You say your heifers are in good BCS. Hopefully you understand what good condition really is and that you are not "barn blind" (loving your own cattle and not seeing their faults).
IF they are in good condition AND you keep them in good condition after calving, you should expect them to start cycling 30-45 days after calving (without pulling calves for 24 hours). IMO the 2nd cycle is the most fertile cyle, but I WILL breed on the first cycle if I'm needing to "up" her due date, otherwise, most of my heifers/cows are bred on the 2nd & 3rd heat to keep the same calving dates each year.
IF your heifers are not in good condition and you do not want to feed them/ can't feed them, then selling their offspring is an OPTION. Not one I would recommend, but it is an option if they are poorly and not cycling, and you do not have the resources to feed them properly.
Actually, in my herd, 2nd calf heifers are the hardest to get rebred. My 1st calf heifers have been raised properly (yes they get grain when weaned thru first winter) and are in great BCS (around 6.5 - 7) at calving. They generally cycle in 30-35 days & rebreed with no problem. And this is done on grass and hay after the point of being bred as virgin heifers.
My 2nd calf heifers, raised a calf and are usually the thinnest at weaning time, and if they don't gain on proper nutrition (hay) thru the winter they will be the thinnest at calving time, making them the hardest to rebreed.
For that reason, I keep my 1st & 2nd calf heifers seperated from the main cows, so that I can feed them the best hay.