Yes. And no. We tried them years ago and they didn't seem to help, plus half of them broke off. It was an expensive experiment. Some producers swear by them but it's just a part of management; you still have to be diligent about control by spraying, rubs, powder, etc. Keep in mind, nothing will work very well if you have a neighboring pasture that has essentially zero fly control - they just come over to your place/your cows and it's double the effort. Additionally, while they protect the face (for a while), they don't offer full protection.
I've posted this pic before of my neighbors cow with "cancer" (that was his explanation when I texted him the pic). Notice the fly tags? And the flies?
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