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We studied 663 cyclic lactating Holstein cows in an industrial farm near Qazvin, Iran, from June 2119 to September 2020. We investigated the effects of equine chorionic gonadotropin (eCG) hormone administration 80 h before the implementation of the Ovsynch and fixed-time artificial insemination (OVS + FTAI) program in the form of the G6G protocol on pregnancy rate and reproductive parameters of dairy cows in the first postpartum insemination under heat stress conditions. The cows were randomly assigned to one of the following protocols and received the following treatments: G6G (prostaglandin F2 alpha (PGF2α)-2d-gonadotropin releasing hormone (GnRH)-6d-OVS+FTAI(GnRH(GnRH1)-7d-PGF2α(PG)-56h-GnRH-18h-FTAI, n = 213), 300 international unit (IU) eCG+G6G (300e+G6G) (PGF2α-2d- GnRH-64h-300 IU,eCG-80h-OVS+FTAI, n = 231), and 500 IU eCG+G6G (500e+G6G) (PGF2α-2d-GnRH-64h-500IU,eCG-80h-OVS+FTAI, n = 219). The overall pregnancy rates of cows in the G6G, 300e+G6G, and 500e+G6G protocols were 29.5 ± 0.03%, 33.7 ± 0.03%, and 35.6 ± 0.03%, respectively. The 500e+G6G protocol increased the pregnancy rate of cows with a body condition score (BCS) ≤ 2.5 compared to the G6G protocol (P = 0.04). The 500e+G6G protocol significantly increased the pregnancy rate of cows that produced > 34 kg milk per day compared to the G6G protocol (P = 0.03). In conclusion, using 500 IU, eCG 80 h before the implementation of the OVS+FTAI program in the form of G6G protocol had beneficial effects on increasing the pregnancy rate of cows that had a BCS ≤ 2.5 or produced > 34 kg milk per day compared to the G6G protocol under heat stress conditions.
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