Cows eat grass, hay, and sometimes grain. Don't feed your cows apples, thinking that because horses eat apples, cows do too. Apples cause bloat in cows, which causes the stomach to swell and makes it hard for the cows to breath. Cows can die from bloat. If a cow or calf starts foaming at the mouth and nostrils, it can be from bloat. Call the vet immediately.
I've fed my cows apples once in awhile over the last couple of years, foolishly. Last night our calf Daisy started foaming at the mouth. I called the vet - and by the time he got here, she was breathing in a less shallow, quick way. Poor little Daisy did fine - after several large burbs and some anti-inflammatories and Pepto from the vet she seems back to normal today. But what a scare. Daisy is my little calf - the one whose mother is at Someday Farm. She is like an orphan - since Giddy pushes her away and only is pleasant to her own calf Blue.
The vet was quite clear - don't feed cows leftover kitchen goodies just because chickens and hogs eat them. Cows don't eat cake like our chickens do. Just feed them grass, hay, and sometimes grain. And make sure if you feed them a little grain, it isn't old grain. Grain should not be more than a month old or mold can grow in it. Yikes.