Amino acid and energy interrelationships in pigs weighing 20 to 50 kilograms

The efficacy of the urea dilution technique for estimating empty body composition of pigs weighing 50 kg was evaluated in experiment 1 (Exp. 1). Blood samples were collected via ear catheter before and at various times (3 to 90 minutes) after urea infusion for plasma urea analysis. Urea space was related ($P \u3c$.05) to chemically-determined total empty body components with few exceptions. Inclusion of additional independent variables with urea space improved the estimation of empty body components. Two experiments were conducted to investigate the interrelationships between amino acids and digestible energy (DE) in pigs weighing 20 to 50 kg. In Exp. 2, diets containing three lysine (Lys) levels that were either adjusted (1.50, 2.35 and 3.20 g/Mcal DE) for five DE levels (3.00 to 4.00 Mcal/kg) or unadjusted (.45,.71 and.96%) for three DE levels (3.50 to 4.00 Mcal/kg) were utilized. The effects of six Lys:DE ratios (1.90 to 3.90 g/Mcal) at two DE levels (3.25 and 3.75 Mcal/kg) were investigated in Exp. 3. Diets were formulated using a constant ratio of corn and soybean meal. Pigs were housed and fed individually; feed and water were available ad libitum. Empty body composition was determined by the urea dilution technique in Exp. 2, and by prediction equations based on backfat in Exp. 3. DE intake was independent of energy content of diets. In Exp. 2, weight gain, gain:DE intake, protein deposition (PD) and PD:DE intake increased ($P \u3c$.001) as Lys levels increased in both the adjusted and unadjusted diets. Weight gain, PD and PD:DE intake decreased ($P \u3c$.001) and fat deposition (FD) increased as DE levels increased when Lys:DE ratios were not maintained. In Exp. 3, there were linear increases ($P \u3c$.001) in weight gain, gain:DE intake, PD and PD:DE intake and a reduction in FD up to 3.00 g Lys/Mcal DE. Energy content of diets had no effect on gain:DE intake, PD and PD:DE intake. The results emphasize the need to adjust dietary amino acid concentrations in concert with energy levels, and indicate that the performance of pigs was optimized at 3.00 g Lys/Mcal DE