Estimates were made of total body protein in 45 unimproved crossbred pigs slaughtered serially between 20 and 200 kg live weight. Allometric functions were used to estimate protein weight at any given body weight and the Gompertz function was used to express live-weight changes in relation to time and to predict weight at maturity. Predicted values for daily protein retention increased rapidly from 20 kg to attain, at around 75 kg live weight, maxima of 130 g for entire males, 120 g for females and 105 g for castrated males. Between 45 and 110 kg live weight rates of protein retention were maintained within 10 g of the maximum rate. Mature live weights were estimated to be 240, 215 and 225 kg for entire males, females and castrated males. The allometric exponents for whole body protein as a function of body weight were 0·963 (s.e. 0·025), 0·927 (s.e. 0·021) and 0·850 (s.e. 0·049) for entire males, females and castrated males respectively.

Without knowledge of potential protein growth, nutrient requirements of pigs cannot be accurately determined. Daily protein retention [Pr] was estimated with 45 crossbred pigs serially slaughtered between 20 and 200 kg body weight. An understanding of growth to maturity is vital for the adequate nutrition of pigs grown to slaughter at heavier weights and for breeding sows, but the determined function: Pr(kg) - 0.125 [±0.009] -0.0002 [±0.0001] mean live weight, having no significant slope, was an inadequate descriptor of instantaneous Pr for pigs of more than 110 kg. Allometry was therefore used to predict protein weight at any given body weight and the Gompertz function to express body weight changes with time and derive values for weight at maturity. Predicted values for Pr attained maxima at 75 kg body weight of 130 g for entire males. 120 g for females and 105 g for castrates. Between 45 and 110 kg body weight Pr was within 10 g of the maximum rate (Figure 1); mature body weights were estimated to be 240, 215 and 225 kg. Allometric expressions for dissected carcass and chemical components as a function of empty body weight are given in Table 1. Figure 2 compares currently predicted potential rates of protein retention with those of Carr et al. [1977] and Thorbek (19751. It is evident that weight and age at maturity are crucial to the quantification of protein growth; such measurements are prerequisite to the provision of a quantitative description of improved pig genotypes.

1. Semi-synthetic diets, with dried microbial cells (Pruteen) as the nitrogen source, were used to measure N retention in 50 kg pigs given different combinations of N intake involving periods of deprivation and enhanced 2. Metabolic faecal N losses were 1.92 g/d (1.26 g/kg dry matter eaten) and endogenous urinary losses were Many pigs undergo a period of growth during which deposition of protein proceeds at below the potential rate; for example, as a result of depressed feed intake following weaning or as a consequence of imperfect matching of dietary protein supply to protein requirements for growth. It is therefore of biological and economic importance to ascertain whether or not pigs can compensate for a temporary reduction in protein deposition rate by undergoing a period of enhanced nitrogen retention when dietary protein supply is more liberal. A previous paper (Whittemore et al. 1978) demonstrated that compensatory protein retention (as measured by N balance) was possible in growing pigs following a 12 d period of N deprivation, and that enhanced N retention rates were associated with improved efficiency of utilization of dietary N. The aim of the present experiment was to examine the phenomenon in more detail by providing growing pigs with a sequence of three diets. The four diets used contained low, medium, high or extravagant levels of protein, and they were offered in combinations to test the effects of changes on N balance during the first three and the following 7 d. These were compared with values obtained from control pigs which received one of the four diets throughout the experiment. It was also of interest to examine possible changes in the biological value of proteins when provided at different levels in the diet.Conventional balance techniques were used to measure N balance after profound changes in N intake. Control treatments involved the provision of very low, moderate, high and very high daily N intakes, the very high level being included to test N-retention response at an extreme level of N supply. While compensatory N retention was observed, the results indicated that this mechanism accords more closely with the short-term replenishment of variable body lean (Fowler, 1976) or labile N stores (Boorman, 1980), than the acceleration of N deposition in carcass muscle growth to allow growing pigs to compensate for previous dietary misfortunes and catch up with unchecked contemporaries. * For reprints.

1. Semi-synthetic diets, with dried microbial cells (Pruteen) as the nitrogen source, were used to measure N retention in 50 kg pigs which had been given only sufficient N (5.3 g/d) to maintain N equilibrium for the previous 12 d. Control pigs were given 33.2 gN/d. 2. Metabolic faecal N losses were 1.62 g/d (1.2 g/kg dry matter eaten) and endogenous urinary losses were 3.90 g/d. 3. Realimentation of N-deprived pigs was achieved with diets providing 20.0, 33.2 and 67.4 g N/d and daily rates for N retention were 11.2, 17.8 and 25.9 respectively; the corresponding value for control pigs was 15.0 g. 'Catch-up' protein growth was demonstrated in pigs given both 33.2 and 67.4 g N/d. In the former instance, this was associated with an increase in the efficiency of utilization of dietary N. 4. The biological value of the protein in Pruteen was 0.85, and it appeared that under conditions of increased demand for N the pig could utilize some of the nucleic acid-N fraction of this protein source.

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