Reducing Dietary Crude Protein: Characterization of Branched-Chain Amino Acid Antagonism in Modern Broiler Chickens

Reducing the crude protein content of broiler diets while maintaining performance has been a primary research area in nutrition. During the late 20th century, the bulk of this research has involved supplementing broiler diets with feed-grade amino acids and proving the economic benefits of their additions. To date, widespread adoption of feed-grade amino acids is limited to methionine, lysine, threonine, and to some extent valine. With valine and isoleucine gaining prevalence in commercial formulation, classic data on the antagonisms associated with the branched-chain amino acids has returned to the forefront of practical broiler nutrition research. Therefore, a series of 23 experiments utilizing 19,692 broilers was conducted in order to ascertain the influence of low crude protein diets and the branched-chain amino acids in broiler production. The first phase, consisting of ten experiments, focused on the practicality of reduced crude protein diets, the effectiveness of their implementation, and the limiting order of amino acids in male and female broilers. The results of these studies indicate that dietary crude protein can be effectively reduced in both male and female broilers with minimal impacts on broiler performance. Furthermore, the 4th limiting amino acid in corn-soybean meal diets was determined to be valine, whereas valine and arginine were found to be co-limiting in wheat-soybean meal diets. Phase two investigated valine and isoleucine requirements and the influence of leucine on broiler performance and carcass traits. Five experiments were conducted to determine the valine requirement for male and female Cobb 500 broilers. Valine requirements were found to be 78 and 77 for male and female broilers, respectively, based on feed conversion responses during a 15 to 35 d grower period. A single experiment was conducted to determine the isoleucine requirement in female Cobb 500 broilers but a requirement could not be identified. No major influence of leucine was observed on male or female performance or carcass traits. Phase three evaluated the classic antagonisms associated with the branched-chain amino acids. Two experiments were conducted using Box-Behnken design to model the antagonism. Significant interactions were determined between isoleucine and valine that could not be replicated when leucine was removed from the model. Based on these findings, a large three factor factorial study was conducted to determine the influence of varying isoleucine and leucine levels. Significant interactions between valine and leucine and isoleucine were found. These interactions were further evaluated in two smaller two factor factorial experiments in which no responses were observed on live performance or carcass traits. Overall findings indicate that the classic branched-chain amino acid antagonism is still of practical concern in modern broilers. The impacts of this antagonism on broiler performance and carcass traits indicate that formulation strategies concerning isoleucine, valine, and leucine must take all three into account as opposed to individual adjustments. Despite the fact that leucine has not been shown to directly affect live performance when evaluated individually, levels should be monitored during formulation and should be considered when making adjustments in dietary valine and isoleucine levels