The composition of branched-chain amino acids modulates response to nucleotide supplementation of low-protein diets in nursery pigs

A total of 160 pigs (initial body weight [BW], 5.79 ± 0.01 kg) were used to investigate the effects of dietary nucleotides and branched-chain amino acids (BCAA) supplementation of low-protein diets on growth performance, blood metabolites, short-chain fatty acids (SCFA), apparent ileal and total tract digestibility of nutrients (AID and ATTD). Following a 3-d adaptation period on a common diet after weaning, pigs were randomly assigned to 1 of the 5 dietary treatments using a randomized complete block design. Each treatment consisted of 8 replicate pens with 4 pigs per pen. The experimental diets included a normal-crude protein (CP) Positive Control (PC) diet with CP at 22%, 20.6%, and 19.4% for phases 1, 2, and 3, respectively; a BCAA-deficient low-CP Negative Control (NC) diet with CP at 16%, 14.6%, 13.4% for phases 1, 2, and 3, respectively; NC diet supplemented with crystalline isoleucine and valine (NC + BCAA); NC diet supplemented with 9 g/kg of dietary nucleotides (NC + NL9); and NC diet supplemented with both BCAA and nucleotides (NC + BCAA + NL9). Average daily feed intake, average daily gain (ADG), and gain-feed ratio (G:F) were recorded. Fecal samples were collected on days 39, 40, and 41 to determine the ATTD of nutrients. Blood samples were collected on day 42 to measure blood metabolites and pigs were euthanized to determine the AID of nutrients and colonic SCFA. Pigs fed the PC diet had the highest (P < 0.0001) overall ADG, G:F, and final BW, with intermediate values in NC + BCAA + NL9 and NC + BCAA, and the lowest values in NC + NL9 and NC. Blood urea nitrogen concentrations decreased (P < 0.0001) with reduced CP, while serum glucose, insulin, and glutathione concentrations remained unaffected. The NC diet had the lowest (P < 0.0001) serum insulin-like growth factor-1 concentration (IGF-1), with partial recovery in the NC + BCAA diet and complete recovery in the NC + BCAA + NL9 diet, compared to PC. The NC + BCAA + NL9 diet tended to increase (P = 0.09) serum uric acid concentration and had the highest ATTD of nitrogen (N), gross energy, and dry matter (P < 0.05) as well as the highest AID of N and phosphorus (P < 0.0001 and P < 0.001, respectively). Additionally, the NC diet showed a tendency to decrease (P = 0.08) the total colonic SCFA concentration. Results suggest that nucleotide supplementation, following BCAA addition to low-CP diets, conferred additional beneficial effects on growth performance and nutrient digestibility in pigs.