I think they ARE effective, as long as they are in contact with the skin and not just hung on a headcollar or rug. Its the constant contact with the skin that allows the chemicals in the tag to be fully absorbed.
I seem to remember there was someone on here at one time who'd bought a whole load of these cattle tags and was "inviting" people to buy one of her/him for a fiver. That was a few years ago now I think. But the problem is that you have to buy a whole box of the things, and a lot of places are very chary of letting you and me have them if they're for horses and not cattle.
I think, tho' cannot say for sure, that Alec Swan on here can offer enlightenment on their correct use?? Alec, are you there hunni??
I tried the Naff Off Citronella tags last year, they seemed to work for a while, and they smell nice. I plaited them into the mane and tail.
But not sure about the cattle ear tags.