Tallow (A) and rape oil (E) were obtained for evaluation. They were blended in the ratios A95:E5, A90:E10 and A80:E20. The three blends together with the two pure fats were each included at 40, 80 and 120 g/kg into a basal diet. 2. The experimental diets were evaluated for apparent metabolisable energy corrected to zero nitrogen retention (AMEN MJ/kg) and apparent fat availability (g/kg) using 6 replicates of a cage of two male broiler chicks 14 d old and 8 replicates of a cage of one Rhode Island cross cockerel approximately 1 year old. Diets were fed for 72 h then removed for 24 h. This was followed by a 48 h period when food was available ad libitum and a further 24 h of starvation. A total collection of excreta was undertaken each 24 h for the latter 72 h period. 3. Evaluation of AMEN and apparent fat availability for the individual fat blends was achieved through both linear and quadratic regression. 4. Synergism, as demonstrated by a higher determined than calculated AMEN value, was detected with fat blends fed to chicks but not cockerels and only at rates of inclusion beyond 80 g/kg. 5. No consistent differences between AMEN determined through calorimetry or AME determined with reference to apparent fat availability multiplied by fat gross energy were detected.
