There are huge variations that exist among the different fat sources and these variations even exist within a well characterized fat such as soy oil. There can be large CV’s for AME in veg oils, animal fats, AV-blends, acid oils, soap stocks, and blended fats. A correct estimation of metabolizable energy content of a fat or oil is critical for precision feed formulation and staying on target of performance goals for poultry. As demonstrated in these three studies, the more information ascertained about the characteristics and quality of the fat, could help explain differences in AME but not always as in the case of soy oil. Predicting AME and digestibility of fats is challenging especially with acidulated soapstock blends of unknown origin where factors such as FFA levels, and U:S ratios can be variable but still not always explain the digestibility and AME of a fat.
In conclusion, there are many variables that have an impact on digestibility of fats such as the age of the bird and incl level of fat. But, for a good estimate of the nutritive value of fats and fatty acid mixtures, the following are important: the U:S fatty acid ratio and characteristics of the fatty acids such as length of the fatty acid chains, intact fatty acids after saponification and NEM, content of free fatty acids, as well as the oxidative state of the fat source.