C. Dalibard
The author is Animal Production Officer in the Feed Resources Group, Animal Production and Health Division, FAO, Rome, Italy.
Sustainable livestock production in arid and semiarid areas
Integrated livestock/agriculture production systems
Draught animal power
Fertilizer production
Weed control
Source of fuel
Role of the feed base in environmental protection
Other positive impacts of livestock on the environment
Animal production and wildlife conservation
Conclusions
Bibliography
Over the last few decades, there has been much emphasis placed on the detrimental effects of livestock on the environment. Livestock have been blamed for water pollution in Europe, deforestation in South America, desertification in Africa and, at the global level, for increasing the greenhouse effect and for reducing biological diversity as well as for feeding on grain (produced in a non-sustainable way) that could be better utilized directly by humans.
These detrimental effects, however, are essentially generated by the way livestock have been managed recently, mostly on a short-term profit basis with no concern for sustainability.
In semi-arid and arid areas, previously existing management systems were disrupted by such factors as governmental ownership of rangelands replacing the traditional land tenure; uncontrolled practices of resettling crop farmers in areas only able to sustain livestock production in the long term; "plundering" of feed resources by outside speculators, often rich businesses interested in making high profits on a short-term basis; repeated droughts; common political will restraining the mobility of pastoral people; and also decreased mobility because of increased insecurity. All these factors interfered with the traditional balance that has permitted pastoral tribes to live for centuries within limited areas that were well managed.
Moreover, wherever it was possible (often favoured by governmental subsidies), the need for short-term profitability lead to the development of intensive animal production systems with no concern for the environmental impact. In these systems, sewage disposal was often a problem as it was too costly to transfer the sewage to crop areas. Consequently, a useful input for crop production was wasted and left to pollute water reservoirs. Like most other intensive mono-production agricultural systems, intensive livestock production cannot be sustainable and environmentally friendly if it is not integrated with other agricultural activities. It must not be forgotten that even though productivity and efficiency of livestock production is very low in developing countries, it can nevertheless be improved with considerably less inputs of fossil energy than is the case in developed countries (Preston and Murgueitio, 1992).
Livestock's negative impact on arid and semi-arid areas has long been exaggerated and has only become controversial in recent years. Sidahmed and Yazman (1994) advocate that traditional pastoral systems are stable since they respond to high climatic variability. There are good prospects for animal production, which is fully compatible with environmental protection in these areas, as demonstrated by the following examples.
Recent studies on rangeland management in arid and semi-arid areas have shown that animal production can be perfectly sustainable if some precautions are taken (Breman and Ridder, 1991). For example, to estimate the carrying capacity, a dry year should be taken as reference; for grazing capacity, only half of the biomass of herbaceous perennial; and for browsing, only 15 percent of the annual biomass production of palatable plants should be considered available. Other management aspects include fire control (strictly used as a rangeland management tool only when necessary), surveillance of the presence of cattle around villages and watering points and protection of fragile soils during the rainy season.
Abel and Blaikie (1990) confirmed that, in the case of Botswana and Zimbabwe, because the intrinsic resilience of rangeland was not acknowledged, much effort had been wasted in trying to stabilize production instead of promoting a "tracking" strategy that better followed variations in rainfall. Very often, traditional transhumant systems have proved to be much more sustainable and productive (per surface unit) than ranch systems in the arid and semi-arid areas because of their high degree of flexibility (Breman and Ridder, 1991). Nevertheless, as confirmed by Niamir (1991), although a reversion to the traditional systems would be ideal, there is still much room for improving these traditional systems and developing locally appropriate techniques. Galaty and Johnson (1990) confirm that, for dry lands, the traditional pastoral regimes are clearly superior to the mixed agricultural systems that agriculturists pressing northwards tend to replace them with, for example, in the Sahel. According to Perrier (1990), rangeland management projects often fail because the wrong approach is taken. Modifications to local conditions, such as the replacement of traditional pastoralist models with centralized management, were usually undertaken in order to accommodate proposed innovations, mainly increased control of grazing resources. Such an approach often underestimated the complexity of the local production systems and usually only managed to disrupt these systems, generating conflicts and the deterioration of range management. Perrier (1990) proposed to revert this tendency by promoting range management proposals to fit the local context, such as repeat-seasonal grazing rather than rotational grazing, based on a systems approach, which would lead to a thorough understanding of the local production systems. Thus, new prospects should arise for innovations in these systems, for example, the introduction of fodder trees and the use of supplements such as multinutrient blocks and urea-treated bush straw. According to herders in the Niger, animals fed with such supplements are less selective when grazing on rangeland, thereby preventing the most palatable species from being overgrazed and, at the same time, contributing to the preservation of the environment (Sourabie, Kayouli and Dalibard, 1995). Another positive impact that ruminants have on vegetation is the dispersal and facilitated germination (after ingestion of pods, acid treatment of the hard seed coat and rejection in manure) of fodder tree species (Acharya and Singh, 1992). The growth of the seedlings is further assisted by the presence of nutrients in manure.
