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The Upjohn Company, Kalamazoo, MI 49001
Abstract
Plasma progesterone (P4) profile and estrous detection were used during three experiments to evaluate the effects of exogenous progestogens on the life span of gonadotropin releasing hormone (GnRH)-induced corpora lutea (CL) in postpartum (pp) beef cows. Experiment 1 utilized primiparous fall-calving cows (n = 28, trial 1); and spring-calving cows (n = 29, trial 2). On d 18 to 27 pp (d 0) all cows received intravaginal devices containing either P4 or no P4 (NP) for 5 d. On d 5 the devices were removed and calves were either removed (CR) or were present (CP) with half of the cows within steroid group. At 50 h after device removal, 500 µg of GnRH was given (iv) to all cows, and weaned calves were reunited with their dams. The induced CL had a normal life span (>16 d) in 17 and 86% (trial 1) and 8 and 79% (trial 2) of NP and P4 cows, respectively. Calf removal did not affect (P>.10) the life span of the CL. In Exp. 2, spring-calving multiparous cows (d 18 to 24 pp; d 0) received either no P4 (NP; n = 19), P4 for 6 d via intravaginal devices (P4H; n = 19) or a single im injection of 300 mg P4 (P4IM; n = 18). At 48 h after device removal or at 8 d after the injection of P4, half of the cows within steroid group received either 500 µg GnRH or saline. Corpora lutea had a normal life span in 0, 11, and 80% of NP, P4IM and P4H cows, respectively, that received GnRH and in 22% of P4-saline cows. In Exp. 3, fall-calving multiparous and primiparous cows (d 25 to 31 pp) received either no progestogen (NP; n = 20), P4 via intravaginal devices for 5 d (P4H; n = 21) or melengestrol acetate (MGA; .5 mg·head–1·d–1 for 5 d orally, n = 15). At 48 d after device removal or at 72 h after the last MGA feeding, all cows received 500 µg GnRH. Progesterone post-GnRH injection was increased (>1 ng/ml) at d 7 in 64, 100 and 100%, and remained elevated at d 14 in 11, 46 and 100% of NP, MGA and P4H cows, respectively. For all experiments plasma P4 was increased (range 2 to 5 ng/ml) when the devices containing P4 were in place, then decreased (<1 ng/ml) by 48 to 50 h after device removal.The administration of P4 via devices designed to deliver about 3 ng P4/ml plasma for 5 or 6 d caused GnRH-induced CL to have a normal life span in a majority of these postpartum beef cows that were in good body condition.
1 Some of the data were presented at the Annu. Meet. of the Amer. Soc. of Anim. Sci. in 1983 (Abstr. 585) and 1984 (Abstr. 436 and 437).
2 Our appreciation is expressed to Dr. E. M. Convey (Michigan State Univ.) for doing part of the LH assays, Dr. E. K. Inskeep (West Virginia Univ.) for doing the FSH assays and Dr. K. A. Ash and Mr. L. F. Krabill (The Upjohn Co.) for statistical analyses.
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