Grass-fed cows are kept on grass pastures, where they can roam freely and eat grass. On the other hand, grain-fed cows are kept in confined spaces called CAFOs (Concentrated Animal Feeding Operations), which are very nasty places. Typically the cows are fed a diet of GMO (genetically-modified) grains based on soy and corn. They are also injected with hormones (to make them grow faster) and antibiotics (to survive the unsanitary conditions). Grass-fed milk contains significantly more Omega-3 fats, Vitamin E, beta carotene and CLA than grain-fed milk.