Everybody says so. Today's dairy cows are very high milk producers. Sheep or goats, on the other hand, are choosier. Some people claim goats aren't selective at all. Yet nobody calls goats dumb.
Goats are just inquisitive, sometimes to their own detriment.
But dairy goats are also high milk producers though sheep are not. One of the most difficult things in nutrition research has been predicting how much dairy cows will eat. Cattle breeders generally agree that characteristics, like a broad mouth muzzle, a deep heart girth, a broad chest floor, and wide, deep ribs, are indications that a cow has the physical makeup to eat a lot and digest a lot for greater milk production. Cattle breeders select for cows that look like they are always hungry and can eat a large volume of feed daily. They report contents of major and minor nutrients, and many equations have been devised to try to predict what a cow will do with a certain feed.