Monensin reduces enteric methane emissions in late-lactation Holstein cows fed high-concentrate diets

Methane is a major GHG produced by cattle. Strategies to reduce enteric methane include supplementation with monensin, which has shown variable outcomes. Therefore, we hypothesized that monensin (Rumensin, Elanco, Greenfield, IN) would reduce enteric CH₄ yield and intensity in lactating dairy cows (∼207 DIM). Twenty cows were enrolled in a 2 4-wk period crossover completely randomized design with repeated measures including 2 treatments: control (CON, n = 10/period) and monensin (MON, n = 10/period). Monensin treatment consisted of 300 mg of monensin in 34 g/cow of dried distillers grains (DDG) whereas CON was DDG alone, both top-dressed daily on the TMR. Cows were milked 3 times/d and housed in a sand-bedded freestall barn with Calan gates to collect individual DMI. A GreenFeed system was used to measure enteric gas output. Daily milk yield and DMI were measured from each cow during the study. Additionally, fat, protein and lactose percentages were measured weekly in milk. Average (±SEM) DMI (MON = 25.4 ± 0.3, CON = 25.4 ± 0.3, kg/d) and ECM (MON = 35.8 ± 1.09, CON = 36.4 ± 1.09; kg/d) were similar between treatments. Conversely, daily CH₄ was significantly reduced with monensin (MON = 207.1 ± 13.1, CON = 257.2 ± 13.1 g/d). Methane yield showed a significant decline with treatment (MON = 8.1 ± 0.4, CON = 9.9 ± 0.4 g/kg DMI). Last, methane intensity relative to ECM tended to decline with monensin (MON = 5.7 ± 0.3, CON = 6.5 ± 0.3 g/kg of ECM). The results suggest that monensin reduces daily enteric CH₄ emissions and yield in dairy cattle, providing evidence to support its use to reduce CH₄ emissions from cattle.