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f Crilical Reviews in Planl Sciences. 14(3):2 13-238 ( 1995)

Agroecology of Tropical Underground Crops for Small-Scale Agriculture Hector R. Valenzuela and Joseph DeFrank Department of Horticulture, University of Ha,«aii at Manoa,

3190

Maile Way, Honolulu , HI

96822

Referee: Dr. Charles A. Francis, Director, Center for Sustain able Agricultural Systems, University of Nebraska, Lincoln

ABSTRACT: The important tropical root and tuberous crops cassava (Manihol esculenta). swee t potato (Ipomoea balOlas), yams (Dioscorea spp.). and the aroids (especiall y Xanlhosoma and Colocasia spp.) represent an important source of relatively inexpensive carbohydrates to large sectors of the population in tropical areas. One

or more tropical root crops are normall y a staple in rural communities and are typ icall y grown on small-scale subsistence fanns. The current status of the agroecology research on these crops. includi ng productivity under polyculture systems. resource (water, nutrients. li ght . space) utilizatio n, to lerance to envi ro nmental stress. pes t dy namics respo nse to habi tat manipulatio n. and alternati ve cultural practices, is reviewed in this paper as the y

re late to the performance of these crops in small-scale tropical agricultural systems. The development of technolog ical recommendations to improve the productivity of tropical root crops in the tropic s is depe ndent on an

understanding of important underlying agroecological principles. The objective of background ecophysio logical work is to deve lop crop-specific technological packages appropriate to low-input subsistence fanning. and to match specific crops with a cropping system that will result in adequate yie lds and in ecological and socioeconomical

sustainability. Because of the close relationship between crops and humans in small-scale farms of the tropics. it is imperative th at agroecology research be holistic. multidisci plinary, and cogni zant of the many soc ioeconomic

and cultural factors that will detenni ne whether improved technologies will be adopted in any given locati on. KEY WORDS: tropical root crops. Manihol esculenta. Xanrhosoma sagiw/olium , Colocasia esculenta. Dioscorea spp., Ipomoea batalas, Arnceae, farm ing systems research. polycultures. intercropping. multiple cropping. rural development, resource utilization. integrated pest management. sustainable agric ulture .

I. INTRODUCTION

Tropical root and tuberous crops contribute significant calories to the diets of over one third of the population in developing countries. 72 Glob-ally, these crops are grown on about 20 million ha ofland, with annual production of over 170 million MT. Their omnipresence throughout the tropics is explained by their long-standing existence in native societies as traditional crops,27 their versatility in the diet of rural communities, their compatibility (from both an ecophysiological and marketing standpoint) with other annual and perennial crops in polyculture systems, and their ability to maintain relatively stable yields in marginal lands characterized by nutrient, water, salt, temperature, and other environmental stresses. Until recently, however, tropical root and tuberous crops, except the upland Peruvian potato

(So /anum tuberosum), received little attention from agricultural researchers, extensionists, and policy decision makers.72.SS. IS7 This is partly explained by the fact that root crops have traditionally been considered a staple for the poor, and "inferior" to grain crops in Western-dominated cultures. For example, during the seventeenth and eighteenth centuries in Europe, So/anum was consumed by the "lower" and socially oppressed people, who were viewed with "disdain" by the dominant classes."" Unfortunately, this prejudice toward root crops still prevails in many tropical regions, especially in areas where grain crops and grain-based products have long been promoted (until recently) by international agencies and local governments. Over the last 20 years, work conducted by international research centers and local governmental agencies has begun to provide needed infonnation on the dynamic productivity of tropical

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root crops under diverse environmental parameters and cropping systems in tropical agroecosystems. The objective of such work has been to match the root crop species of interest with the environmental conditions in an agroecosystem. 176 This requires a better understanding of crop response to soi l quality and fertility, seasonal c limatic patterns, primary and secondary pest organisms, and their interactions (e.g., climate x soil fertility ; soil fertility x pest population dynamics) . Models to statistically analyze d ata obta in ed in hi gh ly comp lex e n vironments,66.84.138.182.207.238 and to predict/evaluate crop growth in polyculturesI 3.32.23 1have tentatively been developed. Because of the close interaction between humans and crops in small farms of the tropics, socioeconomic and cultural factors must be considered together 180 to understand specific crop patterns, crop cycles, and production technologies adopted by agricultural communities. This paper reviews the current agroecological research on root and tuberous crops of economic importance in the tropics : cassava (Manihot esculenta), sweet potato (Ipomoea batatas), the aroids [mainly taro (Colocasia esculenta) and coc oyam (Xa nthosoma sagittifolium)] , yams (Dioscorea spp.), and other minor root crops, with specific focus on the crop productivity of small farms.

