Combining equine chorionic gonadotropin and temporary calf removal can increase luteal progesterone but does not improve pregnancy per AI in suckled Nellore cows.

The objective was to determine whether combining equine chorionic gonadotropin (eCG) and 48-h temporary calf removal (CR) at the end of an estradiol and progesterone (P4)-based estrous synchronization protocol for fixed-time AI (TAI) improves pregnancy per AI (P/AI) in suckled multiparous and primiparous Nellore cows. Suckled cows (n = 2643) with adequate body condition score (>95% with BCS ≥ 2.5 on a 5-point scale) were assigned to receive CR, 300 IU of eCG, or eCG + CR on day -2 (D-2) followed by measurement of follicular diameter and estrus expression by TAI (D0), serum P4 concentrations on D7, pregnancy per AI (P/AI) on D30, and calf weaning weights. At TAI, eCG-treated cows had larger follicle diameter (13.5 mm) than CR-treated cows (13.2 mm vs. eCG + CR: 13.1 mm, SEM: 0.2), P < 0.05. Estrus expression was greater in CR and eCG + CR treated cows than in eCG-treated cows (eCG: 59.5b% vs. CR: 77.8a% vs. eCG + CR: 75.0a%). Among primiparous cows, eCG-based treatment (eCG and eCG + CR) resulted in greater serum P4 concentrations on D7 than CR alone. Despite these physiological differences, P/AI was similar among treatments (eCG: 49.8% vs. CR: 53.6% vs. eCG + CR: 49.4%). Calves from cows treated with CR and eCG + CR were lighter (unadjusted weaning weights: 184.1 and 186.2 kg, respectively) than calves from eCG-treated cows (189.2 kg). In conclusion, combining eCG and CR did not improve P/AI compared with either treatment alone. Temporary 48-h calf removal increased estrus expression but did not improve fertility and reduced calf weaning weight. In contrast, eCG alone resulted in heavier calves at weaning, supporting its use over temporary 48-h calf removal in estrous synchronization programs when calf weaning weight is also considered.