Use full size (Ex-Large) eartags at birth. Red for heifers & yellow for males. Put birth date (2-15) on top (actually at the base of the "neck" of the tag), Put sire's name under that at top of the main part of the tag. Than put calf's permanent tattoo ID - Cow # + year letter.
As your herd grows with cow families, this system gets complicated. Cow family 29 can have 29E, 29G, 29M etc. When they calve, cow 29E's calf is E29R, cow 29G is G29R. You have to get creative with this system as years progress, but being able to look at an animal and ID the cow family is very important to us. We actually have a 29 cow family and it is soooo large now, I have had to drop the 2 and make some of the offspring a 9 number because I limit it to 4 digits (for a tattoo), so if I have E29M and G29M, I made their calves EM9R and GM9R. (actually if one of the calves was a heifer, I would have made it M29R since she might be staying in the herd.) Confused??? you should see my husband :shock:
But back to your original question, I put birthdate, sire & ID on color coded tags.