We have about 30 cows and our criteria for retaining heifers is to keep ones that were born in the first 30 days of our caving season (April-May) and they must weigh over 575 lbs at weaning (6 months). Prefer to keep heifers out of your best cows. Keep good records of when they calve, problem births, and Average Daily Gain for each calf. You will soon be able to rank your herd and identify your best cows. Keeping the earlier born heifers will naturally select for fertility. Numbers are important, but select animals that are appealing to you as well, whether it's body shape, personality, or color.