Cows have evolved over millions of years to eat almost nothing but grass. Farmers feed cows grains in order for them to gain weight or so they produce more and/or sweeter milk. Grains are hard for cows to digest and lead to chronic digestive problems.
Another reason a farmer might feed their cows grains is because the cows are confined and have no access to fields or grass. Have you ever driven past feed lots with cows crammed together, standing in their own waste, with no grass in sight? Well that could easily be an organic dairy farm.   _{Your Comment}