Nutrition affects fat-free body composition in broiler chickens.

The independence of fat-free body composition from nutrition is assumed in most models that simulate animal growth. This assumption has not been investigated extensively. We studied the allometric relationships between water and ash with protein in growing broiler chickens and tested whether the amounts of water or ash at a given protein weight was affected by nutritional factors. Two experiments, each with a 2 x 9 factorial design, were conducted using male broiler chickens of two body weight ranges [200-800 g (expt. 1) and 800-1600 g (expt. 2)]. The treatment factors were two levels of feed intake and nine dietary ideal protein to protein-free energy ratios (PE-ratio). Protein was balanced for amino acid content. The allometric relationships of water and ash with protein were different for carcass and organs. The relationship between water and protein was not affected by nutrition, except for a 7% reduction in water weight at a fixed protein weight in the carcass in expt. 1 at the lowest compared with the highest PE-ratio (P < 0.001). The relationship between ash and protein was strongly affected by nutrition. The lowest PE-ratio increased ash weight at a fixed protein weight in the carcass by up to 28%, compared with the highest PE-ratio (P < 0.001). We conclude that, at least for modern meat-type animals, nutrition can significantly affect fat-free body composition at a certain fat-free body weight. The nutritional effects on fat-free body composition could be incorporated into models of the chemical body composition of growing animals.