DIFFERENTIAL EFFECT OF AGE ON METABOLIZABLE ENERGY CONTENT OF HIGH PROTEIN-LOW ENERGY AND LOW PROTEIN-HIGH ENERGY DIETS IN YOUNG BROILER CHICKS

1. An experiment was carried out with 6-d old male broiler chicks in a n attempt to understand better the effect of age on dietary AME(n) of high protein-low energy (HPLE) and low protein-high energy (LPHE) diet s which are used in the determination of the AME(n) content of grains when substituted for the entire diet. The experiment was carried out i n a split-plot design in which the effects of 2 diets (HPLE-reference and LPHE-test, containing maize) on food intake, faecal excretion, die tary AME(n) and the utilisation of the nutrients were evaluated in 3 a ge periods (A = 11-13, B = 15-17 and C = 20-22 d of age). 2. Chicks fe d on the HPLE diet consumed significantly less food than those fed on the LPHE diet during periods A and B, but not in C. They also produced a significantly larger amount of droppings during periods B and C. Fo od-to-droppings ratio, which was consistently and significantly lower in chicks fed on the HPLE diet, decreased markedly in period C only in these birds. 3. Absolute and relative retention (RR) of dry matter (D M) and starch, and RR of nitrogen (N) from birds fed on the HPLE diet, were consistently and significantly lower than from those fed on the LPHE diet, but fat retention (absolute and relative) was higher. RR of DM and of N in period C was significantly lower than in periods A and B, while RR of fat and of starch was not affected by age. The effect of age on RR of N was observed only with the HPLE diet. 4. AME(n) of t he HPLE diet, but not of the LPHE diet, in period C was significantly lower than in periods A and B, resulting in a significant interaction between age and diet and a general reduction with age. The calculated AME(n) contents of the maize in periods A and B were essentially the s ame (14.91 and 14.85 MJ/kg, respectively), and lower than in period C (15.28 MJ/kg). It is concluded, therefore, that because of its effect on AME(n) of the HPLE reference diet in chicks older than 17 d, bird a ge is of considerable importance in the determination of AME(n) in gra ins when substituted for the entire diet.