Dietary fiber concentration modulates the efficacy of phytogenic and antimicrobial rumen modifiers on ruminal fermentation and methane emissions in cattle.

This study evaluated the effects of dietary fiber level and rumen modulators on feeding behavior, methane emissions, and ruminal fermentation parameters in cattle. A single 4 × 4 Latin square design with a 2 × 2 factorial arrangement of treatments was used, involving four ruminally cannulated cattle evaluated over four experimental periods. The treatments combined two dietary neutral detergent fiber (NDF) concentrations (12% and 50% of dry matter) with two feed additive strategies: monensin plus virginiamycin (VM) or a phytogenic additive (LM+). Diets containing 12% NDF represented forage-free conditions, whereas diets containing 50% NDF represented conventional fiber-based feeding systems. Dry matter intake, rumination time, ruminal pH, and liquid passage rate were greater in animals fed 50% NDF diets (P < 0.05). Methane emissions were substantially higher in 50% NDF diets, averaging 153.1 versus 46.9 g/day, and 12.8 versus 8.8 g/kg DMI for 50% and 12% NDF diets, respectively. Within the 50% NDF diets, LM+ reduced methane emissions by 26% compared with VM (130.0 vs. 176.3 g/day; P < 0.05). Within the 12% NDF diets, LM+ increased total short-chain fatty acid concentrations by 34% compared with VM (95.0 vs. 70.9 mM) and increased butyrate concentration (20.2 vs. 11.9 mM), whereas VM reduced ruminal ammonia nitrogen concentration (12.4 vs. 16.9 mM; P < 0.05). Lactate concentrations were markedly greater in 12% than in 50% NDF diets (10.7 vs. 1.3 mM; P < 0.05), regardless of additive. Dietary NDF concentration was the major factor influencing ruminal fermentation, feeding behavior, and methane emissions, although several responses depended on diet × additive interactions. These results indicate that the efficacy of phytogenic and conventional antimicrobial rumen modulators is strongly dependent on dietary fiber concentration and the prevailing ruminal fermentation environment.