Effect of ovarian characteristics and propylene glycol supplementation on pregnancy rates per AI in suckled beef cows subjected to a 7-day CO-Synch fixed-time AI protocol across seasons, body condition scores, and postpartum intervals

The present study evaluated the independent effects of ovarian characteristics and oral propylene glycol (PG) supplementation on pregnancy rates in suckled beef cows subjected to the 7-day CO-Synch + CIDR fixed-time AI (FTAI) protocol across different seasons, body condition scores (BCS; 5-point scale), and parturition-to-FTAI intervals. Cows received a CIDR insert and 10 μg GnRH on day -10, followed by 25 mg PGF2α at CIDR removal on day -3, and another 10 μg GnRH 72 h later (day 0). In Study 1, the ovarian characteristics of 36 cows were assessed transrectally on days -10 and 0 relative to the FTAI. In Study 2, cows were assigned to a control (n = 118) and a treatment group (n = 57). The treatment group received 1 ml PG/kg body weight mixed with 1 kg rice bran from days -10 to -1, whereas the control group received the same amount of rice bran without PG. Season, BCS, and the parturition-to-FTAI interval were included as covariates. In Study 1, cows with a dominant follicle diameter ≥ 9.00 mm at AI had a higher pregnancy rate than those with a diameter < 9.00 mm (50.0 % vs. 7.1 %, P = 0.014). In Study 2, PG supplementation increased pregnancy rate compared with the control group (68.42% vs. 51.40%, P = 0.036). Pregnancy rates were lower during the hot season in cows with BCS < 2.5, and in those with a parturition-to-FTAI interval < 90 days (P < 0.05). The numerically highest pregnancy rates were observed during the cool season and among cows with a BCS ≥ 2.5, whereas the lowest rate occurred in control cows with a parturition-to-FTAI interval < 90 days (P < 0.05). In conclusion, the pregnancy rate in cows subjected to the 7-day CO-Synch + CIDR FTAI protocol was influenced by follicle diameter and propylene glycol supplementation. Variation in pregnancy outcomes among PG-supplementation cows was further affected by season, BCS, and the parturition-to-FTAI interval. Propylene glycol supplementation enhanced pregnancy rates, particularly during the cool season and in cows with a BCS ≥ 2.5.