Answer:
It's not really important whether you feed a cow right before it gets butchered. When it gets butchered, the stomach contents are going to be cut out anyway, and the meat kept to be eaten.
But, if you mean to fatten up a cow before butchering, you may wish to feed it a little bit of grain (around 10 to 20 lbs per day) along with hay and/or grass for a few weeks before sending her to the butcher.