Growth in sheep. II. Efficiency of energy and nitrogen utilization from birth to 2 years

Abstract

SUMMARYThirty sheep were pen-fed from age 2 days to nearly 2 years. Fifteen were fed ad libitum and the others were restricted to half that rate, age for age; daily food intakes were recorded. Sequential estimates of body composition were made on each sheep so that protein and energy gains could be obtained for specified age ranges.In the sheep fed ad libitum, food intake increased several fold in the first few months of life but was relatively constant from 4 months despite large increases in body weight. Thus intake per kg¾ declined progressively as the sheep grew.The data for the well fed and restricted groups were combined within each age range and linear regression was used to relate body growth (protein and energy) to food intake, variates being expressed as multiples of body weight (kg¾). Energy gain was zero when intake was 250–590 kJ metabolizable energy/day kg¾, depending on age. Wool growth per unit food intake was constant for each sheep throughout the experiment but was twice as great in some sheep as in others and was higher in the restricted group.Efficiency of energy and protein utilization declined at weaning but otherwise did not vary much with stage of growth. Approximately 40% of energy storage was in protein during milk feeding; after weaning, the fraction declined from 25 to 10% (30 to 20% including wool protein) as the sheep grew.