I . The requirement of the artificially-reared pig for crude protein (nitrogen x 6.38; CP) was estimated between 2 and 28 d of age. The pigs were given one of five diets, each containing 270 g soya-bean oil/kg and either dried skim-milk and dried whey in different proportions to supply 150, 180, 210 or 240 g CP/kg, or dried skim-milk alone which supplied 270 g CP/kg. The energy content of the diets was approximately zz MJ digestible energy/kg.
2.The diets were obtained as spray-dried powders, reconstituted in water, and fed as a liquid containing zoo g DM/I. The pigs were fed at hourly intervals to a scale based on live weight. The protein requirement of the artificially-reared pig has received little attention, and reported estimates vary considerably. Part of this variation is attributable to differences in the age of the pigs, the energy content of the diet, and the source and quality of the protein in the diet.Protein requirements decrease with age. Reber et al. (1953) reported that up to 22 d of age 410 g crude protein (nitrogen x 6.38; CP)/kg diet was needed for maximum performance and N retention, but at 49 d of age 200 g CP/kg diet produced optimal performance. Peo et af. (1957) also found that the age of the pig had a similar effect; 300 g CP/kg diet was required for maximum growth rate between 7-21 d of age, but only 200 g CP/kg diet between 7-35 d of age. Protein requirements are also related to the energy density of the diet. A requirement of 250 g CP/kg from 2-28 d of age was reported when the diet contained less than 10 g fat/kg (Manners & McCrea, 1962), but if the fat content of the diet was increased to 210 g/kg, then the protein requirement was increased to 310 g/kg (Manners & McCrea, 1963).Protein sources vary in digestibility, which will affect the requirement for CP. In the very young pig, milk protein is highly digestible. During the first month of life, the apparent digestibility of N in cow's milk was 0.99 (Braude et al. I970), compared with 0.92 for soya-bean protein and 0.88 for fish protein (Cunningham & Brisson, 1957). The apparent digestibility of non-milk proteins increases with age; the digestibility of soya-bean protein increased from 0.78 at 14 d of age to 0.82 at 35 d of age, whereas the digestibility of milk protein remained constant at 0.96 (Hays et al. 1959).
