A multivariate Box-Behnken assessment of elevated branched-chain amino acid concentrations in reduced crude protein diets offered to male broiler chickens.

In a Box-Behnken assessment of elevated branched-chain amino acids (BCAA), 13 nutritionally equivalent maize-based diets were offered to a total of 390 off-sex male Ross 308 broiler chickens from 7 to 28 days post-hatch. The BCAA concentrations investigated in reduced-crude protein diets were 12.5, 15.5, 18.3 g/kg leucine (125, 155, 183); 8.9, 10.2, 12.5 g/kg valine (89, 102, 125) and 7.2, 8.9, 10.8 g/kg isoleucine (72, 89, 109), where their relativity to 11.0 g/kg digestible lysine are shown in parentheses. Determined parameters included growth performance, relative abdominal fat-pad weights, nutrient utilisation, apparent digestibility coefficients, disappearance rates of 16 amino acids and free amino acid systemic plasma concentrations. Increasing dietary leucine linearly depressed weight gain and quadratically influenced FCR where the estimated minimum FCR of 1.418 was with 14.99 g/kg leucine. Polynomial regression analysis and surface response curves of determined parameters were generated for significant (P < 0.05) BCAA variables, based on lack of fit (P > 0.005). Quadratic and cross-product responses were observed for weight gain, FCR, AME, AMEn, N retention and apparent digestibility of 13 amino acids. Relative fat-pad weights declined linearly with increasing isoleucine and valine. The lowest N retention was estimated at a combination of 15.25 and 10.50 g/kg leucine and valine respectively whilst the highest mean digestibility coefficient (0.793) of amino acids was estimated at a combination of 15.74 and 10.47 g/kg of leucine and valine respectively. The remaining parameter minima or maxima responses were not able to be determined since they were outside the extreme BCAA treatment levels. Increasing dietary BCAA significantly increased apparent ileal digestibilities and disappearance rates of BCAA. Systemic plasma concentrations of valine increased (P < 0.001) with increasing dietary valine but leucine was not influenced (P > 0.25). Systemic plasma concentration of isoleucine was maximised (P < 0.001) only when accompanied by elevated dietary leucine. Also, dietary treatments influenced (P < 0.05) apparent disappearance rates of all the essential amino acids analysed, with the exception of methionine. Whilst overall growth performance was not disadvantaged (P > 0.10) by elevated BCAA levels, compared with 2019 Ross 308 performance objectives, polynomial regression analysis suggested both interaction and antagonism between BCAA.