I have a methane powered horse. It's a turbo!
So you've been supplementing the hay with some beans, hmm?
Actually, cows do produce "gas" as well as milk. What we would need would be a multiple purpose udder -- milk, low-fat milk, regular gas, and high test.
Cows would become a lot more popular.
Did you know that they've bred goats that produce spider silk in their goat milk? My weird ideas are not quite as strange as some may think.