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Alteration of Ovarian Activity in Cycling, Pregnant and Hysterectomized Heifers with Exogenous Estrogens^1,2,

U. S. Department of Agriculture and Nebraska Agricultural Experiment Station, Lincoln

Abstract

Ninety-nine yearling Hereford heifers were used in a 5 x 3 factorial experiment to study the effects of various estrogens on ovarian activity. The heifers were randomly assigned to a cycling, pregnant or hysterectomized group and to one of the following hormone treatments within each of these groups: control, 50 mg. estradiol-17β, 50 mg. estradiol-17, 50 mg. estrone, or 5 mg. estradiol valerate. Estrogen treatments were administered as a single intramuscular injection on day 5 in the cycling and hysterectomized heifers and day 35 in the pregnant heifers. All heifers were laparotomized at the time of estrogen injection, and corpora lutea were marked with India ink for later identification. Heifers were ovariectomized 7 days after the hormone treatment.

Mean corpora lutea weights between the cycling, pregnant and hysterectomized heifers were not significantly different. Estrogen treatment reduced (P<.05) corpora lutea weights in all three reproductive groups. Estradiol-17β appeared to be the most effective in this respect. Progesterone levels were consistently higher (P<.05) in the hysterectomized heifers compared to the cycling and pregnant groups. Both progesterone concentration and progesterone content were reduced (P<.05) following estrogen treatment.

Estrogen treatment reduced ovarian follicular fluid weight and caused a general depression in the number of follicles less than 15 mm. in diameter.

Heifers in different reproductive states responded similarly to the different forms of estrogen administered for all but one of the indices (number of 6 to 10 mm. follicles) of ovarian function studied. This is evidenced by the small and non-significant reproductive state x hormone treatment interactions.

¹ Published with the approval of the Director as paper No. 1445, Journal Series, Nebraska Agricultural Experiment Station.

² This investigation was supported in part by PHS research grant (A-5665) from the National Institute of Arthritis and Metabolic Diseases, U.S. Public Health Service.

³ Fort Robinson Beef Cattle Research Station, Animal Husbandry Research Division, ARS, Crawford, Nebraska.

⁴ Department of Animal Science, University of Nebraska, Lincoln, Nebraska.