What is a cows prey and diet?
Cows are not predators, hence they do not have prey. As to what cows eat, that question has already been answered in a separate question, which can be seen below in the Related questions section.
Though I cannot give you the balanced diet of a herd of cows , I can give you and define a balanced diet for a herd of cows. A balanced diet for cows is also called a complete ration, and is a ration that meets all the nutritional requirements for a herd of cows based on their weight, physiological and reproductive needs, breeding, reproductive/lactation stages, age, class, and feeds available.
It's all because of an ancient predator-prey relationship that is always at play, even at the domesticated level. Humans are predatory animals, cows are prey animals. Cows are killed for their meat because humans, being natural predators (though not in the same sense as wolves, wild and domestic cats, spiders, and many other predator/carnivorous species are), like to eat meat from cows, which we call beef.
Usually it's from a sudden switch from a high-fibre diet to one that is low in fibre and high in energy and protein, like switching cows from a hay-only diet to a fresh pasture. The grass that cows eat get digested in the rumen, and the by-product that does not get used or is not needed are expelled as feces.