Taste was my first thought as well, but rather than fishy, I would think if anything it might almost act as a tenderizer, adding natural sodium and umami to the

Taste was my first thought as well, but rather than fishy, I would think if anything it might almost act as a tenderizer, adding natural sodium and umami to the meat. That does assume that the cows are eating it in a quantity where it makes any difference, but if yellow flowers can change the color of the milk, I don't know why seaweeds wouldn't do something to the taste.