Fixed-time artificial insemination of postpartum beef cows at 72 or 80 h after treatment with the MGA® Select protocol

The objective was to determine the appropriate timing of fixed-time artificial insemination (AI) following administration of the MGA(R) Select protocol. Cows at two locations (Location 1, n = 114; Location 2, n = 97) were assigned to fixed-time AI at 72 or 80 h by age, body condition score (BCS), days postpartum (DPP), AI technician, and sire. All cows were synchronized with the MGA(R) Select protocol, consisting of oral administration of melengestrol acetate (MGA; 0.5 mg/ hd per day) for 14 days, GnRH (Cysotrelin,(1) 100 mug, i.m.; Day 26) 12 days after MGA withdrawal, followed in 7 days with PGF(2alpha) (PG; Lutalyse,(2) 25 mg i.m.; Day 33). Cows were inseminated at 72 h (n = 108) or 80 h (n = 103) after PG and GnRH (100 mug) was given at insemination. Location was not significant and, therefore, was removed from the model. Mean BCS (5.2 +/- 0.1, 72 h; 5.3 +/- 0.1. 80 h) and DPP (34 +/- 2, 72 h; 35 2, 80 h) did not differ (P > 0.1) between treatments. Serum progesterone concentrations 7 and 1 day prior to MGA were used to determine pretreatment cyclicity: cows with at least one sample with progesterone greater than or equal to 1 ng/ml were defined as cyclic (33/108, 31%, 72 h, versus 32/103, 31%, 80 h; P > 0.1). Cows with serum progesterone concentrations greater than or equal to 1 ng/ml on the day of PG were defined as responding to the synchronization protocol (74/108 (69%), 72 h versus 69/103 (67%), 80 h; P > 0.1). Although pregnancy rates were higher (P < 0.05) for cows inseminated at 72 h (69/108, 64%) versus 80 h (52/103, 50%) after PG, pregnancy rates at the end of the breeding season did not differ (P > 0.1) between treatments (98/108 (91%), 72 h; 88/103 (85%), 80 h). In conclusion, pregnancy rates were higher when postpartum beef cows synchronized with the MGA(R) Select protocol were inseminated at 72 h versus 80 h after PG. (C) 2003 Elsevier Inc. All rights reserved.