Most all of them are grass fed until they go into the feed lots to be fattened. A cow is a ruminant. The process produces gas. Cows that are put out on a new pasture especially would have gas like you wouldn’t believe. If they were allowed on that new pasture for too long each day until they were used to it they would produce so much gas that they would have to be treated for it. It was called bloat. A tube would be put down into their stomach to release the gas.
Hmmm, there are some strictly grass-fed beef/dairy farmers that rotate their cattle to new pasture every day; guys like Joel Salatin and Dennis Stoltzfoos.
I realize that the conventional farmers raise their cattle on grass until they go to the feed lots. They can get bloat on grain as well.