With as small a group as you have, the logistics of keeping replacements heifers separate as a group, forgetting hay feeding, dictate selling all heifers and buying in replacement bred cows to fit your preferred calving season. Herds with large numbers of heifers who are run in a separate stocker enterprise, and bred for a very short period, 21-30 days, may be able to take advantage of the genetic improvements of well selected maternal bulls. When you consider needing a calving ease bull for a handful of heifers, supplementing them through winter better than even dry cows, providing better spring nutrition than calving cows, getting them bred in a tight window, carrying them on a better plane of nutrition than a cow for the next year, watching like a hawk as the calve, assisting when necessary, correcting those that won't take their calf and then supplementing them better than cows to get them bred back, the hardest thing to do in the cattle business, retained heifers are not worth it for small producers. Buy good quality bred cows, sell the heifers. Buy growth bulls. Be a terminal herd. You will be money ahead.