The objective of the study was to examine the impact of increased branched-chain amino acids (BCAA) blends to lysine ratio on growth performance, nutrient utilization, caecal short-chain fatty acid (SCFA), and protein and fat accretion in the pectoralis major muscle in broiler chickens fed a reduced-protein diet. A total of 270 Cobb 500 male broiler chicks were allocated into 18-floor pens, with 15 birds per pen on d 0. All birds received the same starter diets (220g/kg crude protein and 3,000 kcal/kg AME) during the first 8d. Thereafter, three experimental diets (D) were provided for the grower (8-28d) phase, namely D1, which was an adequate-protein diet (190g/kg crude protein) with the recommended Lys to BCAA ratio (100:294), D2 was a reduced protein diet (170g/kg crude protein) with recommended Lys to BCAA ratio (100:294), whereas D3 had the same protein content as D2 but with a 10-point decrease in Lys and a proportionate increase in BCAA, that is Lys: BCAA of 90:304. The ratios of individual AA in the BCAA group were kept constant in all the diets. Birds and feed were weighed on d8 and 28. Ileal digesta and ceca content were collected from 5 birds per pen on d21 for AA digestibility and SCFA analysis, respectively. On d28, the pectoralis major was collected from one bird per cage to analyze protein and fat composition. The birds fed D3 had lower (P < 0.05) weight gain and greater FCR than the other diets, whereas birds fed D1 and D2 had similar weight gain and FCR values. The birds receiving D1 and D2 had greater (P < 0.05) AA digestibility values than D3. However, digestibility was similar for D1 and D3 except for Asp, His, Lys, Pro, and Tyr, which were lower (P < 0.05) for D3 than for D1. There was no treatment effect on the digestibility of Arg, Met, Thr, and Gly. In addition, the birds fed D1 had (P<0.05) greater N digestibility compared to those receiving D2 or D3. The cecal content of propionate and total SCFA tended (P < 0.10) to be lower in birds receiving D3, but there were no treatment effects on the cecal content of the other SCFA. The birds receiving D3 had (P<0.05) greater fat and dry matter, and by contrast, lower protein content in the breast muscle than D1.
In conclusion, reducing the Lys ratio by 10% with a proportionate increase in BCAA level in a reduced-protein diet produced a detrimental effect on growth performance and amino acids digestibility and increased the fat content of breast muscle; therefore, BCAA did not have a corresponding proteogenic effect as the Lys under the reduced-protein conditions of the current experiment.