Modeling changes in ruminal fermentation caused by changes in ruminal lipid concentrations

A method was developed which separated ruminal lipid into neutral lipids, FFA, and polar lipids using solid phase extraction techniques and selective elution to separate the individual lipid classes. Lipid extraction of culture solids was in excess of 95% of total lipid in cultures. Lipid classes were separated from extracts using modifications of an existing procedure which uses aminopropyl bonded phase columns. Single lipid classes were verified by thin layer chromatography with comparison to known standards. Quantification of fatty acids in individual fractions was by gas liquid chromatography and resulted in excess of 95% recovery of fatty acids from a standard lipid mixture.;In vitro cultures were conducted with varying fat sources and levels. Lipids in these cultures were assayed for neutral lipids and FFA using the separation procedure developed. Lipid concentrations were then regressed against depression of fermentation to generate mathematical equations for prediction of fermentation depression. Neutral lipid fatty acids and saturated FFA were not related to depression of fermentation. Unsaturated FFA were related to depression of fermentation in a nonlinear fashion, which was best described by a Michaelis-Menton relationship. This relationship gave a maximum depression of 53.1% and a concentration of unsaturated FFA at 0.5 maximal depression of 3.21 mg unsaturated FFA per g of ruminal DM.;Ruminally fistulated cows were used to test the prediction equation with in vivo data. Cows were fed linear increases in soybean oil. Ruminal lipids were assayed for neutral lipids and FFA. Depression of fermentation was also estimated from volatile fatty acid concentrations and using in situ techniques. Soybean oil had no effect on apparent digestibility of DM, OM, N, or NDF but decreased apparent digestibility of fatty acids. Increasing soybean oil decreased VFA concentrations and decreased acetate to propionate ratio. Ruminal concentrations of total fatty acids and total saturated fatty acids increased nonlinearly as soybean oil was increased in diets. Total unsaturated fatty acid concentrations increased linearly as soybean oil was increased. Neutral lipid concentrations in the rumen did not respond to increased soybean oil. Ruminal FFA concentrations increased as soybean oil increased.