It is now possible to virtually eliminate cow methane emissions by manipulating the type of water they drink and the feed they have access to.
Changing the environment of the Rumen, changes what can live there.
Research suggests that elevating the dissolved concentration of molecular hydrogen to 100µM (0.2 ppm) or more, will thermodynamically inhibit methanogenesis.
This suggests that an easy delivery mechanism would be for the animal to drink hydrogen (and oxygen) saturated water.