How Does The Presence of Livestock on Land Increase Net Methane Production?

It’s a hot topic, and I’ve just read this article that debunks the claim I’m about to make – But even though I read it, I still don’t really understand it. I’m happy to change my opinion if I understand the facts.

Suppose we have a closed system, imagine a pasture in an aquarium. Light goes in, grass grows, dies, decays, is recycled, all that good stuff. I imagine the air going into the aquarium is pretty atmospheric, and the air exiting has been changed depending on the amount of CO2 that is absorbed through photosynthesis, the amount of O2 is absorbed through plant respiration, and the amount of greenhouse gasses produced from decaying plant material in the thatch/soil.

(I think I have that right – my biology education doesn’t go past high school level)

Now suppose we add a cow into the mix. If anything, to me, the cow reduces net emissions while it grows because less grass decays, and is instead turned into cow. Once the cow is mature, in this closed system, it’s only eating and "processing" the grass that was already part of this natural cycle of growth/decay.

Here’s where I think I am possibly going wrong:

1. Does the cow increase the amount of "decay"? Does eating the grass make it re-grow faster, thus speeding up that natural cycle?
2. Or, does the cow output different (more harmful) greenhouse gasses than the grass would normally produce through decay?

In short, how does the cow increase emissions? It can only have eaten what has already been grown – And what has grown is doomed to someday decay and be re-released into the atmosphere anyway. I don’t particularly like having contrary opinions, so help me out!