There are a lot of different scenarios and circumstances that will affect what you should feed and how much. What feeds & supplements are available & affordable? What kind of body condition are the cows in currently? What's expected of these females besides producing a calf, milking, and rebreeding? For example, first-calf-heifers are still growing and probably need more or better supplementation compared to mature cows in the same environment and under the same management.
Our mature cows get decent quality hay throughout the winter and whatever pasture might be available. The only other supplementation they ever get is free choice complete mineral and sometimes a liquid protein supplement. We do separate our first calvers and give them 2-3 pounds per head per day of a 15% coarse feed. That seems to help them keep growing and breed back on time, and occasionally we'll add to that group any old or thin cows, a cow that has twin calves, or a three-year-old second calver if she fits.