In arid and semi-arid areas, livestock are often considered to be only an animal production system, with no integration into other agricultural activities. But, in reality, the mobility of livestock in these systems usually implies great interaction with crop production systems, especially through manuring contracts between farmers and herders, providing draught animals and utilizing crop residues as feed. As pointed out by Toulmin (1983), given the lack of chemical fertilizers in the Sahel, access to manure is of the utmost importance for promoting short cycle crop production and ensuring strengthened food security, despite rainy season uncertainties.
Most positive contributions of livestock to the environment are related to their role in integrated sustainable farming systems. Some examples are given below, although it is not intended that these constitute an exhaustive list covering all systems where livestock contribute positively to environment-friendly farming systems. Considered separately, a livestock production system generally demands less fossil energy than a crop production system does, which often relies heavily on fertilizers and herbicides that are manufactured and spread using expensive fossil fuel.
With the rapidly growing world population, the demand for grain is also increasing. Areas previously devoted to fodder crops have now been taken over by grain production, mainly to satisfy the needs of the population, with livestock relying more and more on crop residues and by-products, as is apparent in China and Egypt.
In Asia, through centuries of experience, the integration of livestock, fish and crops has proved to be a sustainable system. In China, for example, the integration of fishpond production with duck, goose, chicken, sheep, cattle or pig raising increased fish production by two to 3.9 times (Chen, 1992). Also recognized were the ecological and economic benefits of fish utilizing animal wastes, a manageable system for small-scale farming. Environmentally sound integration is ensured in these systems: livestock droppings and feed wastes can be poured directly into the pond, where they constitute feed for fish and zooplankton; livestock manure can also be used to fertilize grass, which also constitutes feed for fish; vegetables can be irrigated from the fish ponds, and their residues and by-products can be used for feeding livestock.
In Amazonian Ecuador, the introduction of sheep grazing on legume forage/fuelwood fallows within traditional farming systems (cassava shifting cultivation) has been shown to greatly benefit the environment. In this way, degraded lands can be rehabilitated for crop production and further use of less fertile lands can be reduced (Bishop, 1983).
Maximizing both integration and recycling ensures the sustainability of animal production/agricultural systems (Preston and Murgueitio, 1992). These authors have shown that a sustainable livestock production system for the humid tropics can reach a high degree of integration. Such a system is now operational at the small-farmer level in Colombia and supports high levels of production (more than 3 000 kg of meat per hectare per year). It includes pig, sheep, duck and earthworm raising (in partial or total confinement), food crops, environmentally protective perennial crops such as sugar cane, nitrogen-fixing trees (Gliricidia septum and Erythrina fusca) or other multipurpose trees (Trichantera gigantea) and water plants (Azolla filiculoides). Feed (sugar-cane juice for pigs and ducks, bagasse and tops for ruminants, fodder from trees and aquatic plants), energy (biogas and tree branches) and fertilizers (effluent from biodigester and humus made by earthworms) are all produced on the farm.
The different components of the role of livestock in a farming system are listed below.
Livestock provide renewable energy for agriculture, saving a considerable amount of fossil energy that otherwise would be used mainly for manufacturing and operating heavy agricultural machinery, as well as for producing fertilizers. For example, it has been calculated that the energy needed for soybean seed-bed preparation and sowing using a tractor is 6.3 times more than that required when using draught animal power. Moreover, the latter is renewable, while this is mostly not the case with the former (Mittal and Srivastova, 1993), and this figure does not take into account the fossil energy spent making the tractor and maintaining it. To a large extent, draught animal power is self-repairing, self-propagating and sustained by by-products produced on the farm. Draught animal power can also play a key role in large-scale production systems, as is the case on a 30 000 ha sugar estate (La Romana) in the Dominican Republic, where some 18 000 oxen haul sugar cane from the fields to the railway system leading to the sugar mill (Preston and Murgueitio, 1992). Sustainability is guaranteed in such a system, since the oxen's main source of energy comes from the sugar-cane tops and, in the end, the manure goes back to the sugar cane.
