Abstract

Replacing synthetic feed additives by plant secondary metabolites (PSM) as essential oils, saponins and tannins has been proposed, due to their potential to reduce methane (CH4) emissions, without adverse effects on ruminal fermentation. This study aimed to evaluate the use of oregano essential oil (OR), quillaja saponin (QS), and quebracho tannin (QT) extracts and their combinations as feed additives on in vitro CH4 production and rumen fermentation parameters. The design was an incomplete factorial arrangement in a randomized complete block with seven treatments using batch culture. Dietary treatments were: control (CON), without plant extracts; OR (0.07% dry matter [DM] basis of the diet); QS (0.05% DM); QT (1.00% DM); and binary combinations QT+QS (1.00 and 0.05% DM, respectively); OR+QS (0.07 and 0.05% DM, respectively); and OR+QT (0.07 and 1.00% DM, respectively). The forage to concentrate ratio was 51:49. Forage was composed of corn silage (42.0%) and perennial ryegrass and white clover hay (9.0%); concentrate was based on high–moisture corn (33.0%), soybean meal (15.0%), vitamin and mineral salt (1%). All combinations decreased the net CH4 and its production (P ≤ 0.038 and P ≤ 0.027, respectively). However, the interaction between QT and QS decreased CH4 yield (P = 0.046), whereas OR and QS interaction, trended to decreased CH4 yield (P = 0.068) and the in vitro DM disappearance (IVDMD; P = 0.055). In contrast, the interaction between OR and QT decreased the IVDMD (P = 0.036). The gas output, partitioning factor and pH, were unaffected (P ≥ 0.066) by PSM, separately or in combinations. The results suggest that QT+QS is the best PSM combination to reduce the amount of CH4 per g DM degraded without adversely impacting rumen fermentation and diet digestibility. Although OR+QT or OR+QS are also an alternative to reduce CH4, its combination may also reduce diet digestibility.