Impact of genetic and environmental factors on birth weight of Fulbe sheep in Cameroon

Sheep production plays an important role in the economy and socio-cultural activities of the people of the North and Far North provinces of Cameroon. The sheep population in this part of the country is about 57.8 % of the national sheep population (15). The Fulbe breed found there is dominantly white in coat colour, has long legs, large and long ears that droop on the sides of the head. The females are hornless and the males have long, spiral and horizontal horns. Husbandry practices are mostly in small flocks located in rural areas, in a subsistence manner, with minimal management and capital investment. Consequently, sheep production is in the hands of the rural farmers whose methods of production may not be appropriate in improving flock size and scale of operation. This, as well as the limited information available on the role of existing environmental factors in determining productivity, seriously affect the farmers’ earnings in this sector of the economy.

Sheep production plays an important role in the economy and socio-cultural activities of the people of the North and Far North provinces of Cameroon. The sheep population in this part of the country is about 57.8 % of the national sheep population (15). The Fulbe breed found there is dominantly white in coat colour, has long legs, large and long ears that droop on the sides of the head. The females are hornless and the males have long, spiral and horizontal horns. Husbandry practices are mostly in small flocks located in rural areas, in a subsistence manner, with minimal management and capital investment. Consequently, sheep production is in the hands of the rural farmers whose methods of production may not be appropriate in improving flock size and scale of operation. This, as well as the limited information available on the role of existing environmental factors in determining productivity, seriously affect the farmers' earnings in this sector of the economy.
The role of environmental factors in determining sheep productivity in this area is very important because of the variability, severity and length of the dry season which directly affect forage production. Knowledge of the role of these factors Will permit carrying out modifications towards improvement strategies aimed at increasing productivity andlor adjusting the records of reliable genetic parameter estimates.
Therefore, the objectives of this study include identification of environmental factors that influence birth weight of Fulbe sheep reared under relatively rural conditions, determination of the maximum lambing period, oestrous and ovulation activities, determination of productivity parameters under such environment and genetic evaluation of ewe performance through statistical adjustment of productivity for significant management and environmental influences.

W MATERIALS AND METHODS
Data on birth weights were collected from 1984 to 1990 from a random bred non-selected foundation population of Fulbe (Peulh) sheep maintained at the Animal and veterinary Research Station, Garoua, Cameroon. Al1 the animals were identified with ear tags. Each progeny was pedigreed only by dam. Individual sire identification of lambs was not possible because the ewes were exposed to several rams simultaneously. Management practices were relatively rural. The animals were allowed to graze on Effets génétiques et de l'environnement sur le poids à la naissance de moutons unimproved pastures during the rainy and dry seasons and were given varying quantities of feed supplementation (cotton seed cake and brewer's dry grain) in the dry season.
This region is located within the Sudano-Guinean zone of Cameroon. The rain pattern divides the year into two distinct seasons: a rainy period that usually begins in April and ends in October, with no consistent pattern of rainfall, and a very severe dry season from November to April. There is such an irregular variation in precipitation that the quality and quantity of forage available for grazing change according to the seasonality and distribution of rain.
Ewes and lambs were identified and the lamb birth weights (kg), sex, parity, type of birth, month and year of birth were recorded within 24 h of lambing.

