My cows know which calf is theirs if a cow needs culled she gets culled.
Both of them?
Easy to improve your herd without tagging or keeping records. A week or two before weaning throw your herd into a small trap. Pair out anything that doesn't fit your program, along with any dry cows. Send those to the sale barn. No buying tags, no getting rolled tagging calves, no spending time going over data, then trying to find the cows you need to cull. Everything taken care of in in a pleasant day or two playing with your cows.