Fish oils as potent rumen methane inhibitors and associated effects on rumen fermentation in vitro and in vivo

The effect of two fish oil (FO) types, FOa (n-3-eicosapentanoic acid (EPA), 18.1%; n-3-docosahexanoic acid (DHA), 11.9%) and FOb (EPA, 5.4%; DHA, 7.5%) and quantity (0, 12.5, 25, 50, 75, 100 and 125 mg FO) on rumen fermentation patterns was evaluated with 24 and 48 h batch in vitro incubations and compared with fermentation shifts induced by soybean oil (SO). The 48 h incubation was essentially a batch culture as it was re-inoculated with rumen fluid, buffer and hay after 24 h. Shifts in rumen fermentation pattern were only observed during the second step of the 48 h incubation for the three oils. The inhibition of rumen methane (CH4) production was influenced by both oil (FOa>SO>FOb) and concentration, although amounts higher than 75 or 25 mg for FOa and FOb, respectively, did not decrease further CH4 production. A maximal CH4 inhibition of 80% was observed. CH4 inhibition seems proportional to the relative amount of polyunsaturated fatty acids (PUFA) and their rate of lipolysis. Lower CH4 production was accompanied by increased propionate and reduced acetate production. SO supplementation was associated with lower net VFA productions (−17%), whereas none of the amounts of FO reduced VFA production. In vivo effects of rumen FOa injection (18 ml, twice daily at 1 h after feeding) on rumen fermentation and NDF digestibility were studied in a cross-over trial with four rumen canulated wethers, offered a hay/concentrate (65/35, w/w) diet, at their maintenance energy requirements. In vivo FOa injection did not alter faecal NDF digestibility (48.5±4.5% versus 46.5±8.2% for control and FOa, respectively, P>0.05), despite a lower 48 h rumen in sacco degradability (57.5±3.3% versus 51.6±5.8%, P<0.05). FOa-induced higher rumen propionate concentrations (211.8±18.0 versus 262.7±20.8 mmol/mol total VFA, P<0.05), suggesting a depression of rumen methanogenesis, confirmed during simultaneous in vitro incubations (344.2±43.4 versus 287.7±36.6 mmol/mol total VFA, P<0.05). Reduced biohydrogenation of FO PUFA, resulting in rumen accumulation of unesterified EPA and DHA might be nutritionally and ecologically relevant in terms of an increased post-ruminal supply of these PUFA for incorporation into animal products and through reduced rumen methanogenesis, the latter representing both an energy loss for the animal and an important greenhouse gas.