In many parts of the world, draught animal power is often the best alternative leading to increased yields of crop production (see Figure). As pointed out by Delgado (1979), the expansion of cultivated area is often limited by a shortage of labour for weeding and harvesting needed for a larger crop output. In the long term, more extensive use of draught animal power for weeding and harvesting will lead automatically to the expansion of cultivated area. Compared with human traction, this type of system contributes to the improved quality and timely execution of farming operations, such as tillage. It also permits the introduction of soil conservation practices to impede excessive soil compaction and erosion (Thomas and Barton 1995).
In mixed-farming systems, livestock are often fed on crop residues and by-products as well as pasture. In turn, some nutrients and organic matter are returned to the soil through livestock manure (see Table), ensuring the maintenance of soil fertility and acting as a soil conditioner. The main results are improved cation exchange, better absorption of water and the prevention of runoff and soil surface crusting. On the other hand, the exclusive use of mineral fertilizers will result in a decrease in base saturation and pH and in the occurrence of aluminium toxicity (Thomas and Barton, 1995). It is therefore clear that manure plays a key role in sustainable crop-livestock production systems. Moreover, small farmers in developing countries often cannot afford any other type of fertilizer, and fallow periods are becoming shorter as population density increases. The direct result of the proper use of manure is increased yields, which may prevent small farmers from further encroaching on fragile areas.
The value of manure as fertilizer could be increased through better animal feeding. There are now various widely spread techniques to upgrade roughage, such as urea treatment, which was practiced in China in 1993 by 3.3 million small farmers, who treated more than 11 million tonnes of straw. It was observed in the Niger that the percentage of crude protein in manure increased from between 4.5 and 5.8 percent with a traditional diet to as much as 7 to 8.3 percent with urea-treated roughage, which allowed the farmers to decrease the use of costly, often imported chemical fertilizers (C. Kayouli, 1995, personal communication). Also in the Niger, farmers using the upgraded manure could reduce the urea used for rice by half and still increase their yields by 40 to 50 percent (Sourabie, Kayouli and Dalibard, 1995).
Livestock can be successfully used for weed control and therefore contribute to the decrease of water pollution by herbicides. The fossil energy that would otherwise be used for making and spreading herbicides is also saved. Livestock are currently used in sugar-cane, oil-palm, coconut, rubber, citrus, kola and mango plantations. While grazing in these plantations, the livestock, most commonly sheep, also provide organic fertilizer. On a100-ha sugar-cane estate in Colombia, for example, a flock of 175 African hair ewes control the weeds, replacing the need for either tractors or herbicides. Other species also useful for weed control are geese, which are used in India, and horses, which, because of a herbicide shortage, have been used successfully in Cuba for the last three years in citrus plantations to control such weeds as Panicum maximum and Hyparrhenia rufa that have a very negative effect on orange yields (M. Sanchez, personal communication). Fish, such as grass carp and silver carp, are very efficient for weed control in drains and canals.
Relative dung and nitrogen production for different animal species (per head per day) - Quantités de déjection et d'azote produites par différentes espèces animales (par animal et par jour) - Producción relativa de estiércol y nitrógeno de distintas especies de animales (por animal por día)
|
Species |
Adult live weight (kg) |
Dung production (kg dry matter) |
Nitrogen content (%) |
Nitrogen production (g) |
Nitrogen production (kg/year) |
|
Buffalo |
460 |
5.8 |
0.80 |
46.4 |
16.9 |
|
Cattle |
350 |
4.4 |
0.73 |
32.1 |
11.7 |
|
Goat |
20 |
0.3 |
1.32 |
4.0 |
1.5 |
|
Sheep |
20 |
0.3 |
0.91 |
2.7 |
1.0 |
|
Chicken |
2 |
0.05 |
3.9 |
2.0×10-4 |
0.07 |
|
Duck |
3 |
0.06 |
3.0 |
0.18×10-4 |
0.07 |
Source: Devendra, 1992.
Livestock production is integrated with forestry mainly for reforestation purposes or to prevent forest and bushfires. In Scandinavia, sheep grazing is used in reforestation areas to control shrubs that tend to suffocate young trees (Johansson, 1982). Goats have been used to create fire breaks and are also widely used to control weeds (Acharya and Singh, 1992). In Europe, the European Union provides some countries with subsidies to encourage grazing in the woody areas of the Mediterranean in order to prevent fires.
