SPECIAL STORY 56 FEED & ADDITIVE MAGAZINE September 2023 Figure 1. Concentration (%) of digestible lysine (Lys), leucine (Leu), valine (Val), and isoleucine (Ile) in corn, corn DDSG, soybean meal, poultry by-product meal, and corn gluten meal. IMPACTS OF LEUCINE ON POULTRY PERFORMANCE Swine nutritionists in the US were the first to consider the impacts of leucine on animal performance as many finishing swine diets contain numerous synthetic amino acids to reduce crude protein and high levels of DDGS which is a high leucine ingredient. Researchers at Kansas State University conducted a thorough literature search and meta-analysis of published BCAA data and developed a predictive model which suggested that adjustments to valine and isoleucine ratios should be made based on the diets leucine level (Cemin et al., 2019). Currently, US broiler diets are higher protein, contain less synthetic amino acids (typically 3 or 4), and lower levels of by-product meals such as DDGS as compared to swine finisher diets. Therefore, it was unclear if the level of leucine in the broiler diets was sufficient to elicit a negative performance response. Maynard et al. (2021) conducted an experiment utilizing a Box-Behnken response surface design investigating increasing levels of dig leucine in a broiler grower diet between 15 and 35 days of age. They reported linear increases in feed conversion ratio with incremental increases in dig leucine from 110% of dig Lysine, to 130% and 150% of dig Lysine (Figure 3). As observed in the figure, the surface curves suggest a plane separation of 4 points in feed conversion ratio between the 110% and 150% leucine diets confirming that in fact, excess dig leucine does result in performance loss of market broilers. Typical US broiler diets will vary in dig leucine ratio usually between 135% and 165% depending on dietary phase and ingredient profile. Digestible leucine will increase in diets as the bird ages and is usually higher in all vegetable diets as compared to diets containing animal protein. Regardless, Maynard et al. (2021) confirms that an increase from 110% to 130% dLeu: dLys will negatively influence Figure 2. BCAA enzymatic pathway
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