What makes you think that the extra limbs were caused by mutations? I know very little about bovine biology, but if we were dealing with a human, I would assume that an extra leg was likely caused by absorption of a sibling in utero. I have never heard of a mutation in mammals causing extra limb development. (Even weirder is the idea of a mutation causing an extra single leg, as opposed to an extra leg pair.) The vertebrate body plan simply does not seem to work that way.
Pure speculation! However, this was a wide-spread occurrence not just one or two cows hinting at some systematic setup. I also don't remember the details as it was many years ago and I was quite young - it's possible that there was a pair of legs.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.