Effects of a selection of additives on in vitro ruminal methanogenesis and in situ and in vivo NDF digestibility

The effects of 18 essential oils, yeast, Q uebracho tannin and Q uillaja saponin on ruminal methane and gas production ( GP ) were studied in vitro . A lactating cow diet was incubated with rumen inoculum . Doses of the additives (mg/L) were: essential oils = 500; tannin = 67; yeast = 8.35 and 16.7; and saponin = 300. Lemongrass, estragole, eugenol, geraniol, limonen, thyme oil and thymol produced less gas (overall mean 33.8 mL/200 mg dry matter ( DM )) than control (43.6 mL/200 mg DM ; P  < 0.001). Methane produced (mL/200 mg DM ) by guaiacol (10.7), lemongrass (9.6), limonene (11.4), thyme oil (10.9) and thymol (2.1) was lower than control (12.5) ( P  < 0.001). Methane percentage on total GP was lower ( P  < 0.001) for cinnamic alcohol (25.4), guaiacol (24.5), thymol (19.7) and vanillin (26.3) than control (28.8). In a second experiment, thymol, guaiacol and yeast were added to the diet of dry fistulated cows to determine in situ neutral detergent fiber digestibility ( NDFD ) of six forages and in vivo dietary NDFD . Thymol and yeast decreased in situ   NDFD after 24 h (33.9% and 33.5% vs. 38.1%; P  = 0.008). Thymol decreased in vivo total tract NDFD (40.8% vs. 51.4%; P  = 0.02). Differences in GP and methane levels were registered within classes of additives. A careful selection of additives may allow for the manipulation of ruminal fermentation.