Same here. In addition to heifer calves cycling and signs of heat, I have a bull calf that I am retaining who comes from early maturity genetic lines that was in with them until Oct 24th in addition to my registered bull. It is very likely the heifer calves were bred. 

Lute at weaning has worked for me for several years without problems since they were also likely bred much earlier than Oct-Nov weaning but might as well be safe.

Thanks again.

Jim