Three experiments were conducted to determine the effect of excess dietary protein on threonine requirement of broiler chicks to 14 d of age (Experiments 1 and 2) and to determine the threonine requirement from 16 to 28 d of age (Experiment 3). Two dietary protein levels were used in Experiments 1 and 2:20% CP in a threonine-limiting basal diet containing wheat, peanut meal, and selected amino acids and 25% CP in the same basal diet supplemented with a mixture of amino acids lacking threonine. A threonine-limiting 25% CP diet based on corn, soybean meal, and amino acids was also included in Experiment 2. The threonine requirement of chicks from 16 to 28 d of age was determined using a single CP level (20%) in Experiment 3. Threonine requirements were estimated by broken line regression analysis of weight gain and feed efficiency. Threonine requirements based on weight gain were 7.7 and 6.7 g/kg of diet in Experiments 1 and 2, respectively, for chicks receiving the 20% CP diets. The requirements increased to 8.6 and 8.2 g/kg, respectively, for chicks fed the 25% CP diets based on wheat, peanut meal, and amino acids. The requirement for maximum weight gain of chicks fed 25% CP based on corn, soybean meal, and amino acids was 7.7 g/kg of diet. However, chicks ate more of this diet, and on an intake basis, the requirement of the chicks fed the 25% CP diets based on wheat and peanut meal or corn and soybean meal did not differ. Requirements based on feed efficiency were equal to, or less than, those based on weight gain in Experiments 1 and 2. Body moisture and fat contents were affected by dietary CP level (P < .01), ingredient composition (P < .01), and threonine content (P < .05). Estimates of threonine requirements based on regression analysis of plasma threonine concentrations were higher than those based on weight gain or feed efficiency. The threonine requirements of chicks fed a 20% CP diet from 16 to 28 d of age were 6.3 and 6.9 g/kg of diet based on weight gain and feed efficiency, respectively.
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