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Effects of PMSG and PGF₂ on Gonadal Hormones and Reproduction in the Beef Heifer¹

Clemson University, Clemson, SC 29631

Abstract

The effects of pregnant mare serum gonadotropin (PMSG) alone or in combination with prostaglandin F₂ (PGF₂) when administered at two different times of the estrous cycle were observed in 36 beef heifers. Twelve heifers received PMSG on day 16 of the estrous cycle (Group I). Twelve heifers were injected with PMSG and PGF₂ on the same day (Group II), and another 12 heifers were injected with PMSG 2 days before PGF₂ (Group III). Six heifers in Groups II and III received PGF₂ on day 9 or 10 of the estrous cycle (mid-luteal phase) and the other six in each group received PGF₂ on day 16 or 17 of the estrous cycle (late-luteal phase). All heifers were mated at the first estrus after treatment.

Prostaglandin F₂, when administered during the mid- or late-luteal phase of the estrous cycle, induced estrus. Progesterone levels decreased in response to prostaglandin treatment at mid- or late-luteal phases. Heifers receiving PMSG alone averaged 5.7 ± 1.4 ovulations compared to 4.6 ± 1.4 ovulations per heifer receiving PMSG two days before PGF₂, or 2.3 ± .7 ovulations per heifer receiving PMSG and PGF₂ on the same day. These results suggest that PGF₂ suppresses the effects of PMSG on ovulation, particularly when PGF₂2 is given the same day as PMSG. Irregardless of treatment, mean estrogen concentrations increased in a similar manner in all heifers treated with PMSG, and the level was significantly higher (P<.05) than in a group of untreated heifers bled at the same time (Henricks et al., 1974).

An important observation was the relationship between the plasma estrogen level at estrus and the number of ovulations in individual heifers. A correlation coefficient of .70 (P<.01) was obtained between these two characteristics. The mean plasma estrogen concentration in heifers with multiple ovulations was 27.2 ± 3.6 pg/ml (mean ± SE) and 14.6 ± 2.4 pg/ml in heifers with a single ovulations. The plasma estrogen level at estrus might be used to predict ovulation rate. Although not unexpected, the plasma progesterone concentration was related in a positive manner to the number of corpora lutea observed at day 16 after mating. Finally, although the plasma estrogen concentration declined to less than 10 pg/ml and remained low in Groups I and III for the remainder of the experimental period (until day 16 after mating), the level in Group II rose to a peak on day 6 after mating similar to the level at estrus indicating follicular activity and possibly ovulation.

¹ Published with the approval of the Director of the Agricultural Experiment Station as Technical Contribution No. 1224.

² Departments of Food Science and Animal Science.

³ The authors gratefully acknowledge the assistance that Dr. Don Lamond (deceased) gave in the planning of this study and the technical assistance of Mr. J. Toler in the statistical analysis. We thank Dr. H. Dobson (University of Liverpool) for the progesterone antiserum and Dr. B. V. Caldwell (Yale University) for the estrogen antiserum used in this study.