Diammonium phosphate supplementation in low-protein diets improves nitrogen utilization and feed efficiency without impairing carcass traits or meat quality in finishing pigs.

Low-protein (LP) diets improve nitrogen utilization efficiency and mitigate environmental pollution, however, excessive reductions in crude protein (CP) result in total nitrogen (TN) deficiency, thereby compromising growth performance. This study investigated the effects of replacing calcium hydrogen phosphate with diammonium phosphate (DP) in an LP diet on growth performance, carcass traits, meat quality, plasma biochemical parameters, hepatic nitrogen-metabolizing enzyme activities, and amino acid (AA) profiles in finishing pigs under TN restriction. Additionally, 15nitroge (15N)-isotope tracing was employed in suckling piglets to track ammonia nitrogen assimilation by analyzing 15N enrichment in plasma and hepatic AA following 15N-DP administration. Sixty-three barrows (Duroc × Landrace × Large White; initial body weight: 74.3 ± 3.8 kg) were randomly allocated to one of three dietary treatments: a normal protein (NP) diet (CP 13.47%), a LP diet (CP 9.06%), or a LP (CP 10.47%) diet supplemented with 1.07% DP (LDP). All pigs had ad libitum access to feed and water for 51 d. The ratio of essential AA nitrogen to TN was 0.43, 0.50, and 0.43 in the NP, LP, and LDP diets, respectively. Pigs fed the LDP diet exhibited higher gain-to-feed (G:F) ratio (P = 0.014), plasma urea nitrogen concentrations (P = 0.001), hepatic non-essential amino acids (NEAA) concentrations (P = 0.028), and carbamoyl phosphate synthetase 1 (CPS-1) activity (P = 0.018), but lower average daily feed intake (ADFI; P < 0.001) compared with those fed the LP diet. The free AA profile of the longissimus dorsi muscle (LM) in pigs fed the LDP diet showed reduced concentrations of Arg and Phe (P < 0.050), but increased the concentration of Pro compared with those fed the NP diet (P = 0.037). To trace nitrogen fate, three suckling piglets received a 15N-DP-supplemented diet. 15Nitrogen enrichment in plasma and hepatic AA at multiple time points confirmed that dietary ammonia nitrogen was absorbed and incorporated into endogenous AA. In summary, dietary supplementation with DP in the LP diet improved nitrogen metabolism and AA synthesis, thereby enhancing G:F ratios without compromising carcass traits or meat quality. However, excessive DP supplementation may suppress feed intake, underscoring the need to optimize its dietary inclusion level to ensure efficient pig production.