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1  Introduction

Increase in the human population and the scarcity of production resources exert severe pressure on the small scale mixed farming system that threatens its existence (PETERS and LAES-FETTBACK, 1995). Increasing demand for animal protein and the ever increasing competition for land resources call for major structural changes in the agricultural sector which is the major source of occupation, subsistence and income in the Tropics. A major objective of such changes will be to increase productivity per animal and per unit area.

With the threat of land degradation, rising demand in animal products should be met by an increase in animal productivity and not by further increase in animal numbers which means increased production per animal through changes in animal genotypes, better feeding and management (MALMFORS et al).

Small ruminants have an increasing role to play in areas where the availability of land and fodder is insufficient (HORST, 1981). They can utilise fodder resources high in crude fibre and present the advantage of high reproductive performance and a small body size that makes it feasible to adapt to extreme environmental conditions (PETERS, 1988).

By increasing small ruminant rearing and productivity, a still higher degree of integration can be achieved in the existing farming systems of the Tropics. The high productivity and low capital investment demand of small ruminant production ensures easy access to animal protein supply, higher liquidity of the smallholder enterprise and a ready supply of easily applied manure. Small ruminant production is a further strengthening of the socio-cultural institutions through its cultural relevance and religious function. KEZIE (1997) reports on the situation in Togo where sale of small ruminants (sheep and goats) is effected to meet family needs especially before the harvest, at the beginning of the new school year and in case of illness and where the preference of sheep as slaughter animals at religious ceremonies is more pronounced. Similar reports pertaining to the situation in Ghana and the importance of local sheep have been made by NGERE (1973) and LONDON (1993).

In drought areas, small ruminants are increasingly playing the role of bridging the gap between good years and drought years, years of plenty and years of hunger.

Besides being a means of supply of meat, milk, and hides, sheep production as a sub-sector of small ruminant production could form a basis of the major export trade as has been the case in Somalia where big livestock (cattle and camels) and small ruminants (sheep and goats) have made up to 80% of the national export trade with the latter numerically making up 75% of all animals (MUMIN, 1986). It has its own specific role within complex and diversified farming systems. The role of science is to bring this more to bear.

1.1 Statement of the problem

Constraints to increased sheep production in the Tropics and Subtropics are of a biological and environmental nature and relate to the available potential, survival in hot tropical environments, and required efforts to increase production.

Sheep production in the Tropics is constrained by the following factors:

  1. Low genetic potential
  2. Seasonality of availability of feed and scarce water resources
  3. High ambient temperature

Most sheep breeds of the Tropics have not been selected for high productivity. This implies low rates of growth and long time to reach physiological maturity. Low milk yield of the ewes reduces the chances of the lambs to survive (BULLERDIEK, 1996).

To be able to survive in hot climates, animals should demonstrate the ability to consume and digest feed stuffs high in crude fibre content and to survive under conditions of seasonal feed availability, water scarcity, high heat and radiation while still retaining the ability to utilise the range (HORST and PETERS, 1983). Feed production, quality and availability are dictated by weather changes and thus take a seasonal pattern. Low productivity of tropical sheep races and seasonal availability of feed do not therefore offer the best combination for increased sheep production. Only fast growing sheep [page 13↓]strains with a higher rate of growth from birth to weaning could utilise seasonally available feed resources including crop by-products more efficiently. The natural ability of the animal to regain weight lost due to feed scarcity upon realimentation provides the possibility for compensatory growth and recovery and efficient utilisation of scarce feed resources characterised by the same seasonal pattern.

Conditions of rearing that are free from thermal stress also generally yield the highest economic returns such that it would be in the interest of the producers to be aware of them and to take any necessary steps to provide the necessary climate in the shed and to select breeds or individuals of animals best suited to a given climatic area (BIANCA, 1961). The tropical environment might pose problems of a thermoregulatory nature and of survival for temperate strains (BIANCA, 1976) of sheep which have mostly been selected for higher productivity over a long period of time. The tropical climate and weather are both characterised by high ambient temperatures especially during the day. Cold nights and winds are a common feature in the dry part of the Tropics.

According to BIANCA (1971) knowledge in the area of bioclimatology of the farm animal needs to be extended to include the effects of such factors as age, sex, breed, level of feeding and level of performance to be adequate. It would then be possible to link theory with practice provided the behaviour of the animals under variable climatic conditions are studied and properly interpreted to solve related problems in the animal production sector.

Unlike field studies, experiments in the climate chamber have the advantage of reducing to a minimum the number of effective climatic factors as well as bringing under control or eliminating altogether non-climatic factors such as nutrition and husbandry and thus making it possible to identify causes and interpret results for practical use (BIANCA, 1971).

Thermoregulation covers all changes taking place in a given animal in response to thermal stress which enables the animal to maintain body temperature within normal limits for its species when exposed to cold or heat (BIANCA, 1977). Such changes are functional (e.g. shivering or sweating), structural (e.g. vlies type) or behavioural such as the search for a less hot micro-climate. Behavioural changes normally form the first pattern of response followed by physiological ones (ibid.).

In view of the long time it would take to select for high productivity within the local tropical sheep breeds and regarding the existing big gap between demand and supply of animal protein, the production of sheep crosses of temperate and tropical breeds becomes all the more relevant in an effort to increase productivity while retaining the ability of the animal to apply adequate thermoregulatory function.

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