Effect of the essential amino acid-nitrogen to total nitrogen ratio on lysine requirement for nitrogen retention in growing pigs

Low protein diets supplemented with essential amino acids (EAA) fed to pigs reduce the excess supply of EAA and nitrogen (N). However, low protein diets may become limiting in non-essential amino acids (NEAA) and N, thus affecting the utilization of EAA for N retention. It has been suggested that the EAA-N:total N (E:T) ratio can give an indication of dietary N sufficiency. An N-balance study was conducted to determine the effect of E:T ratio on the Lys requirement for maximum N retention. A total of 80 growing barrows (19.3 ± 0.21 kg initial body weight) were randomly assigned to 1 of 10 diets (n = 8) in 8 blocks in a 2 × 5 factorial arrangement. Diets consisted of a low ratio (LR; E:T of 0.33) or a high ratio (HR; E:T of 0.36) with graded Lys content (0.82%, 0.92%, 1.02%, 1.12%, and 1.22% standardized ileal digestible [SID]). After a 7-d adaptation, a 4-d N-balance collection was conducted. Blood samples were obtained on d 2 of the collection period 2 h after the morning meal for plasma urea N (PUN) analysis. Data were analyzed using the MIXED model procedure with fixed effects of ratio (n = 2), Lys (n = 5), and their interactions. The experimental block (room) was included as a random effect (n = 8). The SID Lys requirement was estimated using PROC NLIN linear broken-line breakpoint model. There was a significant interaction between E:T ratio and Lys (P < 0.01), where LR diets had a higher N retention than HR diets, while increasing Lys linearly increased N retention (P = 0.01) in both HR and LR diets. The marginal efficiency of utilizing SID Lys (P < 0.01) reduced with increasing Lys content, while the efficiency of utilizing N (P < 0.05) increased as Lys increased. The SID Lys required to maximize N retention of pigs fed HR diets was estimated at 1.08% (R2 = 0.61) and LR diets at 1.21% (R2 = 0.80). The current results indicate that N may be limiting in diets with a high E:T ratio, limiting N retention. Supplying additional dietary N, as intact protein, can increase N retention, resulting in a greater Lys requirement.Low protein diets supplemented with essential amino acids (EAA) can improve growth performance, but dietary non-essential amino acids (NEAA) and nitrogen (N) content may be limiting factors. This limitation may ultimately affect the efficient utilization of EAA for optimal N retention and growth performance. As a benchmark, appropriate quantities of EAA and total N (TN) must be provided, using the EAA-N to TN ratio (E:T) to indicate that both are supplied in sufficient amounts. The present study generally observed a linear increase in N retention with increasing dietary Lys, and N retention was greater in the low E:T as compared with high E:T diets. A greater Lys requirement was observed in the low E:T compared with the high E:T-fed pigs. A low E:T ratio with Lys above current recommendations is warranted to maximize N retention.