Comparison between pigs and poultry in the prediction of the dietary energy value of fats

Fats and oils of different degrees of saturation and free fatty acid (FFA) content were blended in fixed proportions to give mixtures varying regularly in the ratio of unsaturated to saturated fatty acids (U/S) and FFA content. Fats were from animal and plant sources and the increase in free fatty acid content was achieved through chemical hydrolysis. Mixtures were evaluated for apparent metabolisable energy (AME—poultry) and digestible energy (DE—pigs) through determination of apparent fat digestibility as assessed through excreta and fecal analysis and fat gross energy (coefficient of apparent fat digestibility×gross energy=AME or DE in poultry and pigs respectively) in a series of metabolism trials using two ages for both species (1.5 and 7.5 weeks of age; 15 and 30–85 kg live weight respectively for the two species with groups being referred to as young and old). The data generated were employed to derive comparative equations where the DE and AME (in MJ/kg in all cases) were predicted from U/S and FFA. Solving equations gave values of 28.4 and 35.2 (young poultry); 33.0 and 36.7 (old poultry) at FFA levels of 50 g/kg (with U/S of 1.0 and 3.5 respectively) and 24.4 and 31.0 (young poultry) and 30.3 and 34.0 (young poultry) at FFA levels of 500 g/kg. Corresponding data for pigs were 32.7 and 36.3 (young), 33.7 and 36.4 (old), 30.4 and 34.0 (young), 31.7 and 34.3 (old). In addition the data revealed that synergism, defined as the phenomenon whereby the dietary energy value of a saturated fat may be improved in the presence of a more unsaturated oil, was not in general detected. Accordingly equations could be based upon U/S and FFA operating additively within one function. Responses obtained indicated that DE values for pigs of both ages were similar and close to those obtained for adult poultry. However there was a significant (P<0.05) difference between these three groups and young poultry. The equations are not applicable to fats and oils which have been heat-damaged during processing.