Effect of Added Fat and Calcium on in Vitro Formation of Insoluble Fatty Acid Soaps and Cell Wall Digestibility

In vitro experiments were conducted to determine the extent to which fatty acids react with divalent cations to form insoluble soaps in rumen contents. The addition of 10% tallow to a hay and corn (70/30) basal substrate increased (P<.01) the insoluble fatty acid soap content, expressed as milligrams/flask, at all incubation times (6, 12, 24 and 48 h), but yet markedly depressed cell wall digestibility because of an equally large increase (P<.01) in nonesterified fatty acids. Supplying additional Ca to the high fat diet as 2% dicalcium phosphate increased the insoluble fatty acid soap content and cell wall digestibility only slightly. Calcium from calcium chloride reacted more quickly with nonesterified fatty acids to form insoluble soaps, and cell wall digestibility was substantially improved as early as 6 h of incubation. Differences in fatty acid composition between the nonesterified and insoluble fatty acid soap fractions suggested that both increasing saturation and increasing chain length increase the extent to which a fatty acid forms an insoluble soap.