Effect of dietary fish oil supplements alone or in combination with sunflower and linseed oil on ruminal lipid metabolism and bacterial populations in lactating cows

Background: Fish oil (FO) alters ruminal biohydrogenation (rBH), causing trans fatty acid (FA) intermediates to accumulate. Plant polyunsaturared FAs (PUFA) also affect long-chain FA metabolism and microbial communities. The effects of the combination of dietary FO and plant PUFAs are not well established. Objective: The aim of present study was to evaluate the effects of FO alone or in combination with plant oils (PO) enriched in 18:2n-6 or 18:3n-3 on rumen microbial ecology and flow of FAs at the omasum. Methods: Four lactating cows were used in a 4 × 4 Latin square with 21-d experimental periods. Treatments comprised a basal grass silage-based diet containing no additional oil (control) or supplements of FO (200 g/d) or FO (200 g/d) plus 500 g/d of sunflower oil (SFO) or linseed oil (LFO). Flow of FAs was determined using the omasal sampling technique. Relative abundance of key bacteria was assessed by qPCR on 16S rRNA genes in omasal digesta. Results: FO supplemented treatments increased the amounts of 18:1trans, 18:2trans, and 20- and 22-carbon unsaturated FAs escaping the rumen. Relative to the control, oil supplements had no effect on the amount of 18:0 leaving the rumen, but LFO decreased the flow of 18:0 at the omasum compared with SFO. Both SFO and LFO increased 18:1trans relative to FO while LFO resulted in the highest 18:2trans flow. Supplements of FO plus POs shifted biohydrogenation towards 18:1trans-10 formation. Alterations in the amount of intermediates at the omasum or rBH pathways were not accompanied by major changes in analyzed bacterial populations. Conclusion: Dietary supplements of FO alone or in combination with POs increase the amount of biohydrogenation intermediates containing one or more trans double bonds escaping the rumen, with major implications for host metabolism and the nutritional quality of ruminant foods.