| Summary: | Cold-pressed sunflower cake (CPSC), by-product of oil-manufacturing, has high crude fat and linoleic acid concentrations, being a promising supplement to modulate rumen fatty acid (FA) profile. This trial studied CPSC effects on ruminal fermentation, biohydrogenation and the bacterial community in dairy cows. Ten cows were used in a crossover design with two experimental diets and fed during two 63-day periods. The cows were group fed forage ad libitum and the concentrate individually. The concentrates, control and CPSC, were isoenergetic, isoproteic and isofat. The ruminal samples collected at the end of each experimental period were analyzed for short-chain fatty acid, FA and DNA sequencing. CPSC decreased butyrate molar proportion (4%, <i>p</i> = 0.005). CPSC decreased C16:0 (28%, <i>p</i> < 0.001) and increased C18:0 (14%, <i>p</i> < 0.001) and total monounsaturated FA, especially C18:1 trans-11 (13%, <i>p</i> = 0.023). The total purine derivative excretion tended to be greater (5%, <i>p</i> = 0.05) with CPSC, resulting in a 6% greater daily microbial N flow. CPSC did not affect the diversity indices but increased the relative abundances of <i>Treponema</i> and <i>Coprococcus</i>, and decreased <i>Enterococcus</i>, <i>Ruminococcus</i> and <i>Succinivibrio</i>. In conclusion, the changes in ruminal fermentation and the FA profile were not associated with changes in microbial diversity or abundance of dominant populations, however, they might be associated with less abundant genera.
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