infact here is Stevie G's full reply
Chewing the cud has nothing to do with them building up of immunity, but it helps the cow to digest its grass more effectively, so i don't see the connection and they don't spit it out on the ground. I understand the principle of feed back and have done it many times before to gilts, but if there is no disease present in the dung there is no guarantee that it will achieve anything. I've feed afterbirth, died piglets, stillborns back to gilts as a means of building up gilt immunity, but that holds higher risks(so only practice it when necessary), as it can make the situation worse instead of better in some cases, so good judgement(and good advice) is necessary. Its just that feedback is too miss rather than hit.
As to getting a resistance to wormer, just periodically use a dfferent one, then that won't happen. Applies to drugs as well.