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 experim ents to identify potential factors that might affect the in vitro rates of lipid lipolysis and biohydrogenation in ruminal contents. The factors exami ned 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 o f inoculum from the donor cow. Lipolysis and biohydrogenation were expresse d as the rates of disappearance of neutral lipid and unsaturated fatty acid s, respectively, from the culture contents over time using a first-order mo del. 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 f rom 2 to 10%. The overall rate of biohydrogenation of C-18:2 was 14.3%/h, b ut 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 C-18:1 we re generally lower (averaged 3.6 %/h) for all fat sources. The rate of bioh ydrogenation of C-18:2 in soybean oil was not affected by the amount of gra in or fat fed to the donor cow, or the time after feeding that ruminal inoc ulum was collected. Based on these findings, high linoleic acid concentrati ons in the diet would possibly reduce biohydrogenation and increase the pos t-ruminal flow of this unsaturated fatty acid. Also, lipolysis may vary con siderably 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 a cids from ruminal contents.