Evaluation of the effect of progesterone CIDR Devices on circulating levels of progesterone in cyclic ewes

A homogeneous group of thirty-one crossbred ewes was used to determine the effect of administering a progesterone Controlled Intravaginal Drug Releasing Device (CIDR) on circulating levels of progesterone in the subsequent cycle following CIDR removal. Circulating progesterone levels were determined for each ewe through daily blood collection via jugular venipuncture. Each ewe underwent a pretreatment 25 day sampling period (Period 1), a 12 day treatment period characterized by the presence of the CIDR (Period 2), and another 25 day sampling period following CIDR removal (Period 3). During the initial period of the study, progesterone levels in peripheral circulation changed (P < 0.0001, effect of day) in accordance with stage of the estrous cycle and were elevated during the luteal phase. In the second period of the study, progesterone levels were elevated (P < 0.0001) in ewes due to exogenous progesterone from the CIDR device (Period 1 versus Period 2: 1.3 ± 0.1 ng/ml versus 2.4 ± 0.1 ng/ml, respectively). After withdrawal of the CIDR in the third period of the study, circulating progesterone levels were not (P > 0.10) different from those observed in the initial period of the study (Period 1 versus Period 3: 1.3 ± 0.1 ng/ml versus 1.4 ± 0.1 ng/ml, respectively). Data collected in this study revealed that treatment with exogenous progesterone via CIDR for a 12- day treatment period does not influence circulating levels of progesterone in subsequent estrous cycles