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Quantitative information on the concurrent changes in major rumen microbial functions induced by dietary lipids in relation with the degree of unsaturation of fatty acids is scarce. During a three-period essay (9 days per period), rapeseed (Brassica napus L., RO), sunflower (Helianthus annuus L., SO) and linseed (Linum usitatissimum L., LO) oils at the high level of 80 g/kg dry matter input (DMI) and a control without oil were compared in four 1-L dual outflow fermenters inoculated with bovine rumen microbiota and maintained on a maize silage-based diet. Neither the fermentation medium pH and redox potential nor starch and protein degradabilities were significantly altered. Oil supply significantly decreased butyrate and methane specific productions of to the benefit of propionate and hydrogen specific productions and decreased the microbial protein outflow and synthesis efficiency while increasing the outflows of ammonia and isovalerate, these effects being amplified by the fatty acids’ unsaturation degree. Besides, with no impact of the degree of unsaturation, oil supply increased the amount of fermented hexose, the productions of all SCFAs except butyrate and the specific production of acetate. The rumen variables apparently sensitive to oil unsaturation degree were thus related either to metabolic hydrogen removal pathways or to protein metabolism.