Ritchey, have them hot stamped with the number.  Tattoo in the other ear to match the tag.  Every calf is tagged with a number that it is identified with until it dies or leaves the place.  Use full size tags in calves and most of them have the original even 10+ years later.  Have been using Ritchey since 2005.  Prior to that we were using Z-tags, but got tired of the ink fading and retention was a problem.  I think UV light might play a role in how quickly the tags fade.  Higher altitude means more UV light and we just couldn't keep the tags legible.  Finally gave up on them and went to Ritchey.  Won't use anything else now.