Nitrogen balance and ileal amino acid digestibility in growing pigs fed diets reduced in protein concentration

Two studies were conducted to assess the effect of dietary protein reduction on N utilization, N excretion, and AA digestibility in growing pigs. The objective was to determine whether pigs fed diets with a reduced CP concentration could maintain the same N retention as pigs fed an adequate diet. The second objective was to test whether reducing dietary CP concentration decreases AA digestibility. In each study, six barrows were allotted to one of six dietary treatments in a Latin square design. Treatments consisted of four corn-soybean meal-based diets containing 15, 12, 9, and 6% CP, a casein-based diet containing 15% CP, and a protein-free diet. Crystalline AA were included in the 12, 9, and 6% CP diets. The indispensable:dispensable AA ratio was maintained at 45:55 with the addition of L-glutamic acid to the 9 and 6% CP diets. The casein-based and protein-free diets were used to determine endogenous total tract N and ileal AA losses. In the first study, total N losses and N absorbed decreased linearly (P < 0.001) as dietary CP concentration decreased from 15 to 6%. Both a linear (P < 0.001) and a quadratic (P < 0.05) decrease in N retention were found with decreasing dietary CP concentration. Nitrogen retained as a percentage of intake and absorbed increased (P < 0.001) as dietary CP concentration was reduced from 15 to 6%. In the second study, six barrows were surgically fitted with a T-cannula at the terminal ileum to determine ileal AA digestibility. For all dispensable and most indispensable AA, apparent and standardized ileal digestibility increased linearly (P 0.01, and for arginine, P < 0.05) as dietary CP concentration decreased. These results indicate that dietary CP concentration can be decreased from 15 to 12% with crystalline AA supplementation to meet an ideal AA profile without adversely affecting N retention, and that decreasing dietary CP concentration from 15 to 6% increases both dispensable and indispensable AA ileal digestibility.