Local outfit around here buys latex paint that is mixed wrong/ugly color, etc. They paint the back end of their heifers & cows with it. Go down the tail head and both hips following the lay of the hair. It's an inexpensive marker and works good. They use a roller to put it on. The person doing the painting gets messy! It's a big enough patch to easily identify what cow has been ridden whether the paint is worn off or the hair is messed up.