GnRH and PGF2α administered at the time of AI differently affect conception rates of non-estrous Nelore cows according to Body Condition Score

This study evaluated the effects of GnRH, PGF2α, and their combination at the time of fixed-time artificial insemination (timed-AI) on pregnancy per AI (P/AI) and pregnancy loss (PL) in non-estrous suckled multiparous Nelore cows. Of 8061 cows submitted to a timed-AI protocol, those expressing estrus (n = 5400) served as a positive control, while non-estrous cows (n = 2661) were randomly assigned to saline solution (Control; n = 791), GnRH (n = 656), PGF2α (n = 667), or PGF2α+GnRH (n = 547) at AI. Pregnancy per AI was diagnosed on Day 41 (D41) and PL assessed on D71. Pregnancy per AI was greater (P < 0.0001) in estrous (59%) than non-estrous cows (34%), and among non-estrous cows a treatment × BCS interaction was detected (P < 0.0001). In cows with BCS < 2.75, PGF2α (34%) and PGF2α+GnRH (31%) achieved greater P/AI (P = 0.006) than GnRH (18%). Conversely, in cows with BCS ≥ 2.75, GnRH achieved the highest P/AI (49%), exceeding PGF2α (26%) and PGF2α+GnRH (28%; P < 0.0001), though not significantly different from Control (41%; P = 0.053). Overall PL was lower (P = 0.042) in the PGF2α+GnRH group (2.5%) than Control (9%). Integrating P/AI and PL outcomes, the final conception rates at D71 demonstrate that BCS influenced the success of the treatment administered at the time of AI. GnRH improved fertility of cows with BCS ≥ 2.75 (Control: 38%, GnRH: 47%, PGF2α: 25%; PGF2α+GnRH: 28%; P < 0.0001) while PGF2α increased pregnancy of cows with BCS <2.75 (Control: 16%, GnRH: 17%, PGF2α: 32%, PGF2α+GnRH: 30%; P < 0.0001). In conclusion, the effectiveness of hormonal treatment at AI in non-estrous Nelore cows depends on BCS: GnRH improved fertility in cows with adequate body condition (BCS ≥ 2.75), while PGF2α-based treatments were more beneficial in cows with poor nutritional status (BCS < 2.75).