Lamb growth performance and factors affecting body weight of Iranian fat-tailed Mehraban breed of sheep

Une étude des facteurs affectant le poids dans la race Mehraban en Iran a été effectuée de 1984 à 1989. Le uoids de naissance. le ooids au sevrage (a 90 jours) et le gain moyen quotidien avant sevrage de ‘973 moutons ont été analyses. Tous les caractères étudiés étaient affectés significativement par l’année de naissance, le sexe, le type de naissance et l’âge de la mère. Les agneaux nés de brebis âgées de 4 à 5 ans avaient le poids le plus élevé à la naissance, pendant la croissance et au sevrage tandis que les agnelles avaient les agneaux les plus légers. Le sexe avait un effet hautement sienificatif sur le noids avant sevrage en faveur des mâles. Les


Introduction
Mutton is the most important source of red meat in Iran, but domestic production cannot meet the consumers increasing demand.Mismanagement and overgrazing of the natural ranges have practically eliminated the possibility of increasing the sheep population in the near future.In order to increase production, efforts must be directed at improvement in feeding, breeding and management of these animals.
TO improve breeding, attempts must be made to Select superior breeding stock to be parents of successive generations in order to make an impact on the Iranian animal population.Selection cari only be effective when animals are compared on an equal basis to identify those that are superior.The growth of animals is influenced by both genetic and environmental factors and their interactions.If non genetic factors are incorrectly evaluated, errors and bias might arise when selecting the superior individuals.It is therefore essential to obtain a knowledge of the factors influencing growth and the present study was undertaken to evaluate the influence of non genetic factors such as age of dam, type of birth, sex, year and month of birth, on birth weight, weaning weight and daily gain from birth to weaning.

Materials and Methods
The lambs used in this study were from Mehraban fat-tailed Iranian breed of sheep.Mehraban are raised on farms in the western part of Iran.Data used in this study were collected over the ô-year period, 1984 to 1989, from a Mehraban flock in the west of Iran.Observations concern 973 lambs, progenies of 18 rams, each being mated on an average of 20 ewes.
The lambing season was between February to May each year.Age of dam, type of birth and sex of lambs were recorded.Single and twins were included in this study.Lambs were allowed to remain with their dams till the weaning age of 90 days.Body weight of lambs at different periods such as birth, every 10 days to weaning were kept (from 10 to 90 days).Ewes were maintained on pasture.

Results and Discussion
Results of the analysis of variante and tests of significance for birth weight, weaning weight and daily gain from birth to weaning are given in table I and the least-squares means for the effects of the age of dam, the lambing year, the type of birth, the sex of lamb and the month of birth are given in table II.
The R' values presented in table II indicate that the main effects, given by order of importance for birth weight, weaning weight and daily gain from birth to weaning are: type of birth, sex of lamb, age of dam, lambing year and mounth of birth.Age of dam had a significant effect (p < 0.05) on the pre-weaning traits.The lamb born of 4 and 5 year-old ewes were the heaviest, where as those born of 2 year-old ewes were the lightest.One year-old ewes reared lambs that had significantly lower rates of growth from birth to weaning and were significantly lighter   4,5,7,8,9,10,16,17).
Pre-weaning traits are greatly influenced by the level of milk production of the ewe and age has a considerable influence on milk production.It was expected that only pre-weaning traits would be influence by the age of ewe, as observed in this experiment.
Sex had the most important influence on the pre-weaning traits (p < 0.001) of all the factors studied.The male lambs were significantly heavier than the female lambs at birth (0.25 kg) and at weaning (1.71 kg) and had a significantly higher rate of growth from birth to weaning (22 g/day).Several authors found significant differences in the birth weight and weaning weight of lambs (1,3,4,10,11,13,14,15,16). Single lambs were significantly (p < 0.001) heavier than twins.Similar results were also reported by other investigators (1,3,4,8,10,Il,13,14,15).The least-squares mean differences for birth weight and for weaning weight of lambs born as singles and twins were 0.4 and 3.90 kg, respectively (table III).The least-squares mean for daily gain of singles and twins were 224 and 185 g, respectively.Differences in year of birth on the preweaning traits were significant (p < 0.05).The maximum differences in the bir-th weight and weaning weight of the lamb born between the best year (1985) and the worst year (1989) were 0.36 and 0.66 kg, respectively.The influence of year on growth of the lambs may have been the result of changes in management, incidence of diseases and other environmental factors.Several authors found a similar trend (1,3,4,8,10,Il,13,14,15).However WILSON (18) repot-ted that year of birth had not significant effect on any pre-weaning trait.Month of birth did not affect any pre-weaning trait.Similar results were also reported by other investigators (3, 18).The first order interactions between type of bir-th,' sex of lamb, age of dam, lambing year and mounth of birth were not significant.The phenotypic correlations between birth weight and'weaning weight, birth weight and average, daily gain and weaning weight and average daily gain were 0.65, 0.53 and 0.96, respectively.The correlation' coefficient was significant and similar to corresponding estimates repot-ted by FARD et a/.(9).
Knowledge of environmental factors which influence preweaning weight and weaning weight should be used to help selection.Since, the effects of sex, type of birth and age of dam influence significantly more weight at preweaning weight and weaning weight than at later ages.If' early selection is practised, correction factors for these, non genetic effects or selection within specific groups' should be realized.The results obtained in this study suggest that the selection to increase weaning weight (908 days) in the experimental herd could be based on the body weight of the lambs before weaning.The main non genetic factors to be included in a mode1 for the estimation of breeding values of the Mehraban breed are: sex,: type of birth, age of dam and lambing year for birth, weight, weaning weight and daily gain.

TABLE I
Analysis of variaace and tests of significance for birth weight, weaaing weight and daily gain from birth to weaning in the Mehraban breed.

TABLE ZZ
R2 of each effectfor birth weight, weaning weight and daily gain from birth to weaning in the Mehraban breed at weaning, compared with the other age.The results correspond to the findings of several investigators (1, 2,