We ear tattoo piglets at 2 to 3 weeks old - old enough to survive, young enough for one person to keep a good grip on. (Mama gets enticed into the stock trailer with a bucket of feed to distract her whilst her offspring utter bloodcurdling screams.) Breeding quality or those going to market as stores get tagged at 9 weeks, a week after weaning so they're used to life without Mum. Weaners moving to a farm get a spray mark, pigs going to the abattoir get slapmarked.