Comparative responses of genetically lean and fat broiler chickens to dietary threonine concentration

1. Genetically lean (LL) or fat (FL) male broiler chicken's were fed on 5 diets containing either 3.80, 4.27, 4.75, 5.22 or 5.70 g true digestible threonine per kg. 2. Threonine deficiency induced a more pronounced reduction in growth in the LL than in the FL but did not influence abdominal fat and breast muscle proportions in either line. 3. Plotting weight gain or protein gain against threonine intake suggests that the requirement of both lines is very similar in terms of mg per g of gain. Thus food intake or appetite should account for differences between genotypes. 4. Requirement for true digestible threonine was estimated as 10.70 mg per g of weight gain or 63.8 mg per g of protein gain, using a linear regression approach. The quadratic polynomial equations suggest that the requirements are 13.9 and 12.4 mg digestible threonine per g of gain for LL and FL respectively.