Methane has a half life of 8 years, and is produced from carbon dioxide and water, specifically it is produced into carbohydrates by plants which are then broken down into methane by certain bacteria in animal digestive systems. It degrades back into carbon dioxide and water through oxidization very quickly in the atmosphere. It's effect on global warming is miniscule compared to carbon dioxide, by measure of the volume of each produced and their persistence in the atmosphere. Methane is a non issue, and is easily made up for by the fact that cows, and the humans that eat them, are carbon sinks also. Imagine if you stopped cattle production and destroyed all those cattle to stop them from creating methane, how much carbon dioxide do you think they'd create as they biodegrade? This would have a significant impact on warming, way way more than the methane does. The existence of cattle (and any and all biomass in general since they're all carbon sinks) is a net positive for warming, by far.