Estrus pre-synchronisation (pre-Synch) using high long-acting progesterone doses in Fixed-Time Artificial Insemination (FTAI) protocols: effects on pregnancy rate in Bos indicus cows

Fixed-Time Artificial Insemination (FTAI) protocols are crucial for improving Bos indicus cattle. The aim of this study was to assess the effect of high long-acting progesterone (pre-synch) doses in FTAI protocols and its impact on pregnancy rate (PR) in Bos indicus cattle. Brahman cows (n=100) were randomly divided into two groups, FTAI-C (C; conventional-protocol; n= 50) and FTAI-P4 (PP4; experimental-Pre-Synch-protocol, using high long-acting progesterone (P4) dose; n=50). A concentration of 200 mg of P4 was applied on Day -8 of the PP4 protocol. On Day 0, the progesterone device (CIDR-1.38 g intravaginal) and Estradiol Benzoate (E2B= 2.0 mg I.M.) were applied in both protocols. On Day 8, the cows received PGF2α (500 µg D-cloprostenol I.M.) + eCG (400 I.U., I.M.) + Estradiol cypionate (E2C= 1 mg I.M.). The CIDR was removed 12h later. Cows showed estrus at 36h post-CIDR removal and the FTAI was performed at 52h (cows showing estrus) and 60h (cows without estrus + GnRH application: 2,500 I.U.). Ovarian evaluation was performed on Day -8 (application of long-acting P4), Day 0 (CIDR application), Day 8 (implant removal), and Day 10 (estrus). PR was evaluated on Day 40. Ovarian diameter (OD), number of follicles (NFLs) follicle diameter (FLD), number of corpora lutea (NCL), and diameter of corpora lutea (CLD) were scored. Data were analyzed by GLMM (SPSS® 25, IBM Corp., USA). No differences were observed between C and PP4 regarding OD, FLD, NFL, CLD, and NCL on Day -8 (p> 0.05); however, differences were observed in OD, NFL, and CLD on Day 0 (p< 0.05). Differences were observed in OD, NFL, FLD, and CLD between protocols on Day 8 and in OD and CLD on Day 10 (p< 0.05). Finally, no differences between C (23/50; PR= 46%) and PP4 (31/50; PR= 62%) were observed (p= 0.06). In conclusion, ovarian structures were similar in both protocols. Although PR slightly improved when using the PP4-pre-Synch protocol, differences between protocols were not significant in Bos indicus cattle. This study was supported by DIRGI-CP2022-005.