In the world of dreams the rules of reality do not necessarily apply. Each dream is unique and it's a direct link to the soul and spirit. Psychology provides its own theories on what dreams mean, but so did tradition and the study of dreams for thousands of years before any relevant science had developed.
Dream Meanings - Cows
If you dream of many fat cows then you will have economic gain, but if they are skinny then keep an eye on your finances. If you slaughter a cow you will overcome some difficulties.