The picture in my profile is an example of what we do. Number and a letter for the year on a yellow tag. We keep records on everything being a registered purebred herd so if we don't already know by memory all we have to do is look in the calving book if we want to know the cow and sire of a calf. We only run about 20-25 cows so we try to use a number range where we have the least chance of duplicating a number that we already have and for the most part it usually works. Another thing we do is tattoo the same in both ears right after birth when we put the tag in. As the calf grows so does the size of the tattoo in the ear. Have got several compliments over the year when someone had to check tattoo either the vet, at a show, or weigh in for a consignment sale that our tattoos are so easy to find and read.
The ear tattoo actually came in very handy many years ago when a pack of dogs chased a cow and calf through a pasture fence and the dogs chased the calf into the creek and killed it. Well the neighbor's pasture they chased it into was questioning if the calf was one of his and not ours (funny how he thought we could mistake his calf with ours because he doesn't have any Herefords) and all we had to was take the mutilated carcass and look in the ear and there was our tattoo to prove whose it was. I'm sure he was hoping it was his so he could go after some compensation for insurance or from the neighbor who owned the dogs. Needless to say between us and some of the other neighbors a lot of those dogs were shot on sight that day as they came back out from the deed they just did or whenever they found their way onto property after that. The Sherriff had already been out a time or 2 before this and warned them that animal control would take the dogs if they could not confine them on their property, they were pretty upset with one we shot as they said he was an expensive dog they purchased and our response was "well I guess that makes us even then for the calf your dogs killed huh?"