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 effect of essential:total (E:T) N ratio on N retention and utilization. Purified 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 utilization gradually decreased as E:T increased. At a constant level of essential N, N retention remained unchanged until the E:T value reached 0.48 and then 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 with increased E:T value. These results suggest that under conditions of optimal protein utilization, essential amino acids are partially degraded and used for the synthesis of non-essential amino acids.