Protein retention in ruminants: changes with body weight and variation with breed, sex and feeding level

Using a great amount of published and unpublished data, this paper analyzes changes in the protein content of the empty body of growing cattle and sheep (appendices 1, 2). Protein content (p. 100 of empty body weight) usually increased from 6-7 p. 100 to 18-20 p. 100 in cattle and to 14-18 p. 100 in sheep during foetal life. After birth, it decreased slowly, reaching 13-15 p. 100 in 70 p. 100 mature-weight animals. Changes in water and lipid content were analyzed in relation to protein content (fig. 1). Protein content (p. 100 of fresh weight) varied between 10 and 14 p. 100 in the adipose tissues, 15 and 16 p. 100 in the offals, 19 and 20 p. 100 in the muscles and skeleton and 28 and 30 p. 100 in the hide. Muscle protein amounted to 48 to 53 p. 100 of whole body protein. Mean daily protein accretion increased in cattle from 100 g after birth to a maximum of 160-200 g/day at 200-300 kg of body weight and decreased afterwards (fig. 3). From 20 g/day in very young lambs, it reached 30-40 g/day at 20-30 kg of body weight. Daily protein accretion in g per kg of metabolic weight (W0.75) was similar in the two species (5.5 g at birth, 2.8 g at 30 p. 100 mature body weight and 1.2 g at 60 p. 100 mature body weight). The lipid deposition was greater than protein accretion after 10 p. 100 mature body weight in sheep, but only after 20 p. 100 mature body weight in cattle. The ratio between lipid deposition and protein accretion was greater in sheep than in cattle. Daily protein accretion, which varied according to sex and genotype, ranged between 150-235 g/day in cattle and between 32-54 g/day in sheep. The protein content of sheep and cattle gain dropped when the feeding level increased. Simultaneously, the lipid content of gain increased and the animals were fatter at slaughter. However, it appears that the composition of sheep gain was less sensitive to feeding level than that of cattle.