Luckily methane, while a potent greenhouse gas, breaks down in the atmosphere quickly. It does break down into CO2 and water, so the question quickly becomes: “are the farts of Midwesterners more potent than the amount of CO2 taken out of the atmosphere by making butter?”
My quick guess is luckily, no, they are not. Some amount of the butter will be stored in fatty tissues which will be sequestered 6 feet underground in a cement box eventually. Most will be shat in liquid or semi-solid form into a toilet to be processed by waste management. As long as they are responsible and compost it into nitrate rich fertilizer we should stay very comfortably ahead of the FBI (Fart to Butter Index).