Assessment of Threonine-to-Lysine Ratio of Male Broilers From 35 to 49 Days of Age

Digestible lysine (dLys) and digestible threonine (dThr) are considered to be the second and third limiting amino acids (AA) after sulfur containing amino acids (dTSAA) for broilers fed corn-soybean meal-based diets. Diets formulated to meet the AA requirements of modern broiler strains are required to maximize performance. Currently, nutritionists formulate poultry diets using the ideal protein concept. Lysine is employed as the reference AA, with all other essential AA formulated into the diet as a ratio to lysine. Formulating diets using this strategy minimizes diet costs and nitrogen excretion by the bird [1]. Thus, it is critical to determine the correct ratios of each essential AA to lysine to maximize growth and performance. Previous research has reported that feeding inadequate concentrations of dThr can limit performance as well as affect the efficiency of dTSAA and dLys utilization by the bird [2]. Initial work on the ideal ratio requirement for dThr conducted by Baker et al. [3] found that bodyweight was maximized with a dThr to dLys (dThr:dLys) ratio of 56 for chicks from 8 to 21 days of age. Kidd et al. [4] recommended a dThr:dLys ratio of 68 for optimum bodyweight gain and breast meat accretion in broilers from 21 to 42 days of age. All previously mentioned studies evaluated the dLys and dThr needs independently. However, Corzo et al. [5] concluded in a study where dLys and dThr estimates were determined using the same flock of birds, feed ingredients, and environmental conditions, that a dThr:dLys ratio of 69 was optimum for maximizing growth and efficiency in Ross × Ross TP16 female broilers from 14 to 28 days of age. Few reports have addressed the optimum dThr:dLys ratio during ages close to slaughter, particularly when evaluating carcass and breast meat yield optimization [5-7], and no studies, between 35 and 49 days of age, have estimated this ratio while conducting concurrent and simultaneous studies.