Effects of amount and source of fat on the rates of lipolysis and biohydrogenation of fatty acids in ruminal contents.

Because the percentage loss of unsaturated fatty acids across the rumen has varied considerably in previous in vivo studies, we conducted five experiments to identify potential factors that might affect the in vitro rates of lipid lipolysis and biohydrogenation in ruminal contents. The factors examined included the amount of fat added to the substrate, the source of added fat, the diet fed to the donor fistulated cow, and the time of collection of inoculum from the donor cow. Lipolysis and biohydrogenation were expressed as the rates of disappearance of neutral lipid and unsaturated fatty acids, respectively, from the culture contents over time using a first-order model. The rate of lipolysis of soybean oil declined from 44%/h to less than 30%/h as the percentage of soybean oil in the culture substrate increased from 2 to 10%. The overall rate of biohydrogenation of C18:2 was 14.3%/h, but declined 1.2%/h for each percentage unit increase in C18:2 added to the substrate. Compared with C18:2, the rates of biohydrogenation of C18:1 were generally lower (averaged 3.6 %/h) for all fat sources. The rate of biohydrogenation of C18:2 in soybean oil was not affected by the amount of grain or fat fed to the donor cow, or the time after feeding that ruminal inoculum was collected. Based on these findings, high linoleic acid concentrations in the diet would possibly reduce biohydrogenation and increase the postruminal flow of this unsaturated fatty acid. Also, lipolysis may vary considerably due to amount and source of lipid added to the diet, but this has little influence on the initial disappearance rates of linoleic or oleic acids from ruminal contents.