Recent advances in ruminal lipid metabolism have focused primarily on manipulation of physicochemical events in the rumen aimed at two practical outcomes: 1) control of antimicrobial effects of fatty acids so that additional fat can be fed to ruminants without disruption of ruminal fermentation and digestion and 2) regulation of microbial biohydrogenation to alter the absorption of selected fatty acids that might enhance performance or reduce saturation of meat and milk. Properties of lipids that determine their antimicrobial effects in the rumen include type of functional group, degree of unsaturation, formation of carboxylate salts, and physical association of lipids with surfaces of feed particles and microbes. The mechanism of how lipids interfere with ruminal fermentation is a complex model involving partitioning of lipid into the microbial cell membrane, potency of the lipid to disrupt membrane and cellular function, physical attachment of microbial cells to plant surfaces, and expression and activity of microbial hydrolytic enzymes. Lipolytic and hydrogenation rates vary with forage quality (stage of maturity and N content), surface area of feed particles in the rumen, and structural modifications of the lipid molecule that inhibit attack by bacterial isomerases.
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