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West Virginia University, Morgantown 26506
Abstract
Two experiments were conducted with anestrous postpartum beef cows (1) to determine the effects of human chorionic gonadotropin (hCG) and gonadotropin releasing hormone (GnRH) on the proportion of cows that formed corpora lutea and on conception rates of those cows at a subsequent estrus, and (2) to examine the effects of progestogen and estrogen on the life span and function of corpora lutea induced by hCG. In Exp. 1, conducted with three herds, ovaries of lactating cows that were 30 or more days postpartum were palpated per rectum. On the day of examination (d 0), those cows without a corpus luteum were injected im with either hCG (2,500 IU) or GnRH (200 µg) or served as controls. Cows with a palpable corpus luteum 12 d after the administration of these treatments were given prostaglandin F2
(PGF2 33.4 mg tromethamine salt, im). All cows were observed for estrus twice daily, on d 8 through 20 after treatment with gonadotropin in two herds and on d 12 through 21 in the third herd. Twelve hours after estrus, cows were inseminated artificially. Treatment with GnRH or hCG increased (P<.05) the proportion of cows in estrus on d 8 through 12 (11 of 37 vs one of 19 control cows) and the proportion of cows with corpora lutea on d 12 (18 of 45 vs four of 30). The overall conception rate was 56% and did not differ among groups. In Exp. 2, 48 anestrous beef cows (19 to 33 d postpartum) were assigned to eight groups of six each. Three factors were studied in sequence: (1) an ear implant of norgestomet (3.35 mg) for 9 d or sham implantation; (2) estradiol-17β (400 µg; im) or corn oil 24 h after removal of the implant, and (3) a second implant of norgestomet for 10 d or sham implantation. Each cow received hCG (1,000 IU; im) 36 h after removal of the first implant, at which time the animals were an average of 37 d postpartum. The second implant was inserted 4 d after the administration of hCG. Ovaries were palpated before the insertion and at the time of removal of the first implant of norgestomet and 4, 7 and 14 d after hCG. Jugular samples for analyses of serum progesterone were taken at the time of each palpation, at the time of administration of hCG and 18 d after hCG. Follicular size at the time of removal of the first implant did not differ between the implanted and sham-implanted cows. Among the cows that formed corpora lutea (29 of 48), those receiving the first implant of norgestomet had a profile of serum progesterone that was higher over the 18 d after hCG than that for sham-implanted cows (P<.05). Neither estradiol nor the second implant of norgestomet altered the life span or level of function of the induced corpora lutea.
1 Division of Anim. and Vet. Sci. Published with the approval of the Director of the West Virginia Agr. and Forestry Exp. Sta. as Scientific Paper No. 1683.
2 Supported by Hatch Project 224 (NE-72). We thank Dr. R. H. Rippel of Abbott Laboratories for providing gonadotropin releasing hormone; Dr. S. E. Mares of G. D. Searle and Co. for providing implants of norgestomet; Dr. James Lauderdale of the Upjohn Company for prostaglandin F2a; Dr. W. V. Thayne for assistance in statistical analysis, and Ms. JoAnn Silipo for technical assistance. We especially appreciate the assistance of W. L. Ferrell in managing the cow herd at Reymann Memorial Farms, in preparation for and during Exp. 2.
3 Claude Worthington Benedum Foundation Fellow in Reproductive Physiology.
4 Manager, Reymann Memorial Farms, West Virginia Univ.,Wardensville.
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