Establishment of 15N-leucine single injection method to determine endogenous amino acid losses and its application in precision feed formulation

Accurate quantification of endogenous amino acid losses (EAAL) is fundamental for determining the true digestibility (TD) of feed ingredients. This study aimed to investigate whether the 15 N-leucine single injection method (SIM) could be used to determine EAAL, measure the TD of different protein sources using this method, and provide a basis for the application of low-protein diets in poultry. In Exp. 1, 21 cecectomized Jingfen roosters (21-wk-old, body weight [BW] 1.54 ± 0.01 kg) were assigned to three treatments with nitrogen-free diet (NFD) group, continuous infusion method (CIM)-CON group, and SIM-CON group, with the latter two groups receiving a standard diet. The experiment was conducted over a 12-d period, with seven replicates per treatment and one bird per replicate. In Exp. 2, 35 cecectomized Jingfen roosters (40-wk-old; BW 1.69 ± 0.04 kg) were allocated to five dietary treatments with soybean meal (SBM) diet, cottonseed meal (CSM) diet, rapeseed meal (RSM) diet, a mixed diet (with protein provided jointly by SBM, CSM, and RSM), and a NDF diet. Each treatment consisted of seven replicates with one bird per replicate, and the experimental period lasted for 4 d. In Exp. 3, 384 one-day-old Arbor Acres broilers (BW 41.06 ± 0.11 g) were distributed into 4 dietary treatment groups, with eight replicates per treatment and 12 birds per replicate. The experimental period lasted for 42 d. The four treatments consisted of diets formulated based on either standard ileal digestibility (SID) or total TD values, at either normal (CON) or low-protein diet (LPD), designated as SID-CON, TD-CON, SID-LPD, and TD-LPD, respectively. In Exp. 1, SIM yielded EAAL and TD values comparable to the CIM ( P > 0.05); except for isoleucine, phenylalanine and cysteine, estimates from the two isotope methods were significantly higher than the NFD method ( P < 0.05). In Exp. 2, with the exception of proline, the TD values determined for mixed diets did not differ significantly from the predicted values derived from single ingredients ( P > 0.05), confirming the additivity of SIM-derived TD values. In Exp. 3, broilers fed TD-CON diets exhibited significantly lower feed: gain ratio (F: G) and reduced specific EAAL (e.g., arginine and lysine) during the starter phase (1–21 d) compared to the SID-CON group ( P < 0.05). In conclusion, this study demonstrated that SIM is a reliable technique for determining EAAL and TD in poultry. Relative to traditional SID-based systems, feed formulations using SIM can improve broiler feed efficiency while reducing SBM inclusion.