I don't think they're missing that at all. The cattle in question are eating feed (mainly corn) not grass. Furthermore the grass in question likely wouldn't "

I don't think they're missing that at all. The cattle in question are eating feed (mainly corn) not grass. Furthermore the grass in question likely wouldn't "decompose" like what I believe that's describing. It probably depends on where you're physically located, but in the north - grass grows for the duration of the season, until it eventually dries out in the fall to go dormant over the winter. I guess I haven't personally tracked that process but I would be very surprised if it's releasing methane in the same way that freshly cut grass left to decompose (via bacteria) would.