Why does a field of wheat produce more energy than a field of cows?

It is more cost effective to produce fields of wheat than a field of beef cows because cows grow slower than plants. Cows also lose a lot of energy through mating, excretion, and respiration, keeping warm, moving and fighting. They also cost a lot more to feed then to provide plants with nutrients. A lot of money is also spent to keep all the cows healthy. Whereas plants take a small amount space, cows take up a lot of space. Also, not all the cow is eaten e.g. bones.
Cows use significantly more energy to grow, metabolize etc., than wheat. Therefore for every Joule of energy obtained fron the sun, humans can harvest significantly more energy from any plant than we can an animal.