Effect of pre-treatment follicular size on reproductive response of dairy heifers after PG2α induced estrus and GnRH administration on the day of artificial insemination

The present study was designed to investigate the influence of pre-treatment follicular size on the reproductive response of dairy heifers after PGF2α injection and GnRH administration on the day of artificial insemination (AI). After transrectal ultrasound examination, fifty-two dairy heifers bearing corpus luteum (CL) were selected and divided into three groups: Group I (n=19) with small follicles (2–5 mm); Group II (n=18) with medium follicles (6–9 mm) and Group III (n=15) with dominant follicle (10–12 mm). Each group was administered 500 mg PGF2α immediately after ultrasound examination (Day 0) and 100 μg GnRH at the time of AI, performed 80 hours after PGF2α injection. A second ultrasound examination was done early morning on Day 3 (72 h) after prostaglandin treatment and presence of a pre-ovulatory follicle and clinical estrus signs were recorded. All inseminated heifers were monitored to a clinical estrus expression by daily observation and new AI was made in the cases of spontaneous estrus detection. Ultrasound pregnancy diagnosis was done on Day 30 and on Day 70 after the first AI. Heifers with medium and dominant follicles showed a better (P<0.05) reproductive response in terms of uterine tone, relaxation of cervix, and pregnancy rate after induced estrus. These findings indicated that pre-treatment follicular size has influence on prostaglandin induced estrus characteristics and pregnancy rate following GnRH administration on the day of AI in dairy heifers.