Effect of dietary fat and calcium source on insoluble soap formation in the rumen

Diets with and without added fat and with different amounts of soluble and insoluble calcium were fed to rumencannulated cows to determine roles of fat and calcium in formation of insoluble calcium soaps in the rumen. Percent of total fatty acids in soap was increased only by addition of soluble calcium (CaCl2). The amount of fatty acid in soap was highly dependent on amount of fat in the rumen and was not changed by calcium addition. Concentration of ionized calcium in rumen content was halved by fat addition and was inversely correlated with rumen pH, whereas proportion of fatty acid in soap was directly related to pH. Rumen pH was lower 8 to 12 h postfeeding when calcium was added to the diet, which was related nonsignificantly with total volatile fatty acid concentration. Addition of fat to diets tended to lower the ratio of acetic to propionic acids in the rumen; this trend was reversed by calcium addition. Beneficial effects of added calcium in high fat diets were not caused by increased calcium soap formation in the rumen.