Source?? Not some looney bin left-wing web site either.
I have NEVER seen the mills add anything when grinding feed. And dairy cows are treated like GOLD.
Are you a farmer??
For example, bovine somatotropin ( growth hormone ) is given to dairy cattle to increase milk production. This hormone occurs naturally but is also given in a synthetic version as well. With the increased amount of BTS there is an increased risk of mastitis in the cattle. With this infection comes the treatments of antibiotics. Hmmm, could that be passed thru the milk?? Sure can and is.
Cattle are also given synthetic estrogens and progesterones to speed up maturity. Could this be passed on as well?? Sure can and is.
You don't have to be a leftist to conclude that when animals raised for human consumption are given these products that inevitably these same products are passed on to us through the meats and dairy products.
As for you never seeing these things added to the feed, you won't. It is injected.