The reason why some say that is because that a reproductive tract and the immune system starts long before the calf is born. On some heifers they may not give as much milk so some think that a heifer out of a heifer will not be as fertile or stay in the herd as long. A heifer is trying to grow, feed a calf, and breed back so a lot going on that COULD take away from the calf. I am not saying it does just that it could.