Cows that are grass-fed are generally considered a more organic option. This is because they rely much less on grain when fed, and grain is considered the norm when raising/mass producing cow products. The reason some people gravitate toward a grass-fed preference is because cows are naturally supposed to eat grass. Their stomachs aren't made for grain which causes health problems in the cows. This, sometimes, can lead to antibiotics or medications being given to cows that are producing milk. In addition, the grain cows are being fed is heavy in corn -an agricultural product that many believe promotes the use of pesticides, etc. Therefore, in theory the cows are eating grain made from corn which could have pesticides on it, then making cows sick (due to pesticides or not), and you are in turn drinking their milk/eating their products.
I'm not sure on the statistics of this, nor am I sure if they are out there! But these are the reasons I know for grass-fed preferences. Here is my source and an article with more information if you'd like to read further. #mce_temp_url#
