"Methane production used to be viewed as a problem at cattle ranches and dairy farms, but methane itself is a highly energetic biogas that can be used as fuel. Harnessing it might prove difficult considering that current social graves do not favor open flatulence even for the sake of renewable energy. However, certain diets richer in alfalfa and flaxseed have been proven to reduce methane production in cows, which could potentially solve that problem (13)."
Also this inspired me to read about how herbivorous mammals digest cellulose and it seems like you probably would want a horse poop transplant, not a cow.
Ruminants have some extra stomachs for hosting the bacteria for digesting cellulose, while for horses and rabbits and such it just happens in the intestine, but is less efficient.