1. Protein and energy requirements of pigs weaned at 2 d of age were studied. In Expt 1, three diets with crude protein (CP; nitrogen × 6.38) and fat contents (g/kg dry matter (DM)) of 268, 260 (diet A); 216, 415 (diet B) and 264, 405 (diet C) were compared, with and without a supplement of 2 g DL-methionine/kg. Diets A and B contained skim milk and soya-bean oil only, in different proportions. Casein was added to diet C to equalize the CP content to that of diet A. In Expt 2, diets containing 290 or 315 g CP/kg DM were used with either 310 or 405 g fat/kg. The diets contained different proportions of skim milk, soya-bean oil and casein. The same diets as used in Expt 2 were used in Expt 3 but fed to a scale based on live weight which was 25% less than that used in Expts 1 and 2.
2. Methionine supplementation had no significant effect (P > 0.05) on the growth rate or feed: gain ratio of pigs from 2 to 28 d of age. The growth rate of the pigs from 2 to 28 d of age was significantly (P < 0.05) reduced when they were given the higher level of either CP or fat, with the exception that the effect of level of fat was not significant (P > 0.05) in Expt 3. The reduction in growth rate was associated with a decrease in feed intake. Feed: gain ratios were unaffected (P > 0.05) by the level of either CP or fat in the diet.
3. The higher protein level did increase the proportion of CP in the carcass DM (Expt 2 P < 0.01, Expt 3 P > 0.05) and gave an indication, which was not significant (P > 0.05), of improved N retention (g/d per kg live weight). The higher dietary fat level increased the proportion of lipid in the carcass DM, and this effect was significant (P < 0.05) in both Expts 2 and 3.
4. Diets containing additional casein were not fully consumed at the level of feeding used in Expt 2. This effect may be associated with the larger amounts of digesta in the stomachs of these pigs. Some differences in the composition of the digesta in the stomach were observed, but these did not affect the performance of the pigs.
5. It seems unlikely that either growth rate or feed:gain ratio (g DM consumed/g live-weight gain) can be improved by protein or energy levels exceeding those in a diet containing (g/kg DM) 730 skim milk (265–290 CP) and 310 fat. In this diet, containing (g/kg DM) 6.4 methionine and 3.5 cystine, sulphur amino acid requirements were satisfied.