I have had pretty good luck moving my herd calving dates up from calving in Feb,-March to starting now in mid Dec. Seemed like I could move up about 3 weeks per year. insert CIDR at about 50 days for 7 days and Lute when removing CIDR. you should be able to insert CIDR at 40 days and still have pretty good luck. I used all AI and bred on visual standing heat. I do not have much experience with bull with beef cows but dairy farmers that used bulls mentioned that if more than two or three cows would come into heat at the same time, the bull may only breed one or two and not the others (maybe not as pretty.) If that is the case, you may want to stagger the heats into a few days apart. Like other post, it may be easier to just replace late calfers with ones that fit the rest of your herd. Good luck. In my case, I would have had to replace the whole herd and it would have been less challenging.