CG yes we have tried the routine of only worming the ones that seem to need it. We do not sell the cow and calf once they are better. But we do watch the family line. We do not routinely worm any more since we feel that the cows as a rule do develop some resistance. We regularly feed DE in the mineral and it is in all the feed that we feed, which is minimal and it seems to take care of most of the problems. Because we do buy and sell we are constantly introducing various genetic deficiencies into our pastures.
Culling just for lack of worm resistance is not practical. It is an added "leaf" in the whole "bale of hay".
We have practiced this very strongly in our sheep. We have 3 "lines" and they each have their strengths. One group is more worm resistant, but don't have the best rams (for horns), and are half nuts to work with. One group produces the best rams heads (horns) but they have the worst feet there are, more foot rot than believeable and very soft hooves. The third group have decent feet tend to be more suseptible for worms and are a bit smaller. We have been crossing the lines, keeping very detailed records, and are now culling the ewes more for feet problems and wormyness. It is a long process and we will never be where they are all "great" but we are working on it. THAT SAID, we are not making our living from them. They are a "hobby". Add to that, we are in an area where the barber pole worm is a big problem and these dall sheep were originally more of a dry land type sheep. Like the Bighorns. One group is from Texas one from NC and I don't remember if we know where the third group (our original ones) was from, although they were a mixed group put together to sell.
So in the meantime, how do you think that all these farmers are going to make a living from their investment if they are supposed to only go buy animals that are worm resistant? And what of the different bloodlines and genetic diversity that will be lost by only going to an animal that meets only these qualifications?
Maybe some only use chemical wormers, but I don't think that anyone just does it for kicks. Chemical wormers cost money, and time to administer.
Until you have the money and time invested, and experience in the cattle business, I think that you are entitled to your opinion, but I don't think you are entitled to tell people who "walk the walk and talk the talk" that they should do such and such. You are not in a position to say, "this is how I have done it and it works for me" ; and I don't feel that you should be saying, this is what you should do....