Threonine requirements of broiler chickens: an experimental validation of a model using growth responses and carcase analysis

1. This paper describes the experimental testing of a model derived from an analysis of published data on the threonine requirements of broiler chickens. The model, published in a separate paper, showed that the age of the bird and dietary crude protein were highly significant determinants of threonine requirements. 2. We tested this model by measuring the threonine requirements of male broilers aged 7 to 21 d and 21 to 42 d fed on wheat-peanut meal diets containing graded concentrations of threonine. The growth of those birds given adequate threonine was compared with that of others fed a typical wheat-soyabean diet to measure the relative value of peanut meal as a protein concentrate. Finally, we measured the dry matter, nitrogen and fat concentrations in the carcase using near infrared reflectance spectroscopy (NIRS) to test the hypothesis that birds given diets deficient in an amino acid produce carcases with more fat. 3. Increasing the threonine concentration of the diet from 5.7 to 7.2 g/kg improved the growth and feed conversion ratio (FCR) of broilers. However, a further increase in dietary threonine to 7.7 g/kg had an adverse effect. The determined threonine requirement agreed with our prediction model. 4. From 7 to 21 d birds given the diet containing 7.2 g/kg threonine ate more, weighed more, had an improved FCR and, in absolute terms, had carcases with more dry matter, fat and protein than did birds given the wheat-soyabean diet. However, after correction for treatment differences in body mass there were no differences in the carcase parameters. Likewise, there were no differences in any measurements between birds given these two diets between 21 and 42d.