Stable Alternative to ShiftingCultivation” (Ibadan, Nigeria: International in-stitute of Tropical Agriculture, 1984).Kirmse, R. D., and Norton, B, E., “The Potentialof Acacia albida for Desertification Control andIncreased Productivity in Chad,” Z3iologicaZConservation, vol. 29, 1984, pp. 121-141.Kramer, John M., “Agroforestry in Africa, ” Tes-timony to U.S. House of Representatives, Com-mittee on Science and Technology, Subcommit-tee on Natural Resources, Agriculture, andEnvironment, Washington, DC, October 1985,Kuchelmeister, Guide, “State of Knowledge Re-port on Tropical Hedges,” Deutsche Gesellschaftfur Technische Zusammenarbeit (German Agencyfor Technical Cooperation), Eschborn, FRG,1986 (draft).Lal, Rattan, “Managing the Soils of Sub-SaharanAfrica,” Science, vol. 236, May 29, 1987, pp.1069-1076.Lundgren, B. O., and Raintree, J. B., “SustainedAgroforestry,” ICRAF Reprint No. 3, Interna-tional Council for Research in Agroforestry,Nairobi, Kenya, August 1983,Lyman, J, K., Sanders, J. H., and Mason, S. C.,“Economics and Risk in Multiple Cropping, ”C.A. Francis (cd.), Multiple Cropping Systems(New York, NY: Macmillan Publishing Co.,1986), Pp. 250-266.MacDonald, L.H. (cd.), Agro-forestry in the Afri-can Humid Tropics, Proceedings of a WorkshopHeld in Ibadan, Nigeria, Apr. 27-May 1, 1981(Tokyo, Japan: United Nations University, 1982),McGahuey, Michael, “Impact of Forestrv Ini-tiatives in ~he Sahel: Effect of Acacia albida” Treeon Millet Production in Chad” (Washington,DC: Chemonics International, January, 1986).

33. Mulongoy, K., “Potential of Sesbania rostrata(Brem.) as Nitrogen Source in Alley CroppingSystems,” Biological Agriculture and Horticul-ture, vol. 3, 1986, pp. 341-346.

34. Nair, P. K. R., Soil Productivity Aspects of Agro-forestry (Nairobi, Kenya: International Councilfor Research in Agroforestry, 1984).

35. Norman, D. W., Newman, M. D., and Ouedraogo,I., “Farm and Village Production Systems of theSemi-Arid Tropics of West Africa: An Interpre-tive Review of Research, ” ICRISAT ResearchBulletin No. 4, Pantancheru, India, 1981.

36. Norman, D. W,, “Rationalizing Mixed CroppingUnder Indigenous Conditions: The Example of

210

37.

38.

39.

40.

41.

42.

43.

44,

45.

Northern Nigeria, ” Journal of DevelopmentStudies vol. 11, 1974, pp. 3-21.Okali, C., and Sumberg, J. E., “Sheep and Goats,Men and Women: Household Relations andSmall Ruminant Production in Southwest Ni-geria,” J.L. Moock [cd.), Understanding Africa’sRural Households and Farming Systems (Boul-der, CO: Westview Press, 1986), pp. 166-181.Palmberg, Christel, “Selection and Genetic Im-provement of Indigenous and Exotic Multi-Pur-pose Tree Species in Dry Zones,” AgroforestrySystems, vol. 4, 1986, pp. 121-127.Porteres, R,, “The Improvement of AgriculturalEconomy in Senegal,” African Soils, vol. 3, No.1, 1954, pp. 13-57.Power, J. F., and Follett, R. F., “Monoculture,”Scientific American, vol. 256, No. 3, March1987, pp. 79-86.Rae, M. R., and Willey, R. W., “Evaluation ofYield Stability in Intercropping: Studies on Sor-ghum/Pigeonpea, ” Experimental Agriculture,vol. 16, 1980, Pp. 105-116.Risch, S. J., Andow, D,, and Altieri, M. A., “Agro-ecosystem Diversities and Pest Control: Data,Tentative Conclusions, and New Research Di-rections, ” Environmental Entomology, vol. 12,1983, pp. 625-629.Root, R. B., “Organization of a Plant-ArthropodAssociation in Simple and Diverse Habitats: TheFauna of Collards (Brassica oleracea) Ecologi-cal Monographs, vol. 43, 1973, pp. !35-124.Sene, El Hadji, “Trees, Food Production and theStruggle Against Desertification,” Unasylva,vol. 37, 1985/4, Pp. 19-26.Siddoway, F, H., and Barnett, A. P., “Water andWind Erosion Control Aspects of Multiple Crop-~in~,” American Societv of A~ronomv SDecial

46,

47!

48,

494

50,

51.

52.

53.

Publication No. 27 (Madison, WI: ASA, 1976),pp. 317-335,Steiner, Frederick, Associate Professor of Land-scape Architecture and Regional Planning, De-partment of Horticulture and Landscape Archi-tecture, Washington State University, Pullman,Washington, personal communication to Roy T.Hagen, January 1987.Steiner, K. G., Intercropping in Tropical Small-holder Agriculture With Special Reference toWest Africa, Deutsche Gesellschaft fur Tech-nische Zusammenarbeit (GTZ), Bonn, West Ger-many, 1982.Trenbath, Brian R., “Resource Use by Inter-crops, ” in: Multiple Cropping Systems, CharlesA, Francis (cd.) (New York, NY: Macmillan Pub-lishing Co,, 1986), pp. 57-81.weber, Fred R., consultant, International Re-sources Development and Conservation Serv-ices, Boise, Idaho, personal communication,1986.weber, Fred R., and Carol Stoney, Reforestationin Arid Lands (Arlington, VA: Volunteers inTechnical Assistance, 1986).Willey, R, W., “Intercropping-Its Importanceand Research Needs, 1. Competition and YieldAdvantages and 2, Agronomy and Research Ap-proaches,” Field Crop Abstracts, vol. 3, 197g,pp. 1-10, 73-85.Willey, R. W., and Rae, M, R., “Genotype Studiesat ICRISAT, ” R.W. Willey and C. Carver (eds.),[international Workshop on Intercropping, Jan-uary 1979, ICRISAT, Patancheru, India, 1981.Winterbottom, Robert, and Hazlewood, Peter T.,“Agroforestry and Sustainable Development:Making the Connection, ” Ambio,, vol. 16, No,2-3, 1987, Pp. 100-110.