I had a similar problem when I was producing straw faster than the cows could eat it. They would eventually just stop eating although they were not full with milk. I had to move the cows to a different grass area to "fix" them. My solution for now was to just not allow them straw. I've also noticed if you have a fence around their area but a worked cereal field is in their "range" they will stop eating grass which makes it tricky to find the perfect area for the cows/cereal production areas.