ENERGY AND NITROGEN INTAKE, EXPENDITURE AND RETENTION AT 32-DEGREES IN GROWING FOWL GIVEN DIETS WITH A WIDE-RANGE OF ENERGY AND PROTEIN CONTENTS

Heat production (HP) and the intake and retention of energy and nitrogen were measured in growing broiler fowl kept at 32-degrees and given diets with metabolizable energy contents from 8 to 15 MJ/kg and crude protein (N x 6.25; CP) contents of 130 and 210 g/kg. The temperature of 32-degrees was chosen for comparison with earlier measurements at 20-degrees to minimize heat produced for the maintenance of body temperature. The effects of diet composition were observed when the same birds were taken from 20 to 32-degrees. The tendency for energy intake to increase with dietary energy concentration was less at 32 than at 20-degrees. The lower heat increments measured for the high-fat diets did not, therefore, confer an increased ability to sustain higher energy intake at 32-degrees. HP was about 17% lower at 32 than at 20-degrees; the change in HP between 20 and 32-degrees was not significantly influenced by diet composition. The absence of significant effects of diet composition on HP, combined with the significant trend in energy intake, produced significant differences (related both to dietary energy and dietary protein concentrations) in total energy retention and in the partition of retained energy between protein and fat. As at 20-degrees, variation in energy retention and in the composition of retained energy were the main responses to variation in dietary CP concentration and energy intake; a significantly higher energy cost of unit protein accretion on the low-CP diets was insufficient to produce an elevation in total HP because the higher unit energy cost was balanced by a lower absolute rate of protein accretion.