EFFECTS OF BIRTH-WEIGHT AND POSTNATAL NUTRITION ON NEONATAL SHEEP - I- BODY GROWTH AND COMPOSITION, AND SOME ASPECTS OF ENERGETIC EFFICIENCY

We investigated the effects of birth weight and postnatal nutrition on growth characteristics of neonatal lambs. Low- and high-birth-weight male lambs were individually reared on a high-quality liquid diet to g row rapidly (ad libitum access to feed) or slowly (ADG 150 g) to vario us weights up to 20 kg live weight (LW). Average daily gain tended to be greater in the high- (mean +/- SE 345 +/- 14 g) than in the low- (3 29 +/- 15 g) birth-weight lambs given ad libitum access to feed owing to slower growth by the small newborns during the immediate postpartum period. At birth, on a weight-specific basis, small newborns containe d 6.4% less nitrogen and tended to have more ash (8.9%) than the high- birth-weight newborns. Daily rates of fat, ash, and GE accretion were greater, and nitrogen accretion tended to be greater in the rapidly gr own large newborns than in their small counterparts. At any given empt y body weight (EBW) during rearing, low-birth-weight lambs contained m ore fat and less ash, resulting in slowly and rapidly grown small newb orns containing 39.3 and 42.7 Meal GE, respectively, at completion of the study (17.5 kg EBW), compared with 34.8 and 40.5 Meal in their lar ge counterparts. The differences in fatness and energy content between the birth weight categories are attributed to energy requirements for maintenance that were approximately 30% lower, coupled with higher re lative intakes in the low-birthweight lambs, during the early postnata l period. At this time, the ability to consume nutrients in excess of lean tissue growth requirements was apparently more pronounced in smal l than in large newborns, which resulted in lower efficiency of energy utilization for tissue deposition. Furthermore, body composition diff erences between the slowly and rapidly reared lambs support the notion of a priority of lean tissue over fat when nutrient supply is limited .