I do mean animal production, and I am assuming a closed system where people find dirt, plant crops, feed those crops to cows. Whatever cows produce has to come from somewhere, so any building materials or carbon emissions are going to come from their feed. In order to produce this carbon in the food, the plants have to absorb and take out of the atmosphere an equal amount of carbon. There are cases where emissions can come from how food is grown with tractors, the production of fertilizer, etc. Those I do not consider directly under "emissions from cows" because they are third party things that often involve the use of fossil fuels, and can change with time to become more efficient or not release greenhouse gases. As for methane being 20 to fifty times the potential of carbon dioxide: methane degrades into carbon dioxide with a half-life of twenty years, not enough time for it to have a significant impact on the atmosphere without us already releasing massive amounts of it at once, which we do not.