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Puberty in Beef Heifers. II. Effect of Injections of Progesterone and Estradiol-17ß on Serum LH, FSH and Ovarian Activity^{1, 2, 3}

Colorado State University^,6, Fort Collins 80521

Abstract

Eight half-sib prepuberal Angus heifers were randomly divided into treatments E, P, or PE. Treatment E (two heifers) was 2 mg of estradiol 17ß injected IM on day 0. The two heifers in treatment P received 20 mg of progesterone IM on day —2. The four heifers in treatment PE were given 20 mg of progesterone on day —2 and 2 mg of estradiol-17ß on day 0. Blood samples were collected from day —9 to day 13 at 6-hr intervals. Samples were collected at 20-min intervals for 4 hr on days —7 and 0 and at 15-min intervals for 2 1/2 hr on day —2. Serum concentrations of LH and FSH were measured by radioimmunoassay (RIA) in all samples collected from day —9 to 6, and progesterone was analyzed by RIA in daily pools of sera for several days prior to day 0 and on day 8, 9, 12 and 13. Injection of progesterone alone in two heifers did not induce behavioral estrus, nor did it have a marked effect on the levels of the hormones studied. Only one of the six heifers receiving E or PE treatments showed standing estrus. Estradiol injections in the E and PE treatments induced LH peaks from 11 to 99 ng/ml that were detected between 12 to 18 hr after the injection. There were no distinct peaks of FSH associated with the LH peaks.

Three out of the four heifers in treatment PE developed normal CL as diagnosed by rectal palpation, serum progesterone concentration on days 8 to 13 and return to estrus on days 19 to 21. The two heifers receiving estradiol without progesterone priming did not develop CL and were not observed in heat within 30 days after the treatment.

¹ Published with the approval of the Director of the Colorado State University Experiment Station as Scientific Series Paper No. 1965.

² Research Supported in part by Rockefeller Foundation.

³ Research supported in part by Western Regional Project W112 Reproductive Performance in Beef Cattle.

⁴ Present Address: INIP, Km 15 1/2 de la Carr. Mexico-Toluca, Palo Alto, Mexico 10 D. F.

⁵ Present Address: Texas A & M University, Agricultural Research Station, St. Rt. 2 Box 43C, Beeville, Texas 78102.

⁶ Department of Physiology and Biophysics and Department of Animal Sciences.