Effect of fish oil and sunflower oil on rumen fermentation characteristics and fatty acid composition of digesta in ewes fed a high concentrate diet

Abstract

Studies in ruminants have shown that supplementing the diet with a mixture of fish oil (FO) and sunflower oil (SO) enhances the concentration of cis-9, trans-11 conjugated linoleic acid (CLA), 20:5 n-3, and 22:6 n-3 in milk due to alterations in ruminal biohydrogenation but the intermediates formed under these conditions are not well characterised. Five ewes fitted with rumen cannula and fed a high concentrate diet were used to examine the effect of a mixture (30 g/kg DM) of FO and SO (1:2, wt/wt) on temporal changes in rumen fermentation characteristics and the relative abundance of biohydrogenation intermediates in ruminal digesta collected after 0, 3, and 10 d on diet. Appearance and identification of biohydrogenation intermediates was determined based on complementary gas-liquid chromatography and Ag+-HPLC analysis of fatty acid methyl esters and gas chromatography-mass spectrometry analysis of corresponding 4,4-dimethyloxazoline derivatives. Inclusion of FO and SO in the diet had no effect on rumen pH, VFA concentrations or nutrient digestion, but altered the fatty acid composition of ruminal digesta, changes that were characterised by time-dependent decreases in 18:0 and 18:2 n-6 and the accumulation of trans 16:1, trans 18:1, 10-O- 18:0, and trans 18:2. Lipid supplements enhanced the proportion of 20:5 n-3 and 22:6 n-3 in digesta and resulted in numerical increases in cis-9, trans-11 CLA concentrations, but decreased the relative abundance of trans-10, cis-12 CLA. Furthermore, detailed analysis revealed the appearance of several unique 20:1, 20:2, 22:1, 22:3, and 22:4 products in ruminal digesta that accumulated over time, providing the first indications of 20 and 22 carbon fatty acid intermediates formed during the biohydrogenation of long-chain unsaturated fatty acids in sheep. In conclusion, FO and SO in a high concentrate diet caused a time-dependent inhibition of the complete biohydrogenation of 18 carbon unsaturated fatty acids resulting in the accumulation of trans 16:1, trans 18:1, trans 18:2, 20, and 22 carbon metabolites in ruminal digesta of sheep, with no evidence of a shift in ruminal biohydrogenation pathways towards trans-10 18:1 formation.