Effect of essential : total nitrogen ratio on protein utilization in the growing pig

Two N balance experiments using growing pigs were conducted to study the ef fect of essential:total (E:T) N ratio on N retention and utilization. Purif ied diets contained casein and crystalline amino acids as the sole sources of N. E:T values ranged from 0.25 to 0.86 while either the concentration of total N (Expt. 1) or essential N (Expt 2) was kept constant. At a constant concentration of total dietary N, N retention and total N utilization were maximized with an E:T value of approximately 0.6, while essential N utiliz ation gradually decreased as E:T increased. At a constant level of essentia l N, N retention remained unchanged until the E:T value reached 0.48 and th en decreased. In Expt 2, maximum total N utilization was attained with an E :T value of 0.66 while N excretion and essential N utilization decreased wi th increased E:T value. These results suggest that under conditions of opti mal protein utilization, essential amino acids are partially degraded and u sed for the synthesis of non-essential amino acids.