Statistical mode1 and analytical technique
The linear mode1 (dam model) used to obtain the least squares means (LSM) estimates for lamb birth weights was as follows: The data were analysed using the Mixed mode1 least-squares and Maximum likelihood computer program (9) which adjusted for fixed effects. This method uses Henderson's method 3 (10) to estimate the dam variante component (02d) and the error variante component (02e) from which heritability (h2d) and its error were computed. The least squares means estimates (LSM) and their appropriate standard errors were also computed using LSMLMW Computer Program (9).
Estimates of the regression coefficients and standard errors of offspring lambing interval and birth weight on the ewe lambing interval and birth weight (hop) were carried out according to the method reported by Mead and Curnow (14). Heritabilities and their standard errors were estimated as described by Falconer (6). Ewes which lambed more than once were considered as separate entries due"to the fact that the age of the ewe has been shown to significantly affect her performance (1, 17). n RESULTS AND DISCUSSION Although, the ewes lambed a11 around the year, there was a variation in the number lambed per month. The highest number of births was registered for the month of October followed by that of February (table 1). This could be an indication that a high rate of conception took place around the months of May and September, respectively. Consequently, the period of maximum oestrous and ovulation activities could be at the beginning and at the end of the rainy season. Ngo-Tama et al. (unpublished results), however, indicated the months of September and November as months of high rate of conception for the same breed. The difference may be due to sample size and seasonal variations. The period of maximum oestrous and ovulation activities may also differ within regions and breeds. The Moroccan Sardi, Beni-Gui1 and Timahdite breeds show maximum oestrous and ovulation activities during the months of June to January (8). Bearing this in mind, it cari be possible to influence the optimum weight for early sexual maturity by giving appropriate feed to the dams during pregnancy and to the lambs up to puberty.
Statistically, there was a significant month effect (p < 0.01) on birth weights. A comparative month effect (p < 0.005) was reported by Ngo-Tama et al. (unpublished results) for the same breed. Fall et al. (7) also reported a month effect (p < 0.001) for tropical sheep. Bathaei and Leroy (1) however, reported a nonsignificant month effect (p > 0.001) for the Iranian fat-tailed Mehraban sheep.
The year effect on birth weight was highly significant (p < 0.001). Similar results have been obtained by various researchers on various breeds (1,4,7,16). The interaction effect of month by year of birth was quite significant (p < 0.001). Bathaei and Leroy (1) reported similar results. The significant effects of month, year and month by year of birth interaction could be due to seasonal variations, associated with nutritional problems, climatic stress, incidence of diseases, herdsman ski11 or probable changes in the genetic make-up of the flock during the long period of breeding. These variations caused changes in the quantity and quality of forage, and in the weight and body condition of gravid ewes. These effects subsequently influenced the lamb birth weights.
The random effect of dam on lamb birth weight was highly significant (p < 0.001). Foetuses of gravid dams tend to receive nutrients in priority and, therefore, are to a degree protected from environmental fluctuations (4), but differences in the genes transmitted from the dam to the offspring, variations in pre-natal environment and changes in the dam nutrition and body condition could seriously affect the birth weights.
Birth types are presented in table II. Though single births were predominant, twinning was also significant. Triplets and quadruplets, however, were not common. Ngo-Tama et al. (unpublished results) obtained comparable results for the same breed. Shelton and Menzies (17) reported a range of 7.7 to 28.9 % and 19.7 to 44.3 % of multiple births for fine-wool sheep maintained in two different stations. The type of birth effect w,as quite significant (p < 0.001). Single lambs were heavier at birth than twins. Similar results have also been reported by various researchers (13,16). This could result from the limited capacity of the ewe to provide pre-natal nourishment for the development of the foetus. Therefore, where twins are concerned, feed supplementation may be necessary to compensate for the deficiencies they might have acquired in their pre-natal environment and while competing for materna1 milk (4, 16).  The percentage of ewes that lambed decreased with increase in parity (table III). This could be an indication of a negative correlation between age of the ewes and lambing percentage.
The age of the ewe has been shown to significantly affect her performance (1, 17). Consequently, under random mating conditions it may be economical to carry out a selection based on breeding to increase ewe fertility (twinning) under improved management as multiple births could result in maximum meat output per unit of input. Selection in this direction, however, Will depend on the economic advantages of feed supplementation (3,4) and its availability during the stressful period to compensate for slow growth of lambs born and raised as twins (16).
Parity was equally a very significant source of variation for the lamb birth weight (p < 0.001). Comparative results have been reported on various breeds by Bologum et al. (5), Fall et al. (7) and Inyangala et al. (11). This is because first parity dams are still growing and therefore must provide for their nourishment and that of the foetus.
The mean, s.e. and range of age at first lambing were 15, 5.8 and 8-28 months, respectively (n = 112   (1) and Fall et al. (7), however, reported significant sex effects (p < 0.001) on birth weights for the Iranian fat-tailed Mehraban and tropical sheep breeds, respectively. The differences observed in the weights of male and female lambs could be due to hormonal differences in their endocrinological and physiological functions.
The heritability estimates and s.e. from the dam variante component and from the regression of offspring lambing interval (n = 44) and birth weight (n = 120) on the ewe are presented in table V. The estimate from the dam component is much larger than the estimate from the hop indicating a substantial bias which could be due to common environment, materna1 andlor dominante effects. On the other hand, this high estimate (0.45) from the dam component seemed to suggest a high variante of milking and materna1 ability in this breed (to be proven). The estimate from the regression of offspring birth weight on the birth weight of the dam appeared to be free from unwanted variations due to environmental differences between sire groups, though the approach did not adjust for environmental factors. Although this estimate compares favourably with that of 0.33 reported bJ Bathaei and Leroy (2)   Climate related environmental factors such as month, year of birth and month by year of birth interaction severely affect the birth weight of the Fulbe sheep (p < 0.001). Maximum oestrous and ovulation activities appear to be important at the beginning and the end of the rainy season. The random effect of dam, and the fixed effects of type of birth and parity are equally very pronounced on the birth weight of the Fulbe lambs. Genetic factors also play a very important role in the determination of the birth weight of Fulbe sheep. This denotes a high estimate of heritability (h2d) from the dam variante component and a moderate estimate from regression (b, ). The moderately high estimate of 0.35 and the low s.e. of 0.1 !! could suggest that the birth weight of the lambs could be used for mass selection. On the other hand, environmental factors seem to influence lambing interval more than is expected of genetic factors as shown by the very low estimate of heritability from regression.