Impact of a progesterone-releasing intravaginal device on plasma progesterone levels in lactating dairy cattle

Abstract

Plasma progesterone (P4) levels were assessed by radioimmunoassay in lactating dairy cows treated with a progesterone-releasing intravaginal device (PRID). Group A cows (N=6) received no treatment. PRIDs were administered to Group B1 (N=3) on day 25 after the first observed estrus (Day 0) and to Group B2 (N=3) on day 31, and were withdrawn 14 days after administration. Blood was collected daily from day 0 until 56 hours after withdrawal of the PRID (Group B), or on alternate days until a second estrus was observed (Group A). The area under the P4 curve (AUC) did not differ for Groups A and B pre-insertion of the PRID, but was significantly greater for Group B post-insertion. The AUC in Group B cows was significantly greater post-insertion than pre-insertion, but AUC did not differ in Group A cows for these study days. Maximum P4 was significantly greater in Group B1 post-insertion than pre-insertion. Results of this study are consistent with clinical studies demonstrating the efficacy of the PRID in estrus synchronization and treatment of infertility, and support the hypothesis that these effects are due, at least in part, to modulation of circulating progesterone levels.