Effect of pH on ruminal fermentation and biohydrogenation of diets rich in omega-3 or omega-6 fatty acids in continuous culture of ruminal fluid

Eight dual-flow continuous culture fermenters (1320 ml) were used in two replicated periods of 7 d (4 d of adaptation and 3 d for sampling) to study effects of pH (high = 6.4 versus low = 5.6) and fatty acid type (omega-3 versus omega-6) on ruminal microbial fermentation, lipolysis and apparent biohydrogenation, flow of fatty acid (FA) and DNA concentration of bacteria involved in lipolysis and biohydrogenation processes. Diets had a 40–60 forage to concentrate ratio and differed in the fatty acid profile which was rich in omega-3 (O3) or omega-6 (O6). Mixed samples from solid and liquid effluents were collected for determination of total N, ammonia N and volatile fatty acid (VFA) concentrations, and the remainder of the sample was lyophilized. These samples were analyzed for dry matter, ash, neutral and acid detergent fiber, FA and purine contents. Low pH reduced fiber digestibility, total VFA concentration, ammonia N concentration and flow, bacterial crude protein (CP) flow and dietary CP degradation, and increased non-ammonia and dietary N flow. Low pH reduced organic matter digestion in the O3 diet. Linoleic (LA) and linolenic acid (LNA) apparent biohydrogenation were reduced by the main effect of pH (LA = 0.82 versus 0.44; LNA = 0.84 versus 0.54). The low pH also reduced lipolysis of LA, although the effect was larger in O6 than the O3 diet. The low pH only reduced lipolysis of LNA in O6 (0.98 versus 0.90). Lowering the pH from 6.4 to 5.6 reduced Anaerovibrio lipolytica (67.0 versus 1.85 pg/10 ng total DNA) and Butyrivibrio vaccenic acid producer subgroup (1149 versus 316 pg/10 ng total DNA) DNA concentrations and increased Butyrivibrio stearic acid producer subgroup (1216 versus 1907 pg/10 ng total DNA). The O3 diet increased ammonia N concentration and flow, and tended (P=0.06) to increase CP degradation. Results indicate that pH inhibited apparent biohydrogenation of LA and LNA, and reduced lipolysis of LA in O6 and O3 diets, but only that of LNA in O6. At low pH, fat from the O6 diet seemed more resistant to lipolysis and biohydrogenation than that of the O3 diet.