| Summary: | ABSTRACT Five experiments were conducted to estimate the digestible threonine requirements of slow-growing broilers: experiment 1, starter phase (days 10 to 28); experiment 2, grower I phase (days 29 to 49); experiment 3, grower II phase (days 50 to 69); experiment 4, finisher phase (days 70 to 84); and experiment 5, which was specifically conducted to determine the production of intestinal mucin over two periods (days 50 to 69 and 70 to 84). Different birds were used in all experiments. A completely randomized design with five treatments and four replicates was used. Treatments consisted of increasing the values of digestible threonine in the diet through basal feed supplementation with L-threonine (98.5%), which was added instead of cornstarch. The following values of digestible threonine were investigated: 0.622, 0.697, 0.772, 0.847, and 0.922% in experiment 1; 0.586, 0.662, 0.738, 0.814, and 0.890% in experiment 2; 0.570, 0.640, 0.71, 0.780, and 0.850% in experiments 3 and 5; and 0.520, 0.595, 0.670, 0.745, and 0.820% in experiments 4 and 5. The digestible threonine values presented quadratic effects on feed conversion ratio in experiments 1, 2, and 3. Digestible threonine values of 0.628 and 0.609% resulted in higher villus height and greater duodenal crypt depth, respectively. Digestible threonine values of 0.762, 0.767, and 0.733% may be recommended for the starter, grower I, and grower II phases, respectively, based on the best feed conversion ratio. In addition, a digestible threonine level of 0.694% may be recommended for the finisher phase based on the highest production of intestinal mucin.
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