comparison of progestin-based protocols to synchronize ovulation and facilitate fixed-time artificial insemination in postpartum beef cows

• Published in: Journal of animal science 2005, Vol.83 (1), p.136-143
• Main Authors: Bader, J.F, Kojima, F.N, Schafer, D.J, Stegner, J.E, Ellersieck, M.R, Smith, M.F, Patterson, D.J
• Format: Article
• Language: English
• Subjects: animal age; Animal productions; Animals; artificial insemination; beef cows; Biological and medical sciences; body condition; Cattle - physiology; Dinoprost - administration & dosage; Dinoprost - analogs & derivatives; Dinoprost - pharmacology; drug injection; estrus synchronization; Estrus Synchronization - drug effects; Estrus Synchronization - methods; Female; Fundamental and applied biological sciences. Psychology; Glucocorticoids - administration & dosage; Glucocorticoids - pharmacology; gonadotropin-releasing hormone; Gonadotropin-Releasing Hormone - administration & dosage; Gonadotropin-Releasing Hormone - pharmacology; Insemination, Artificial - methods; Insemination, Artificial - veterinary; Lactation; Logistic Models; melengestrol; Melengestrol Acetate - administration & dosage; Melengestrol Acetate - pharmacology; ovulation; postpartum period; Pregnancy; Pregnancy Rate; progesterone; Progesterone - blood; prostaglandins; Prostaglandins - administration & dosage; Random Allocation; Terrestrial animal productions; Time Factors; Vertebrates
• ISSN: 0021-8812; 1525-3163
• DOI: 10.2527/2005.831136x

The experimental objective was to compare pregnancy rates after fixed-time AI in postpartum suckled beef cows following administration of two progestin-based protocols to synchronize ovulation. Cows (n = 424) at three locations (n = 208, 122, and 92 per location) were stratified by age, BCS, and days postpartum (DPP) and assigned randomly to one of the two treatment protocols. The MGA Select-treated cows (MGA Select; n = 213) were fed melengestrol acetate (MGA, 0.5 mg x cow(-1) x d(-1)) for 14 d and carrier for 8 d, and then GnRH (100 microg i.m. Cystorelin; d 26) was injected 12 d after MGA withdrawal, and PG (25 mg i.m. Lutalyse) was administered 7 d after GnRH. Cows assigned to the 7-11 Synch protocol (7-11 Synch; n = 209) were fed carrier for 15 d and MGA for 7 d, and then injected with PG on d 22 (d 7 of MGA), GnRH on d 26, and PG again on d 33. Artificial insemination was performed at fixed times for cows in both treatments at 60 or 72 h after d 33 PG for 7-11 Synch and MGA Select groups, respectively. All cows were injected with GnRH (100 microg of i.m. Cystorelin) at AI. There was no treatment x location interaction for age (P = 0.90), BCS (P = 0.64), or DPP (P = 0.93), and the results were therefore pooled for the respective treatments (age [7-11 Synch, 5.5 +/- 0.2; MGA Select, 5.5 +/- 0.2], BCS [7-11 Synch, 5.7 +/- 0.1; MGA Select, 5.6 +/- 0.1], and DPP [7-11 Synch, 41.1 +/- 1.1; MGA Select, 42.1 +/- 1.1]). Blood samples were collected 8 and 1 d before MGA or carrier to determine pretreatment estrous cyclicity (progesterone >or=1 ng/mL; 7-11 Synch, 59/209 [28%]; MGA Select, 54/213 [25%]; P = 0.50) and again on d 33 PG to evaluate treatment response as a percentage of cows with progesterone concentrations in serum >or=1ng/mL (7-11 Synch, 184/209 [88%]; MGA Select, 177/213 [83%]; P = 0.15). Pregnancy rates resulting from fixed-time AI did not differ (P = 0.25) between treatments (7-11 Synch, 128/209 [61%]; MGA Select, 142/213 [67%]), nor did pregnancy rates (P = 0.77) at the end of the breeding season (7-11 Synch, 198/208 [95%]; MGA Select, 204/213 [96%]). These data indicate that pregnancy rates were comparable after fixed-time AI, following administration of the 7-11 Synch and MGA Select protocols. Both protocols provide opportunities for beef producers to use AI and eliminate the need to detect estrus.