Use of melengestrol acetate and gonadotropins to induce fertile estrus in seasonally anestrous ewes.
Safranski T.J. | Lamberson W.R. | Keisler D.H.
Ewes of three genotypes (Hampshire, n = 59; Rambouillet, n = 36; crossbred, n = 57) were used to determine the efficiency of melengestrol acetate (MGA) and(or) PG.600 (a combination of pregnant mare's serum gonadotropin and human chorionic gonadotropin) in inducing fertile estrus in seasonally anestrus ewes. Ewes were assigned randomly, within genotype, to treatments in a 2 X 2 factorial arrangement. Treatments were control, 0.125 mg of MGA given twice per day for 9 d (MGA), a single 5-mL injection of PG-600 (PG-600), and the combination of treatments MGA and PG-600 (MGA/PG-600). Feeding of MGA began on May 14, 1990, and ended on May 23. Injections of PG-600 were given immediately after the last feeding of MGA or vehicle on May 23. All ewes were exposed to fertile, brisket-painted rams on May 24 (d 0) for 40 d. Ewes were checked for estrus twice daily for 9 d. Laparoscopy was performed, to assess ovulation rate (OR), on d 6 for ewes that were not detected in estrus and on d 12 for ewes that exhibited estrus. Percentage of ewes mated was increased by MGA (P < 0.001). Ovulation rate of ewes exposed to rams was increased by PG-600 (P < 0.01) and this effect was enhanced by MGA (P < 0.05), whereas MGA alone tended to decrease OR (P < 0.10). Melengestrol acetate decreased the interval to lambing by 6.5 d (P < 0.05). In addition, MGA increased the percentage of ewes lambing (P < 0.001) and the number of lambs born per ewe exposed to rams (P < 0.001). The number of lambs born per ewe lambing was unaffected by treatment. These data indicate that use of MGA can facilitate twice-per-year lambing. The use of PG-600 did not enhance ewe productivity beyond that achieved using MGA alone.
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