Growth performance of male broilers fed diets varying in digestible threonine to lysine ratio from 1 to 14 days of age1

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Threonine (Thr) is the third limiting amino acid for broilers fed corn-soybean meal–based diets. Digestible Thr to lysine (Lys) ratios utilized by commercial nutritionists vary from 0.63 to 0.68 in the starter period. Low Thr to Lys ratios used during the early growth period can limit performance. This study examined growth responses of broilers provided experimental diets varying in digestible Thr from 1 to 14 d of age. Dietary treatments were formulated at 8 digestible Thr to Lys ratios (0.55, 0.58, 0.61, 0.64, 0.67, 0.70, 0.73, and 0.76). Two diets (dilution and summit) consisting of corn and soybean meal were formulated to be adequate in all other amino acids with the exception of Thr and Lys. All diets were formulated to contain 1.13% digestible Lys, which is slightly below the digestible Lys requirement for male broilers from 1 to 14 d of age. Digestible Thr to Lys ratios were estimated using broken-line methodology. Linear (P = 0.003) and quadratic (P = 0.005) increases in body weight gain (BWG) were observed for broilers fed progressive additions of digestible Thr. Additionally, linear (P < 0.001) and quadratic (P = 0.018) improvements in feed conversion were observed as digestible Thr consumption increased. Based on linear broken-line methodology, optimal digestible Thr to Lys ratios for Ross × Ross 708 male broilers for BWG and feed conversion were 0.66 and 0.69, respectively. These data support a minimum digestible Thr to Lys ratio of 0.67 for male broilers from 1 to 14 d of age when averaged for BWG and feed conversion.

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