Are you talking about cow farts? When cows eat grass, the carbon they emit was already in the system. It's just like any other organism staying alive. This is different than burning fossil fuels, the carbon of which until they are extracted hasn't been in the system for millions of years.

To the extent that the beef industry burns fossil fuels, that's something you could criticize, but it's hardly unique to the beef industry. Cattle would eat less intensively-cultivated corn and more grass if the corn farmers weren't subsidized so much. If no corn was cultivated at all, cattle would still be a pretty efficient way of sustainably extracting food from marginal grassland.