The use of cow dung as fuel in India and other countries results in an enormous amount of trees being saved, and its ash can be used as fertilizer. When water is not a limiting factor, however, dung can be utilized much more efficiently for fuel production through biodigesters.
Zero-grazing livestock production systems can be environment-friendly when feeds are grown on-farm and slurry is collected in biodigesters, the effluent of which may be used either as crop fertilizer or for fish production in ponds. For the former there is the added advantage that weed seeds have been inactivated during the digestion process.
The Grameen Bank in Bangladesh - which is very famous for its successful credit schemes for the rural poor, especially the "cattle lease" programme for many women - is now linking this activity to a biogas programme since the biodigester is considered to be the centrepiece of an integrated farming system, where the effluent entering ponds increases fish yields by accelerating algae production. Since very cheap and simple biodigesters made of plastic sheets have recently been developed through FAO projects, they are now more accessible for rural families than the more expensive cement or steel ones.
In Cambodia, an FAO project has recently demonstrated that, where there is adequate manure from at least three cows or eight pigs, the amount of gas produced is sufficient to replace about 75 percent of the fuelwood normally used by an average family of six persons (FAO, 1995). On-farm production of biogas, therefore, can satisfy most of the basic energy needs of a household, at least for cooking. This would obviate much of the need for fuelwood, which is becoming a scarce resource in many developing countries and must be gathered very far away, involving high energy costs for collection and transport, while jeopardizing the environment through excessive collection.
The feed base of animal production can play an important role in environmental protection. It consists, of either the main energy sources, such as food-crop by-products and residues and sugar cane, or the main protein sources, such as multipurpose trees and water plants.
If not used as animal feed, agricultural residues such as straw, stover and groundnut or cowpea hay are burned, polluting the atmosphere. Some agro-industrial by-products are already widely known as feed, for example, cereal brans, oilcakes, molasses, brewer's yeast and beet pulp. Others, such as sisal leaf waste, filter-press mud from the sugar industry, coffee, citrus, pineapple or tomato pulp, olive cakes and palm oil sludge, are usually produced in large quantities as well, and their disposal is often a problem. Most of these by-products could be used as feeds.
By-products from livestock industries such as slaughterhouse wastes (poultry offal, bones, blood), whey from the cheese industry, hatchery by-products and fish and shrimp wastes can be utilized in ruminant feeding. Even excrete from one animal species can be used as feed for another. Poultry manure or litter, which is rich in crude protein, can be fed as silage to ruminants after processing to ensure its sterilization.
In Cuba, domestic organic wastage derived from human consumption and other wastes originating from the food industry are collected throughout the country and cooked on an industrialized basis in small factories to feed pigs reared in the vicinity (Ly, 1993).
High biomass and perennial fodders (sugar cane and trees) act as carbon sinks, counteracting the greenhouse effect. They also impede erosion. The best use of these fodders permits the reduction of land area per livestock unit.
Legumes, such as fodder trees, planted together with crops or used as ground cover in coconut, rubber or oil palm plantations, play a key role by fixing atmospheric nitrogen and increasing water absorption in the soil, which prevents erosion and decreases the need for artificial fertilizers.
Multipurpose trees integrated into a livestock production system can contribute to the protection of the environment in various ways. They impede erosion and increase nitrogen in the soil (nitrogen-fixing trees), thereby decreasing the need for agrochemicals (fossil-energy consuming). They can also supply domestic energy, such as wood, together with biogas, and preserve local vegetal biological diversity. There is already a large list of useful multipurpose trees known throughout the world, such as Gliricidia sp., Prosopis sp. and Leucaena sp. Nevertheless, there are many other species that would be worth propagating, although these are still to be identified through better investigation of indigenous knowledge.
Water plants, such as Azolla sp. and Lemna sp., also play a key role in animal production systems. They are a protein-rich feed, but they are also useful for purifying surface waters polluted with organic matters (livestock and human wastes), as carbon sinks and for fixing atmospheric nitrogen (azolla) (Preston and Murgueitio, 1992).
