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Ovarian Responses to Intramuscular or Intravenous Administration of Prostaglandin F₂ in Control and FSH-Treated Beef Heifers

U.S. Department of Agriculture^,2,3,, Clay Center, NE 68933

Abstract

Sixty Simmental crossbred heifers 18 to 20 mo of age were detected in estrus and assigned at random to a 2 x 2 factorial design study with 30 controls, and 30 given FSH. Half of each group was given prostaglandin F_2a (PGF) i.v. and the rest i.m. Injections of follicle-stimulating hormone were started on d 7 to 14 of an estrous cycle and continued for 5 d or until ovariectomy; PGF was administered either i.v. or i.m. at 48 (25 mg) and 60 (15 mg) h after the initial FSH injection. Control females received a similar PGF treatment on a day between d 9 and 15 of the estrous cycle. Blood samples were collected from all animals immediately before PGF administration and every 12 h thereafter until ovariectomy. Within each of the four experimental subgroups, ovariectomies were performed at either 24, 48 or 72 h (five/time group) after initial PGF injection. Ovarian and corpus luteum (CL) weights were recorded as well as number and size of follicles and number of ovulations. Regression of the CL was slower (P < .05) after administration of PGF i.v. than i.m. (CL weight was 2.6 vs 3.3 ± .2 g for i.m. and i.v. groups, respectively). Exogenous FSH increased estradiol-17ß (E2) concentrations, and FSH-treated heifers had more (P < .05) early ovulations than control heifers did. Ovulations in FSH-treated heifers had begun to occur by 24 h after i.v. and i.m. PGF injection. This investigation showed that route of PGF administration can alter CL regression, because females receiving PGF i.v. had heavier CL weights and a smaller percentage reduction in concentration of P4 at 24 h after PGF injection.

¹ The authors are grateful to Betty Petitjean, Suzanne Wise and Claire Huebert for technical assistance; to Becky Gerlach, Gale Johnson, Margie McAlhany, and Doug Weichman for care of cattle; and to Lisa Meyer and Donna Griess for stenographic work. Cooperation of the Nebraska Agric. Exp. Sta., Univ. of Nebraska, Lincoln is acknowledged.

² ARS, Roman L. Hruska U.S. Meat Anim. Res. Center, Reprod. Unit.

³ Mention of a trade name, proprietary product or specific equipment does not constitute a guarantee or warranty by the USDA and does not imply its approval to the exclusion of other products that also may be suitable.