Nutrient Digestibility and Nitrogen Metabolism Studies at Different Stages of Growth with Fat and Lean Type Swine Fed Two Levels of Protein

Six lean- and six fat-type barrows were visually selected at weaning and allotted in a 2 × 2 × 3 factorial arrangement to either a high- or low-protein level sequence. Digestion and N-metabolism trials were conducted with these pigs at three stages of growth (41, 70 and 95 kg). No significant differences were found in the digestibilities of dry matter, gross energy or protein within trials regardless of the protein content of the diet or the relative leanness of the pig. Nutrient digestibility generally improved somewhat with increasing age, and protein digestibility was somewhat higher when pigs were fed at the higher protein level. Lean-type pigs fed the higher protein diet retained similar amounts of N daily (22 g/day) at body weights of 40 to 70 kg, but a lower amount (16 g/day) at 95 kg of body weight. Daily N-retention in fat-type pigs fed the high-protein diet decreased with increasing body weight (22, 16 and 13 g/day at 41, 70 and 95 kg of body weight, respectively). Daily N-retention was similar in both fat- and lean-type pigs when the low-protein level was fed. No improvement in carcass leanness was found when the high-protein diet was fed to fat-type pigs, indicating that the low level of protein was adequate for maximum carcass leanness in fat-type pigs. However, the higher protein level substantially improved carcass leanness in lean-type pigs.