All these components also contribute to increased biological diversity within farming systems, as compared with modern intensive agricultural production systems, especially through the reduced need for pesticides. Even the environment-friendly alley farming system (nitrogen-fixing trees and cash crops) gains in sustainability if associated with animal production through the use of foliage as feed (Attah Krahn, 1991). This is confirmed by the recently developed three-strata forage system in the dry areas of Bali in Indonesia (Nitis et al., 1993). This system includes, on the periphery of small square crop fields of 0.16 ha of maize, soybean and cassava, the use of three species each of grasses and ground legumes (as first stratum, 0.09 ha), and a 200-m circumference area with three species of shrub legume (as second stratum, 2 000 plants) and three species of fodder trees (as third stratum, 42 plants) in order to supply livestock with green feed throughout the year. The advantages of this system are increased livestock production; increased biological diversity (12 fodder species); production of fuelwood at farm level; greater returns from organic matter and nitrogen to the soil and prevention of erosion caused by excessive grazing of crop fields after harvest; and better results with scavenging poultry, honey and snail production. Taken together, these contribute to the protection of the environment and the sustainability of the system.
There are fragile areas, in the north of the Sahel, for example, where the integration of livestock into farming systems, by ensuring much more regular income than from crop production alone, can provide small farmers with enough security so that they are not forced to expand cultivation into areas where the probability of harvesting is very low but the erosion effect is high.
Most crop production systems generate seasonal incomes, but the integration of livestock production would result in supplementary and regular incomes from the sale of milk and eggs, as well as provide emergency incomes, whenever necessary, from the sale of animals. This additional income would also allow farmers to make their production systems more sustainable and environment-friendly through, for example, conversion from the slash-and-burn system to the sedentary production system (Sidahmed and Yazman, 1994), better preparation and fertilization of crop fields, implementation of soil protection measures such as terracing to avoid erosion, the use of selected seeds and planting of legume fodder trees These investments could not be afforded otherwise.
The integration of livestock into crop production systems also offers increased prospects for local labour, which would slow down the drift from the land and the never-ending expansion of big cities.
Mainly in southern and eastern African countries, animal production systems, both those of cattle and wildlife, have proved to be sustainable and also to preserve biological diversity. A survey taken of a group of Masai in the Serengeti National Park in the United Republic of Tanzania has demonstrated the possibility of a successful coexistence between wildlife and Masai herds within the conservation area (Homewood, Rodgers and Arnhem, 1987). In the Kenya rangelands, McDowell et al. (1983) found that the conversion of an existing cattle ranch into one comprising a mixture of game and cattle was the best form of range utilization.
In developed countries, new legislation will oblige polluters of the environment to pay for its cleaning, and, if these laws are really going to be applied on a large scale, the spread of environment-friendly animal production systems should be encouraged. Much interest has been shown in environmentally attractive livestock systems that remain, at the same time, productive and profitable. In fact, this will be one of the major themes of the international conference on environmental enhancement through agriculture to be held at Tufts University, Medford, Massachusetts, the United States, from 15 to 17 November 1995.
In sub-Saharan Africa, trypanosomiasis transmitted by the tsetse fly has influenced the distribution of people and livestock, and, as a result, tsetse-infested area shave remained relatively unexploited. However, since pressure from the tsetse fly is expected to decrease in the near future because of new biological control techniques, the prospects for the promotion and implementation of sustainable integrated crop-animal production systems are promising.
There is still ample room for further development of new farming systems or new components within existing farming systems. Animal production can become fully compatible with the fostering of biological diversity. On a worldwide basis, the feed base of animal production includes thousands of different species of fodder plants, and yet, so far, only 700 of them have been described in the FAO publication Tropical Feeds (1993). Clearly, more efforts are needed to collect (tapping especially indigenous knowledge), validate and make available information on the other species. Of the total biomass production, less than 1 percent is used as feed (Speedy, 1995). Therefore, there is still a huge amount of biomass that could be used for animal production in order to meet future food demand.
Taking advantage of ruminants' specific digestive physiology, these animals could play a major role in this respect. As mentioned by Leng (1993), one of the first tasks will be the development of related skills among small farmers.
Abel, N.O.J. & Blaikie, M. 1990. Land degradation, stocking rates and conservation policies in the communal rangelands of Botswana and Zimbabwe. Pastoral Development Network Paper 29a. London, UK, Overseas Development Institute.
Acharya, R.M. & Singh, N.P. 1992. Role of goat in conservation of ecology and livelihood security. In Proc. V International Conference on Goats, 7 March 1992, New Delhi, India. Edited by R.R. Lokeshwar.
Attah Krahn, A.N. 1991. Fodder trees and shrubs in tropical Africa: importance, availability and patterns of utilization. In T.R. Preston, M. Ropsales & H. Osorio, eds. Integration of livestock with crops in response to increasing population pressure on available resources. Ede, the Netherlands, Technical Centre for Agricultural and Rural Cooperation (CTA).
