3  Main findings from survey of literature and objective

The comparison of results from literature and their interpretation is complicated by the different species, breeds and sometimes the sex of animals used as well as the type, duration and intensity of the treatment administered. Nevertheless, the following short summary of literature reflects to some extent the basic relevant points:

1. Crossbreeding of tropical sheep with exotic breeds selected for high performance could increase productivity under tropical conditions (MOHAN et al., 1985; FERNANDES and DESHMUKH, 1986; NGERE, 1973; NGERE and ABOAGYE, 1981; GATENBY et al.,1997; FOOTE et al., 1983; GOODE et al., 1983; SHELTON, 1983; NURSE et al., 1983).
2. Crossbreeding could especially increase the survival and growth rate of the lambs (NURSE et al., ibid.).
3. Hair sheep and their crosses do not only show high fertility but are adapted to the tropical environment as well (BRADFORD BRADFORD and FRITZHUGH, 1983; PASTRANA et al., 1983; ZARAZUA and PADILLA, 1983; GOODE et al., ibid.; ADEMOSUN et al.,1983; BERGER, 1983).
4. It is not clear to what extent crosses produced by breeding between hair sheep and wool breeds of temperate zone origin could adapt to the conditions of high ambient temperature in the tropics (BRADFORD and FRITZHUGH, ibid.).
5. The advantage in growth performance of C1 kids over the pure Somali ones was reflected more after weaning than during the lactation phase (ABEBE, 1996).
6. Restricted feeding has been associated with higher gut fill (CARSTENS et al., 1991; TOUKOUROU, 1997) and lower weight of liver and intestines (BURRIN et al, 1990; CARSTENS et al., 1991; TOUKOUROU, 1997); and reduced plasma glucose concentration (BURRIN, ibid.; NAQVI and KRAF, 1991). It has also been associated with increased levels of growth hormone (DRIVER and FORBES, 1981; HAYDEN et al., 1993; ), plasma urea nitrogen and nonesterified fatty acids (HAYDEN et al., ibid.); as well as with a reduction in concentration of calorigenic hormones. From being only a pathologically related phenomenon, previously, OSBORNE and MENDEL (1916) associated compensatory growth with nutrition and, consequently, with animal production. Upon realimentation, many workers have also reported that compensatory growth had occurred but the question of how this comes into being has remained evasive (refer to McMANUS et al., 1972; THORNTON et al., 1979; THOMSON et al., 1982; RYAN, 1990; HOGG, 1991; HAYDEN et al., 1993;) for a long time.
7. Thermoregulation to balance heat production with heat loss is a pre-requisite to the sustenance of the life of warm-blooded organisms (BIANCA, 1971). The parameters of rectal temperature, breathing rate and hormonal secretions are useful indicators of climatic stress as a result of adverse climatic conditions (MULLER et al., 1994b). Panting may, however, not represent a direct response to high ambient temperature (BLIGH, 1959) such that its effects need to be supported by other more reliable parameters like rectal temperature, sweating and others. Thermoregulatory activity as a result of strong adverse effects of climate and weather involves use of energy which implies loss in production (BIANCA, 1971, 1976, 1977). With constant level of high ambient temperature, productivity is adversely affected above 30°C/40%RH (SCHAFFT, 1993). Over long periods of time, the homeorhetic effects of thermoregulation influencing food partitioning and mobilisation of body reserves set in (ZIEGLER, 1988; SCHAFFT, 1993). The effect of various combinations of temperature and relative humidity with regard to digestibility of feed, energy balance and rate of passage does not seem to be lineal and is therefore largely hard to predict (see STEIN, 1991; KAISER, 1992); the effect on growth and carcass is also not lineal (STELK, 1987). The change-over from day temperature to night temperature causes a compensatory increase in body temperature accompanied by cardiac acceleration and a reduction in skin temperature (BIANCA and NÄF, 1977).
8. High ambient temperature around 30°C has been associated with decrease in rate of passage of cell wall and lignin (STEIN, 1991); feed intake (STELK, 1987; KAISER, 1992) and more especially that of roughage; weight of liver, back and thinning (STELK, 1987); milk yield (BURMEISTER, 1988) and milk fat of dairy cattle (ZIEGLER, 1988). It has also been associated with a rise in rectal temperature independent of type of treatment and in [page 29↓]combination with extremes of diet high in either raw fibre or concentrated feed (KAISER, 1992); in breathing rate (STEIN, 1991; KAISER, 1992); and increase in weight of the heart (STELK, 1987). Ambient temperature level of 15°C has been associated with increased energy expenditure for thermoregulation (KAISER, 1992) and highest negative N-retention of -30.61% compared to 30°C (-10.64%) in adult wethers (ibid.).

This work tries to assess to what extent the rearing of crossbred lambs produced by crossing between Cameroon and German mutton and milk breeds could be justified under tropical conditions as a means of increasing productivity there. The productive performance of the crosses was compared with that of the pure Cameroon.

Also considered was the effect of diurnally fluctuating temperature with high temperature (31°C/50%RH) during the day and low (15°/70%RH) during the night on the growth, feed intake and carcass quality of lambs exposed to specific tropical temperature level over a long period of time and involving a restriction to reflect seasonal availability of feed under tropical conditions.

In the literature, the non-linear effect of high ambient temperature on the reaction pattern of animals has been stressed. This raises the problem of predicting its effects under varying conditions of environment, rearing and management. The importance of paying particular attention to specific ambient temperature conditions with potential for increased animal productivity therefore becomes all the more relevant. This approach is briefly justified as follows:

1. It is production oriented and involves use of growing animals.
2. It involves the use of crossbred lambs foreseen as a basis for increasing productivity under tropical conditions and tries to assess the possibility of rearing under the same.
3. It is an effort to study the reaction pattern of weaned lambs to a specific tropical ambient temperature level and is in that context therefore closer to practice, considering as it does, the effects of temperature on a relatively long period of time.
4. It considers that the complexity of growth and specific alternating ambient temperature (on daily basis) has not received enough scholarship.

The objective of this work was to assess the pre-weaning growth performance of pure Cameroon and crossbred (C1 and C2) lambs, the milk yield performance of pure Cameroon and C1 (Cameroon X Mutton) ewes. It was also to assess the post-weaning productivity and physiological reaction of pure Cameroon and crossbred lambs subjected to different feeding levels at stall ambient temperature and at diurnally alternating ambient temperature in the climate chamber i.e. high ambient temperature during the day and low at night. The performance and reaction of the crossbred animals was compared with that of the pure Cameroon hair sheep.The work tries to give answer to the following questions:

1. What is the pre-weaning growth performance of pure Cameroon lambs and crossbred (C1 and C2) lambs produced by mating Cameroon rams onto German mutton and milk sheep?
2. What is the milk yield performance of pure Cameroon ewes and of C1 ewes produced by crossing between the Cameroon and German mutton breeds - using the indirect lamb suckling method?
3. What is the post-weaning growth performance of pure Cameroon and crossbred (C1 and C2) lambs subjected to varying levels of feeding under stall ambient temperature conditions?
4. What is the physiological pattern of reaction of pure Cameroon and crossbred (C2) lambs subjected to a high (31°C/50%RH) and low (15°C/70%RH) rhythm of ambient temperature in the climate chamber, during the day and at night, respectively, and during three (3) phases of feeding viz. alimentation, restriction and realimentation (High-Low-High)?