Hay is basically just dried pasture plants, so as long as the cows don't get more than a tiny amount of grain, it should be the next best thing to milk from cows eating fresh pasture. Part of the problem with cows fed large amounts of grain, soy, cottonseed meal, and all the other bizarre 'concentrate' feeds they give to cows, is that they are destructive to the normal functioning of the cows' digestive system. Too much grain causes their digestive tract to get too acidic and changes the microbial population (proper microbes are the lynchpin of bovine digestion), which favors the growth of organisms that can be dangerous to people, such as acid-resistant e.coli (i.e. 0157:H7). Hay made from grasses or alfalfa is digested the same way as fresh pasture, basically. Do they grow the alfalfa themselves? If not, do they know how it's grown? In some areas, alfalfa can be a heavily sprayed crop, depending on the pests, soil fertility, etc., so you might want to ask about that. I presume you've checked out the cleanliness of the milking procedures.