Another thing that I think makes a lot of difference in calving heifers is how much exercise they are getting before calving. If the are stuck in a small lot for extended periods they seem to have far more calving problems than if they are active and walking a lot until right before calving. And personally I would rather see that first calf be tiny and born easy than big and hard on momma. The heifers have their whole life ahead of them and I prefer to give them every advantage to breed back even if it means smaller, scrubbier calves the first time around. Just my dumb guy opinion.