Master of Science (MS)


Animal Science (Animal, Dairy, and Poultry Sciences)

Document Type



Two experiments studied the effects of pretreatment with estradiol benzoate before treatment with a dopamine antagonist on prolactin secretion and reproductive traits in mares during 1) the seasonal anovulatory period and 2) the normal breeding season. Experiment 1 was performed in winter with 17 mares selected for low follicular activity. Nine mares received estradiol benzoate injections every other day for a total of 10 injections; 8 mares received similar injections of vehicle. Ten days after onset of injections, all mares were placed on a daily injections of sulpiride (250 mg) for 45 d or until ovulation. Plasma prolactin concentrations were higher (P < 0.001) in mares receiving estradiol than in control mares for all assessments from d 12 through 36. Mean day of ovulation was 73.6 d for control mares and 29.0 for estradiol-treated mares (P = 0.016). It was concluded that estradiol treatment greatly enhanced prolactin secretion in response to sulpiride in seasonally anovulatory mares and hastened the date of first ovulation by an average of 45 d. Experiment 2 was designed to assess the efficacy of a single injection preparation of another dopamine antagonist, domperidone, for increasing prolactin secretion in cyclic mares in the summer. The experimental design and procedures used in Experiment 1 were repeated, except that a single shot of 3 g of domperidone was administered on d 11 rather than 45 d of sulpiride injections. Day 0 was the first day of estrus for each mare. Prolactin concentrations were higher (P < 0.05) in mares receiving estradiol than in control mares from d 12 through 25 and after a thyrotropin releasing hormone injection on d 21. It was concluded that estradiol enhanced the prolactin response to a single injection of 3 g of domperidone in cyclic mares in the summer in a manner similar to the estradiol enhancement of prolactin secretion in response to daily sulpiride injections in anovulatory mares in winter. Thus, the single injection of domperidone could possibly replace the daily sulpiride injections used in Experiment 1 to induce ovulation in seasonally anovulatory mares; this needs to be tested in future experiments.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Donald L. Thompson, Jr.