Long-acting progesterone supplementation and administration of gonadotropin releasing hormone in early diestrus improves pregnancy rates in Nellore cows with low body condition score and subjected to timed artificial insemination.

This study aimed to evaluate the relationship between the body condition score (BCS), pre-ovulatory follicle (POF) and corpus luteum (CL) characteristics, progesterone (P4) concentrations, and reproductive success in Nellore cows subjected to timed artificial insemination (TAI). Two strategies were tested to enhance P4 concentrations and conception rates in cows with a low BCS. After TAI (Day 0), 863 cows were assigned to three groups: P4LA (n = 281; 150 mg long-acting progesterone injection on Day 7), GnRH (n = 285, 10 µg buserelin acetate on Day 7), and control (n = 297; no supplementation). Ultrasound assessments were performed on Day 0 and Day 7, and the cows were classified as high (BCS ≥ 3; n = 160) or low (BCS < 3; n = 101). No significant differences were observed in the POF or CL characteristics between the BCS groups. However, ovulation rates (P = 0.04) and P4 concentrations on Day 7 (P = 0.02) were higher in cows with high BCS. On Day 16, P4 concentrations were not different between high-BCS cows and those treated with GnRH or P4LA (P = 0.16). Among the low-BCS cows, the P4LA group had higher P4 concentrations than the control group (P = 0.04). In the control group, cows with low BCS had lower P4 concentrations than those with high BCS (P = 0.02). The conception rates of the low-BCS cows were lower in the control group than in the treatment group (P = 0.01). In conclusion, administering GnRH or P4LA on D7 post-TAI enhanced P4 concentrations and improved conception rates in Nellore cows with a low BCS.