In experiment 2, progesterone was administered to cycling, lactating, Holstein cows during the luteal phase of the estrous cycle as in
experiment 1. Luteolysis was induced as in experiment 1. The occurrence of an endogenous LH surge and ovulation were monitored for 7
days. Concentrations of progesterone after luteolysis were 0.13, 0.30, 0.70 and 1.20 ng/mL for cows treated with no CIDR, CIDR-28,
CIDR-14 and CIDR-0, respectively. LH surges and ovulation were detected in 5/5 controls, 3/7 CIDR-28, 0/5 CIDR-14 and 0/5 CIDR-0
cows. It was concluded that low concentrations of progesterone can reduce the ability of either endogenous or exogenous estradiol to
induce a preovulatory surge of LH and ovulation.
2007 Elsevier Ltd. All rights reserved.