I agree, if your cows are losing too much BC, it is always cheaper to feed her calf than to feed her. Cows must get back into BCS of at least 5.5 during last gestation. It affects quality of colostrum, energy to calve, energy for the newborn to survive, and breed back.
I have a 60 day calving, so my calves are 6-8 months of age when I wean so my cows have some great fall grazing to flesh up for winter.
A few calves get weaned early at 3-4 months of age for show cattle. But, I do it slow. They spend the day in the barn without dam, and out at night with dam for a week of two - until they are eating grain well.