Influence of source and amount of dietary fat on digestibility in lactating cows.

Digestibility of commercial fat supplements was determined in two experiments with high (59% of diet DM) forage diets. Experiment 1 was a preliminary trial in which six Jersey cows were in two 3 x 3 Latin squares to evaluate two formulations of calcium soap at two intake levels (500 and 1000 g/d). The two formulations were compared with control (0 supplement) within squares; the squares differed in amount of soap supplemented. Mean apparent digestibilities of fat were not influenced by source or amount of fat supplemented and averaged 82.5, 84.3, and 83.4% for control, 500, and 1000 g/d. In Experiment 2, six Jersey cows were in a 6 x 6 Latin square to compare effects of various commercial fats on digestibility of diet components at 2.85 and 5.7% (DM) added fat. Higher fat decreased digestibility of P and fatty acid. Fatty acid intake affected fatty acid digestibility quadratically, and variability among cows increased at higher fatty acid intake. Endogenous fecal fat was 55.9 g/d; apparent digestibility of fat was relatively constant at 80 to 82% between 2 and 5% of diet DM, whereas marginal true digestibility decreased linearly (4.4% units/100 g fatty acid consumed). There were no differences among fat sources in fatty acid digestibility. Rumen VFA were not influenced by dietary fat.