II. CROP DESCRIPTION AND MARKET VALUE IN SUBSISTENCE AGRICULTURE A. Cassava In terms of economic significance, cassava is the number one root crop of the tropics . Cassava is grown in over 80 countries and accounts for 75 % of all tropical root crops. World cassava production of > 160 million MT is led by Africa (46%), followed by Asia (32%) and Latin America (2 1%). On an area basis, cassava compares favorably to major world grain crops in terms of labor efficiency, calories, and biomass production (Table 1). However, the asexual propagation of cassava through stem cuttings is laborious, and the postharvest transportation and storage losses from root decomposition are high relative to grain crops. Cassava is grown predominately on small farms 214

of Latin America (75% of farms I 218

in maize/cassava polycultures,I7I·181in addition to greater marketable yields and economic returns compared with monoculture.'o7.181 The leaf area index of cassava is a good indicator of yield performance of component intercropping species. For example, okra and egusi melon, Cirrulltls lanattls, when intercropped with high leaf-area-index cassava cultivars, yielded only S3 and 70%, respectively, of those intercropped with low leaf-area-index cu ltivars.92 The area x time equivalency ratio (ATER) index 83 should be used if LER values are inappropriate because of significant differences in crop cycles of the cropping systems in question. The ATER index accurately showed a greater productivity for the short-season maize, peanut, and rice monocultures, whereas the LER misleadingly indicated a higher productivity fo r the year-round, cassava-based polyculture relay systems.'7

4. Time of Planting

Studies in Colombia and Australia indicated that cassava could be grown efficiently in a polyculture with short-cycle legumes such as Phaseolus223 and soybeans 34.3s Cassava is not as compatible with pigeonpeas, a long-season legume, as with soybeans, a shorter-season legume. 35 Rapidly growing legumes such as cowpeas, peanuts , and soybeans are well adapted to intercrop with cassava during the first 100 d after planting cassava; competitive effects on cassava yields are minimal.l7.34.108. 133 Intercropping legumes with cassava at 240 d after planting is also recommended because at this late stage of the cassava's growth the plant is less sensitive to crop competition, whereas the legume component yields only '=20 to 3S % less in polyculture than in monoculture.sl Planting time can also be modified to reduce competition from aggressive companion species. 198

5. Maize Systems In Nigeria, cassava yielded 28% less when intercropped with maize than when grown alone. However, cassava yields were reduced by only 3, 6, and 9% when intercropped with okra, melon, or

okra/melon, respectively.94 Maize yields in the Nigeria experiment actually increased 20% in the cassava/maize system compared with maize monoculture. 94 Agroclimatic conditions had an effect on the productivity of po lycultures in Nigeria. 169 When cowpea, maize, cowpea/rice, maize/ cowpea, rice, and maize/cowpea/rice combinations were intercropped with cassava, mean cassava yields were 8.2 MT ha- I (30% of cassava monoculture yields) in a savannah compared with 23.9 MT ha- I (78% of cassava monoculture yields) in a rainforest region, indicating a greater limi tation of resources in the savannah. 169 The cassava/ maize, cassava/cowpea, and cassava/maize/cowpea mixtures also had greater LER than a cassava-only crop in the Umudike savannah region. 169

6. Economic Returns A cassava/peanut experiment in Kerala, India, resulted in greater cassava yields, greater tuber starch content and crude protein content, greater P and K levels in the soil solution after the experiment completion, and greater economic returns from the mixture than in the cassava monoculture.26 In a 4-year polyculture study of cassava with maize, soybean, sunflower, green gram, and peanuts, the latter intercrop provided the most consistent yields and economic returns. Cassava yields were not affected significantly by the different mixtures. 145 Greater compatibility in cassava/peanut than in cassava/cowpea mixtures is attributable to the greater economic returns from peanuts and to the greater competition from cowpeas, which reduced cassava yields.118.208 Cassavalbean systems, as well as polyculture of cassava with other crop species, have higher economic returns than cassava monocultures. In fact, family labor is better distributed in the polycultures because of the convenient staggering of the varied crop cycles, cultural management demands, and other components of polycultures. 33 .\27.208 Polyculture of cassava with several legumes in northeast Thailand showed that cassava yields were not affected by the intercrops and that the greatest returns were obtained with either the cassava-only crop or the cassava/peanut system. Drought stress on the legumes during the growing

