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Evaluation of a fixed-time artificial insemination protocol for postpartum suckled beef cows¹

Department of Animal Science, University of Missouri, Columbia 65211

² Correspondence:
S132 Animal Science Research Center (phone: 573-882-7519; fax: 573-884-4545; E-mail:
PattersonD{at}missouri.edu).

Treatment with melengestrol acetate (MGA), an oral progestin, prior to administration of gonadotropin-releasing hormone (GnRH) and prostaglandin F₂ (PG) effectively synchronizes estrus and maintains high fertility in postpartum beef cows. The objective of this experiment was to determine whether treatment with MGA prior to a GnRH-PG-GnRH protocol would improve pregnancy rates resulting from fixed-time artificial insemination (AI). Multiparous crossbred beef cows at two University of Missouri-Columbia farms (n = 90 and n = 137) were assigned by age and days postpartum to one of two treatments. Cows were fed carrier (1.8 kg•animal^-1•d^-1) with or without MGA (0.278 mg•kg^-1) for 14 d. All cows were administered GnRH (100 µg; intramuscularly) on d 12 after MGA or carrier withdrawal and 7 d before PG (25 mg; intramuscularly). All cows received a second injection of GnRH and AI 72 h after PG. Mean days postpartum for MGA and control cows at the initiation of treatment were 39.6 and 38.9 d for herd 1; and 51.9 and 50.9 d for herd 2, respectively (P > 0.70 within herds). Blood samples were collected from all cows at 10 and 1 d before the feeding of MGA or carrier began and at the times GnRH and PG were administered. Concentrations of progesterone in serum at the initiation of treatment were elevated (>1 ng/mL) in 0% of MGA and 7% of control cows in herd 1, and 54% of MGA and 49% of control cows in herd 2 (P > 0.05 within herds). Pregnancy rates to fixed-time AI were determined by transrectal ultrasonography 50 d after AI. Pregnancy rates in herd 1 were 58% (26/45) and 51% (23/45) for MGA-treated and control cows, respectively (P = 0.52), and 63% (44/70) and 45% (30/67) for MGA-treated and control cows in herd 2, respectively (P = 0.03). Differences in pregnancy rates to fixed-time AI were significant (P = 0.04) when data from the two herds were combined (with MGA = 70/115 [61%]; control = 53/112 [47%]). There was no difference (P > 0.20) in final pregnancy rates (timed AI plus 45 d exposure to bulls) between treatments, within herds, or when herds were combined. In summary, pregnancy rates resulting from fixed-time AI may be improved with treatment of MGA prior to a GnRH-PG-GnRH protocol.

Key Words: Artificial Insemination • Beef Cows • Estrus • Synchronization

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