Effect of Magnesium Oxide on In Vitro Fermentation Parameters and Methane Emission from Low and High-Grain Diets in Ruminants

CH4 emissions from cattle are a major contributor of greenhouse gases in the world. Finding additives to mitigate CH4 production is essential to achieve sustainable livestock operations. MgO may reduce carbon dioxide (CO2) production under anaerobic conditions and, theoretically, can decrease CH4 synthesis. To evaluate the effects of MgO on CH4 production and dry matter digestibility (DMD) using an in vitro ruminant model, two experiments were conducted. The first experiment evaluated the combined effect of replacing sodium bicarbonate (NaHCO3) with MgO and of CO2 flushing (as a C source) on CH4 production and fermentation parameters. The second experiment evaluated the addition of three MgO sources (A, B, and C at 50 mg/g of DM) to one of three substrates (100% corn, 50:50 corn/forage, or 100% forage) in three independent incubations. In both experiments, CH4 concentration and production, gas production, pH, and DMD were individually determined in each bottle per incubation. Data for Experiment 1 were analyzed under a completely randomized design with a 2 × 2 factorial arrangement; for Experiment 2, data were analyzed using the MIXED procedure from SAS following a randomized complete block design with a 3 × 4 factorial arrangement. In Experiment 1, replacing NaHCO3 with MgO reduced gas production and CH4 concentration while increasing pH. In contrast, CO2 flushing increased CH4 concentration and decreased pH. In Experiment 2, increasing forage inclusion reduced CH4 production and gas production, which is potentially associated with the reduced DMD in forage. Supplementation of MgO at 50 mg/g of DM reduced gas production and CH4 yield without significantly affecting DMD. Source A reduced CH4 production per gram of incubated dry matter by 44.6% relative to the non-supplemented control and reduced CH4 yield per gram of DMD. Overall, MgO modified fermentation responses and reduced in vitro CH4 production without significantly affecting DMD. Source A seemed to mediate an aggressive CH4 yield reduction in forage-based substrate. Despite these promising results, further studies measuring fermentation products and evaluating animal responses are needed to clarify the mechanisms and practical applications of MgO addition in cattle diets.