Variation in energy and nitrogen utilization by sheep between weaning and maturity

Abstract

A diet was given at several levels to four sheep at 2,4 and 10 months of age (fasting, fleece-free liveweights 15-28 kg) and to six adult sheep (6-year-old; 47 kg). Complete balances of energy and matter were measured at each feeding level and during fasting at each age, to determine whether energetic efficiency changed as the animals matured. Despite variation amongst sheep, when intakes were expressed relative to body weight an overall inverse relationship was apparent between digestibility and plane of nutrition, but the effect of increasing intake diminished as the sheep grew. The partition of digestible energy between losses (urine and methane) and metabolizable energy (ME) varied with level of feeding except in the youngest animals, but was not otherwise affected by the age of the sheep. The net result was that ME per unit intake was maximal when dry matter intake was c. 2 % of liveweight, and the decrease at higher planes of nutrition diminished with age. Allowing for the effect of intake, energy balance in MJ/day decreased as liveweight increased, but per unit metabolic size (kg +) energy balances were greater in the adult sheep. The efficiency of use of ME for maintenance was 73-80% with no consistent age trend. Efficiency of utilization of surplus ME for growth increased from 32 �- 2 % at 2 months to 55 � 2 % at 10 months and 6 years as the contribution of protein synthesis to energy balance decreased from 50 to 13 %. Partition of ME between maintenance, protein gain and fat gain gave 76-95 and 56-60 kJ/g as the estimated cost of protein and fat synthesis respectively.