A UK study of milk from conventionally-raised vs. pastured cows has discovered that the milk from pastured cows contains more antioxidants and vitamins than that from conventional dairy farms:
An al fresco diet in cows results in milk with up to 60 per cent higher levels of conjugated linoleic acid (CLA9) which has been linked to a reduced risk of cancer, according to research from Newcastle University. The same study found 39 per cent more omega-3 fatty acid and 33 per cent more vitamin E, which are also thought to reduce the risk of cancer and cardiovascular disease.
Grazing provided 84% of food for cows on organic farms during the summer. In winter, when pastured cows in the study weren't able to graze, the differences between their milk and that from conventionally-raised cows was smaller.