The effects of interesterification and fatty acid supplementation on the nutritive value of homogenised fats in milk replacers for calves were studied in two digestibility experiments, each of a 6 x 6 latin square design. Using a level of 20% added fat, the mean apparent digestibilities of the total dietary lipids were: butterfat, 96.5 %; interesterified butterfat, 95.5 %; tallow, 88.7 %; interesterified tallow, 93.1 %; tallow supplemented with trimyristin and triolein by interesterification, 92.3 %; tallow supplemented with trimyristin and triolein by simple admixture, 89.9 %. The high digestibility of butterfat is not dependent apparently on any specific non-random distribution of its constituent fatty acids. The digestibility of the tallow was lower and interesterification just failed to give a significant improvement. The inclusion of additional myristic and oleic acids had no significant effect.
Using a level of 15 % added fat, the mean apparent digestibilities of the total dietary lipids were: tallow, 79.7 %; interesterified tallow, 89.2 %; interesterified, reduced stearic acid, tallow 87.9 %; interesterified, reduced stearic acid, added butyric acid, tallow, 96.3 %; interesterified, added butyric acid, tallow 94.7 %; tallow admixed with tributyrin, 84.0 %. The inclusion in tallow of butyric acid by a process involving interesterification resulted in substantial increases in digestibility to values similar to those obtained for butterfat and interesterified butterfat in the first experiment. The improvement was not achieved by simple admixture of tributyrin with tallow. Reduction of the stearic acid content in tallow to a level similar to that in butterfat had no significant effect on fat digestibility.
The lower digestibility of tallow than of butterfat was due mainly to lower digestibility of palmitic and stearic acids and a higher percentage of the latter acid in tallow. Modifications which effected an improvement in the digestibility of tallow generally did so by improving the digestibility of these acids.