I believe there's a new bovine vaccine (perhaps not commercially available yet but well along the pipeline) against the gut bacterial species responsible for cows churning cellulose into methane. It works! I watched a video recently. I had been concerned that this might affect cellulose digestion. That the cows might appear malnourished/smaller, etc. But the same-age adult cows seemed equal side-by-side to the unvaccinated guys (to my amateur eye).
My remaining question (still unanswered) is whether these vaccinated cows have less efficient digestive systems and might require more grazing space, which would be quite counterproductive.