Yes they will have alot of milk. What I do is get a second calf and put on them so they don't get mastitis and lose any quarters, and then supplement them so that both calves do good. The second calf will more than pay for the extra feed even in this market. I'll pull the second calf at 4 or 5 months,and then let her own calf stay on to finish out to later weaning and stop supplementing.Takes a little effort to get the calf grafted on but then they are good to go. If you are supplementing them, and they have good grass, they should breed back. If you only have one calf on them, and don't supplement them, they will milk all the fat off their back and as dun said, might not breed back , plus they will have too much milk for the calf in the beginning and might get mastitis.