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The efficacy of melengestrol acetate (MGA) to shorten the vernal transition of mares by synchronizing and accelerating the first ovulation of the year after 60 days of phototherapy was determined by ultrasonographic monitoring. A total of 16 mares in late transition were fed two doses of MGA (100 and 150 mg/mare/day) for 10 days. A luteolytic dose of prostaglandin was administered to each mare one day after the end of MGA treatment. The presence and duration of oestrus, follicular growth, uterine oedema and presence of ovulation were monitored by ultrasonography and the cervical tone was evaluated by rectal palpation. Results revealed that ovulation was detected in 87.5% of the mares treated with 150 mg MGA/mare/day for 10 days, while 62.5% in mares receiving 100 mg MGA/mare/day for 10 days. This was statistically different (P=0.03) from the untreated control mares which showed an ovulation rate of 20%. Mares that received 150 mg MGA/day for 10 days had a mean treatment to ovulation interval of 13.1+or-5.97 days after the end of treatment, while mares that received 100 mg MGA/day for 10 days had a mean of 25.6+or-10.50 days (P=0.01) to ovulation. It is concluded that MGA can be used for synchronizing and hastening the first ovulation of the year in mares.
Lopez-Bayghen, C. Zozaya, H. Ocampo, L. Brumbaugh, GW Sumano, H.
Department of Physiology and Pharmacology, Faculty of Veterinary Medicine and Zootechnics, National Autonomous University of Mexico (UNAM), Av. Universidad 3000, Coyoacan, Mexico City 04510, Mexico.