To each his Own. Every creature in Nature instinctively knows what is the proper food for it to eat.
Cows eat grass.
Certain birds eat seeds and insects, others like hawks and eagles eat meat. Some creatures are extremely selective, others eat a wider variety. Whatever their food line, they know what it is and stick with it. They eat what nature designed their digestive system for over the millions of years of their development. Should they do otherwise, they would soon be in dire trouble.