Effects of large follicle numbers at the onset of short-term timed artificial insemination protocol in lactating dairy cows

This study aimed to compare the fertility of lactating Holstein cows with only 1 large follicle (LF, diameter ≥10 mm) with those with ≥2 LF and corpora lutea at the start of short-term ovulation synchronization and timed artificial insemination (AI). Cows with a corpus luteum ≥20 mm in diameter and either 1 LF (1F) or ≥2 LF (2F) verified via ovarian ultrasonography were included in the study. Cows were randomly categorized into groups receiving either estradiol benzoate (EB) or GnRH as ovulation induction agents. Cows received a luteolytic dose of PGF_2α, followed by administration of EB 24 h later or GnRH 56 h later. Timed AI was performed 24 to 28 h after EB administration or 16 to 20 h after GnRH administration. We analyzed data from 605 AI in 554 cows (1F-EB: n = 193, 2F-EB: n = 100, 1F-GnRH: n = 213, 2F-GnRH: n = 99). Logistic regression analysis revealed a significant interaction between the number of LF and the ovulation induction agent. Pregnancy per AI was significantly lower in the 1F-GnRH group than in the other 3 groups (1F-EB: 40.9%, 2F-EB: 44.0%, 1F-GnRH: 28.2%, 2F-GnRH: 44.4%). In conclusion, the fertility of cows with 1F was lower than that of cows with 2F when GnRH was administered as the ovulation induction agent in the ovulation synchronization and timed AI protocol. However, the fertility of 1F cows receiving EB was comparable to that of 2F cows receiving either GnRH or EB.