An evaluation of elevated branched-chain amino acid inclusions on the performance of broiler chickens offered reduced-crude protein, wheat-based diets from 7 to 28 days post-hatch

The physiological functions of essential branched chain amino acids (BCAA) are important in broiler nutrition, but antagonistic BCAA interactions may become evident. Given the increased implementation of reduced-crude protein (CP) diets, this study aimed to investigate whether elevated leucine inclusions in reduced-CP diets would advantage broiler growth performance, provided concomitant elevations in isoleucine and valine were employed. A total of 378 off-sex male Ross 308 broilers were allocated to 9 dietary treatments, each with 7 replicate cages and 6 birds in each. All diets were wheat-based with 190 g/kg CP. The dietary treatments comprised 3 leucine inclusions (12.71, 15.02, 17.33 g/kg) and 3 isoleucine plus valine inclusions (17.21, 20.32, 23.45 g/kg) in a 3 × 3 factorial array. The assessed parameters included growth performance, relative abdominal fat-pad weights, nutrient utilisation, apparent jejunal and ileal digestibility coefficients and disappearance rates of starch, protein (N), amino acids and free amino acid concentrations in systemic plasma. Elevated BCAA inclusions did not enhance growth performance; indeed, elevating isoleucine plus valine from 17.21 to 20.32 g/kg in the 15.02 g/kg leucine diet significantly compromised weight gain by 6.82% (1325 versus 1422 g/bird). However, elevating isoleucine plus valine inclusions from 17.21 to 20.32 g/kg decreased relative fat-pad weights by 8.49% (9.05 versus 9.89 g/kg; P = 0.046). Elevating leucine levels significantly depressed energy utilisation, AME and AMEn, by 0.39 MJ in both instances. Dietary enrichment with leucine had diverse impacts on amino acid digestibility coefficients coupled with free amino acid plasma concentrations. Increasing dietary leucine concentrations improved ileal digestibility of leucine by 4.74% (0.906 versus 0.865) but also significantly increased ileal digestibility coefficients of another 10 amino acids by an average of 3.37% ranging from 1.60% (methionine) to 4.96% (threonine). Overall, elevations of BCAA in wheat-based, reduced-CP diets did not advantage broiler growth performance. The likelihood is that several factors are responsible for this lack of response and consideration is given to these possibilities in this paper.