As someone who researches this topic, I want to correct some errors. First, it is primarily eructations, burps, that cause the majority of methane emissions from cows, not farts. Second, the response by berickf is wrong. The fact that methane is being released instead of carbon dioxide by plant decomposition is an issue and does contribute to increased greenhouse gas (GHG) levels and global warming. Methane (CH4) is a stronger GHG than carbon dioxide, so when a plant absorbs CO2 from the atmosphere for its growth and then this carbon is ultimately returned to the atmosphere in the form of methane via cow burps we are essentially switching out CO2 for CH4, and given CH4 is stronger than CO2 it is a net negative affect. Fossil fuels are not the only GHG emissions of concern, peatlands release a lot of CH4 as well. This was stored carbon that is now released as CH4, a more powerful gas than if CO2 was released.
As far as limiting the emissions of CH4 from cows, there is a lot of research on the topic and depending on what you feed cows you can reduce the amount of CH4 they produce. BUT, you must examine all areas on the farm. Creating more GHG elsewhere to reduce CH4 from the cows burps does not help the situation.