Sample collection and analysis
Whole blood samples were taken in a random subset of heifers by coccygeal venipuncture and collected into Vacutainer (Becton, Dickinson and Company, Franklin Lakes, NJ, USA) tubes just before CIDR removal and stored at
4 1C. The blood was centrifuged for 15 minutes at 2500 Â g,
and serum was collected and stored frozen at À20 1C until
being assayed for the concentration of progesterone.