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I heard in a recent shiur that Dayan Weiss zatzal paskened that an animal that delivered its young via caesarean section is considered a treife. Therefore its milk cannot be drunk. When supervising milking the shomer checks that any such animals are locked away before milking.
How do those those who permit cholov akum put up with this possibility? Is it through bitul?
I'm not sure that all Cholov Yisrael dairy producers do this; there have been a number of questions in the past about whether many dairy cows are treifa.
In a theoretical sense, we know that some dairy cows -- let's say more than 1.6% but less than 50% -- are treifos. If I have one cow on my farm, Bessie, then rov says assume Bessie is kosher, so Bessie's milk is 100% kosher. Okay I walk into a modern dairy setup with a thousand cows' milk all mixed together; if 4% of those cows are treifa, does that mean my milk is 4% treifa and 96% kosher? If so it's not kosher! Rabbi J. David Bleich writes about this in Tradition Spring 2008. The answer seems to be based on a ruba de-leisa kaman. If I don't know that any particular cow on my farm is treifa, even if I know that the general population in theory has a 4% treifa rate, then I assume each of my cows is 100% kosher, and thus the milk is 100% kosher.
So your question would only be if a mashgiach walks onto a dairy farm and knows specifically that there are one or more treifa cows here. Which I doubt is usually the case in industrial-scale chalav stam production.
From the Chashash Terefa point of view, the unsupervised milk is better. Because nobody would try to check if a cow is Terefa. Even if they decide to eat it, the non Jews would not verify if the animal is terefa. So we have both Chazaka and Rov.
The first is Chazaka, Milk cows are greater than one year, so they wasn't terefot when they born. The chazaka is a Chazaka demeYkara. This point is a Machloket Rishonim in Gemara Chulin 11a. See Tosfot paragraph Atia. Tosfot rules that this Chazaka is not valid because the non-terefut was not clear at time. We know that the cow wasn't terefa before 12 month because a terefa cannot live 12 months.
Even if the Chazaka per se is not valid, we have a Rov. Most animals are healthy. Moreover, Rav Shimshon bar Shimshon says there is a Chazaka that come from the strength of Rov (he think that Rov is a Berur equivalent). That is, even if a cow becomes Terefa and you have cheese from it, Rashba (there) says that from the strength of the Chazaka we say that it became terefa after the confection of the cheese.
Anyways, in non supervised dairy, there is no case of discovery of Terefa. Even more, if it was the case, milks of all cows are mixed and Zil batar Ruba, the mixture of Chalav Terefa with a majority of Chalav not Terefa is cancelled.
