The objective of this study was to examine the effect of oleic (OA), linoleic (LA) and α-linolenic (ALA) acid used as supplements (3.5% wt/wt) to a diet containing 80% lucerne and 20% barley on rumen fermentation and lipid metabolism in an artificial rumen (Rusitec). The experiment lasted 12 days with 6 days of stabilization period. The fatty acid (FA) supplementation to a mixed diet did not affect any parameters of rumen fermentation (degradation of DM, NDF, ADF, total VFA production, production of acetate, propionate and butyrate). The methane production was decreased numerically (NS) by FA supplements (OA, LA, and ALA by about 8, 8.3 and 13.2%, respectively). The stoichiometric parameters of rumen fermentation such as NM (nitrogen incorporated by microflora), OMF (organic matter fermented) and EMS (efficiency of microbial protein synthesis) were affected by unsaturated C18 FA to a different extent. EMS calculated from NM and OMF was significantly (<i>P</i> < 0.01) increased by OA, ALA and decreased (<i>P</i> < 0.01) by LA. The lipid metabolism was also affected by C18 FA supplements. The concentration of total FA and proportion of LCFA (long chain fatty acids, > C<sub>18:0</sub>) increased and proportion of MCFA (medium chain fatty acids, C<sub>14:0</sub> − C<sub>17:0</sub>) decreased after OA, LA and ALA addition. The biohydrogenation (BH) of fatty acids was characterized by increased (NS) accumulation of stearic acid and trans isomers C18:1 and lower SFA/UFA ratio in the effluent. The concentrations of two main BH intermediates, TVA (trans 11 C<sub>18:1</sub>) and CLA (cis 9, trans 11 C<sub>18:2</sub>), significantly (<i>P</i> < 0.001) increased (TVA − 1.06, 1.29 and 1.10; CLA − 0.32, 0.43 and 0.36 mg/g rumen fluid DM, respectively) after OA, LA and ALA supplementation compared to the control (TVA − 0.7; CLA − 0.23).
Effect of unsaturated C18 fatty acids (oleic, linoleic and α-linolenic acids) on ruminal fermentation and production of fatty acids isomers in artificial rumen