Bishop, J.P. 1983. Tropical forest sheep on legume forage/fuelwood fallows. Agroforestry Syst., 1: 79-84.
Breman, H. & Ridder, N. de, eds. 1991. Manuel sur les pâturages des pays sahéliens. ACCT, CABO-DLO and CTA. 485 pp.
Chen, Y.C. 1992. Integrated livestock-fish production in China. Proceedings FAO-ITP International Workshop on Integrated Livestock-Fish Production Systems, Kuala Lumpur, Malaysia
Delgado, C. 1979. An investigation of the lack of mixed farming in the West African savannah: a farming system approach. In K. Shapiro, ed. Livestock production and marketing in the Entente States of West Africa: summary report. USAID/CRED, University of Michigan.
Devendra, C. 1992. Integrated animal-fish-mixed cropping systems. Proceedings FAO-ITP International Workshop on Integrated Livestock-Fish Production Systems, Kuala Lumpur, Malaysia.
FAO. 1987. Agriculture: toward 2000. Rome, FAO.
FAO. 1993. Updated electronic version of Tropical feeds by Bo Gohl, 1980. A.W. Speedy, ed. Rome, FAO.
FAO. 1995. Rapport terminal du projet PCT «Plan d'urgence pour la sauvegarde du bétail au Cambodge».
Galaty, J.G. & Johnson, D.L., eds. 1990. The world of pastoralism: herding systems in comparative perspective. New York, NY, USA, the Guilford Press. 436 pp.
Homewood, K., Rodgers, W.A. & Arnhem, K.T.I. 1987. Ecology of pastoralism in Ngorongoro Conservation Area, Tanzania. J. Agric. Sci. (UK), 108: 1, 47-72; 65 ref.
Johansson, T. 1982. Sheep grazing on reforestation areas. In O. Saastamoinen et al., eds. Multiple-use forestry in the Scandinavian countries. Proceedings of the symposium held in Rovaniemi and Saariselka, Finland, 13 to 17 September 1982.
Leng, R.A. 1993. L'application de la biotechnologie à l'alimentation animale dans les pays en développement. FAO Animal Production and Health Paper No. 90. 132 pp. Rome, FAO.
Ly, J. 1993. The role of monogastric animal species in sustainable use of tropical feed resources. World Conference on Animal Production, Edmonton, AB, Canada.
McDowell, R.E., Sisler, D.G., Schermerhorn, E. C., Reed, J.D. & Bauer, R.P. 1983. Game or cattle for meat production on Kenya rangelands. Cornell International Agriculture Monograph No. 101. 77 pp.
Mittal, J.P. & Srivastova, N.S.L. 1993. The role of human and draught animal power in crop production. Proceedings of a workshop in Harare, Zimbabwe, 18 to 22 January 1993. Siloe Research Institute, Agricultural and Food Research Council. Commission of the European Communities. Rome, FAO.
Niamir, M. 1991. Traditional African range management techniques: implications for rangeland management. Pastoral Development Network Paper 31d. London, UK, Overseas Development Institute.
Nitis, I.M., Lana, K., Suarna, M., Sukanten, W. & Putra, S. 1993. Increasing beef cattle production through three strata forage system. Proceedings of an International Conference on Animal Production with Local Resources. Bureau of Animal Production and Health, Ministry of Agriculture, China.
Perrier, G.K. 1990. The contextual nature of range management. Pastoral Development Network Paper 30c. London, UK, Overseas Development Institute.
Preston, T.R. & Murgueitio, E. 1992. Strategy for sustainable livestock production in the tropics. CIPAV-SAREC, Cali, Colombia, 89 pp.
Sidahmed, A.E. & Yazman, J. 1994. Livestock production and the environment in developing countries. In J.A. Yazman & A. Gray Light, eds. Proc. International Telecomputer Conference on Livestock Research and Development, November 1992 to April 1993. Morrilton, AR, USA, Winrock International.
Sourabie, K.M., Kayouli, C. & Dalibard, C. 1995. Le traitement des fourrages grossiers à l'urée: une technique très prometteuse au Niger. Wld Anim. Rev., 82(1): 3-13.
Speedy, A.W. 1995. First Electronic Conference on Tropical Feeds and Feeding Systems. Rome, FAO.
Thomas, D. & Barton, D. 1995. Interactions between livestock production systems and the environment. Impact domain: crop-livestock interactions. Rome, FAO. (In press)
Toulmin, C. 1983. Herders and farmers or farmer-herders and herder-farmers? Pastoral Network Paper 15d. London, UK, Overseas Development Institute.
