In vitro examination of DHA-edible micro-algae: 2. Effect on rumen methane production and apparent degradability of hay

In 24 h in vitro incubations with 25 ml of buffered rumen fluid, four doses (0, 20.8, 41.6 and 83.3 mg) of a DHA-edible algae product, Aquagrow-DHA, were examined in combination with sunflower (20 mg) or linseed oil (20 mg). One treatment contained no Aquagrow-DHA, linseed or sunflower oil and one was supplemented with Aquagrow-DHA (166.6 mg) only. Addition of lard ensured that total fat content was constant among incubations. Increasing amounts of Aquagrow-DHA progressively inhibited CH4 to a maximum of 80% (P<0.001). Increased CH4 inhibition was accompanied by decreased acetate (P<0.001) and butyrate (P<0.001), and increased propionate (P<0.001) proportions, but also by depression of the short chain fatty acid (SCFA) production by 30% at the highest Aquagrow-DHA level (P<0.001). However, SCFA inhibition only occurred when the Aquagrow-DHA dose exceeded 41.6 mg and CH4 production was reduced by about 30% (P<0.05) at this dose. Inhibitory effects were linked to the amount of unesterified polyunsaturated fatty acids, particularly docosahexaenoic acid. Incubations under partial H2 atmosphere suggested that a direct toxic effect to rumen methanogens was caused by higher amounts of unesterified DHA (>41.6 mg Aquagrow-DHA/incubation) only, whereas suppression of rumen methanogenesis at lower doses seemed to be indirect through a lack of substrate. This indicates bacteria providing H2 to the rumen methanogens to be more sensitive to unesterified DHA than rumen methanogens.