Effect of Tannins and Monensin on Rumen Fermentation and Feed Energy Partitioning of Nellore Cows

This study aimed to evaluate the effect of Acacia mearnsii tannins and monensin combination on rumen fermentation parameters and feed energy partitioning of Nellore cows. In a 2 × 4 factorial arrangement, 8 cannulated Nellore cows were distributed in 2 contemporary 4 × 4 latin square design and received 8 diets that differed in the level of tannin inclusion (0.00, 0.75, 1.50 and 2.25% of feed dry matter (DM)) and pres-ence of monensin. Monensin was daily administered to each cow in one square (about 32 mg/kg DM). Ac-cordingly, the experiment was conducted in 4 periods of 28 days each. Therefore, daily dry matter intake (DMI), gross energy intake (GEI), rumen solid mass, neutral detergent fibre (NDF) degradability, rumen pH, rumen fermentation products and rumen protozoa were measured and the feed energy partitioning was evaluated. The only interaction (antagonistic) observed between monensin and tannins was on the minimum rumen pH (P=0.0173). Monensin reduced acetate:propionate production ratio in 28.80% (P=0.0007). Tan-nins linearly reduced DMI, GEI and NDF degradability, but linearly increased rumen solid mass (P<0.05). Tannins had a quadratic effect on the time (min/day) the rumen pH was below 5.8 and 6.0, as well as the pH area (h.pH/day) below 5.8, 6.0 and 6.2. Tannins had neither effect on rumen ammonia nor on protozoa count (P>0.05). Tannins linearly reduced CH4, acetate, butyrate and total short chain fatty acids (SCFA) production as well as the gross energy release in form of CH4, but linearly reduced the energy release in intestine and linearly increased feed energy loss in faeces. Monensin and tannins had independent effects on rumen fermentation parameters, as well as on feed energy partitioning, therefore, no synergy was observed. The use of A. mearnsii tannins up to 2.25% of feed DM reduced CH4 production (up to 34.7%), but did not improve feed energy efficiency.