PSI-4 - Onset of estrus anticipated in beef cows subjected to the 7&7 protocol and its variations

Abstract: Standard procedure for optimal pregnancy rates (P/AI) in artificial insemination (AI) is to AI 12 hours (h) after the onset of estrus. For fixed-timed AI (FTAI) 7&7 protocol, prior literature recommends AI at 66±2h (Andersen et. al., 2021). However, the basis of this protocol, the 7-day CO-Synch+CIDR®, recommends AI from 60-66h (Bridges et al., 2008). This study aimed to characterize the onset of estrus in beef cows subjected to 7&7 protocol and its variations. Primiparous (n=111) and multiparous (n=253) beef cows, across two farms (Farm_1; Farm_2), were enrolled in one of the three treatments: 7&7 (n=128), 7&7G (n=135), or 7&7Max (n=125). All protocols had a P4 implant (CIDR®, 1.38 mg, Zoetis, NJ) inserted on D0. 7&7G and 7&7Max also received GnRH (100mcg; Factrel®, Zoetis, NJ) on the same day. All treatments received a GnRH shot on D7, while only the 7&7Max protocol received an additional shot of PGF (25mg, Lutalyse®, Zoetis, NJ). On D14 all three treatments received a PGF shot, estrus detection patches (Estrotect™, Rockway INC, WI), and had the CIDR withdrawn. Estrus detection was performed every 6h during the daytime starting 24h after CIDR pull. Cows underwent AI 12-18h after heat detection patches were over 50% activated. Cows that show heat 54h and up after CIDR removal were bred at the fixed time (66±2h) and received a GnRH shot. Pregnancy diagnosis was performed by ultrasonography D32 post-AI. Statistical analysis was performed using Proc Glimmix for heat time and P/AI, Proc Lifetest for the survival curve, and chi-square for heat incidence using SAS 9.4. Cows that did not show heat until the 66h were removed from the heat analysis. There was no difference in P/AI regardless of protocols and sire (P >0.05). However, cows bred 12-18h (73.61±2.92%; P=0.008) from the onset of estrus had higher P/AI than cows that did not show heat or shown heat at the 66h (60.51±3.94%). Primiparous (49.80±0.91h) came in heat earlier than the multiparous (53.64±0.68h; P=0.001). Survival curve also indicated a tendency for the primiparous to come in heat earlier than multiparous (P=0.07). There was interaction between farm*parity (P=0.01), with Farm_1 primiparous having earlier onset of estrus (48.39±1.23h) than multiparous from Farm_1 (55.17±1.18h) and Farm_2 (52.16±0.71h; P=0.04). The percentage of cows that came in heat at ≤48h (39.43%; P< 0.0001) was greater than cows that expressed estrus at 54h (20.10%), 66h (19.07%) and no heat (21.39%). These results indicate cows that came in heat by 48hr, would have been bred 18-24hr after onset of estrus. In conclusion, the current recommendations for AI time when using the 7&7 and its variations may not be ideal for one-third of the cows, which may impair their chances of getting pregnant.