season may have reduced their competitiveness when in tercropped with cassava. Concurrent trials in farmers ' fie lds showed greater economic returns in the cassava/mung bean system compared with the cassava monoculture. 72 In Indonesia , a coco nut , cass ava, and Stylosanthes guyanensis (a pasture legume for on-site cattle grazing) polyculture produced higher total biomass than a cassava monocu lture. 156 Cassava yields were reduced in the polyculture compared with the monoculture, but overall economic returns were increased in the polyculture. In addition, the coconut trees produced 18% more nuts in the cassava- pasture-cattle system than in the bare-ground coconut monoculture. 156 Mean yield of cassava in the Philippines is 6.2 MT ha- I and ranges from 2 to 15 MT ha- I . 18o Similar yields are reported in Kwango-Kwilu, Zaire. 69 Experimental yie lds in the tropics have reached up to 90 MT ha- I . The introduction of successful production technology packages in Cuba and in Colombia, including cultural practices to manage the bacterial blight Bacillus manihotis, has resulted in increased cassava yields from 7-8 to >20 MT ha- I . 125 This example illustrates the potential for increasing yields of root and other underground crops in subsistence tropical agroecosystems through the introduction and transfer of appropriate technologies. 170

B. Sweet Potato Crops normally intercropped with sweet potato in the tropics as cash or staple crops include rice, soybean, maize, bean, peanut, barley, oil palm, cola, taro, coconut, vegetables, tobacco, and sugarcane.85 Sweet potato yields reportedly are reduced when intercropped with taller crops, with shade-tolerant sweet potato cultivars being least affected. 51 In polycultures, sweet potato responds linearly to intercepted radiation throughout its growth cycle. Density of tall companion crops, such as maize, therefore should be reduced to maintain intercropped sweet potato yields,5l Sweet potato frequently is grown as a relay or a second crop, making use of residual fertilizers applied for previous cash crops, such as vegetables. 51 In Taiwan, sweet potato cuttings are planted every four or five rice rows about 40 d 219

',
375,000 hal are grown for household use as staples, as a substitute for rice or maize, or as a feed. ISO Root crops are often left in the ground until needed. The lengthy stay (> 12 months) in the ground of some root crops results in greater root expansion and thus maximizes crop nurrient uptake from nutrient-deficient soils.232 This also avoids the need for storage and postharvest shelf management. Root crops in the tropics are commonly planted as insurance crops to assure a steady food supply to rural communities in case drought or other unfavorable environmental conditions result in failure of the main cash component crops.23.33.127,149.152.166.217 Resource-use studies in polycultures indicate that crop stress tolerance imparts yield stability in areas that experience frequent deficits of limiting resources. 102 The fact that root crops have coevolved with traditional farming systems in the tropics appears to enable them to accumulate adequate carbohydrates (as well as adequate proteins where the foliage is consumed) in harsh agroecosystems under which other horticultural or agronomic crops would fail (Table I). In the Philippines, most root crops are 221

grown in soi ls that are deemed unsuitable or "not as productive" when planted to rice or maize. 180 The Andean root crops Oxaiis, Ullucus, and Tropaeoium are grown at elevations of 3000 to 4500 m that frequently experience frosts. 86 However, research is needed to properly identify the dynamics of resource utilization of tropical root crops in agroecosystems unfavorable for ideal crop growth (light, water, nutrient, and temperature stress, crop competition, pests, and interactions of these factors with the socioeconomic realities).' 76 Detailed informati on of resource utilization over time by component crops will better explain the complementary role of the individual component species in extracting from and returning specific resources to the agroecosystem, a process termed "annidation".'52 Such information is required to improve the design of small farm tropical agroecosystems and to better understand rates of fertility depletion and nutrient cycling. For example, limiting data from the Kwilu area in Zaire indicate a net annual loss of 7425 MT N, 3674 MT P, and 33,024 MT K in the form of cassava leaf and root harvests for human consumption, cassava stems used as firewood, and field burning resulting in N losses. 69 These losses occur in an area of 79,000 krn 2, which translates into per hectare nutrient losses of ca. 94 kg N, 46.5 kg P, and 418 kg K in an area with a population density of 23.51km2.69

A. Cassava Cassava is grown in the dry tropics and is popular in traditional agroecosystems due to its drought tolerance under high temperatures «30°C), and low atmospheric humidity (vapor pressure deficit