Two hormonal protocols for timed artificial insemination in mares under different residence times of the progesterone intravaginal device

Summary: The study aimed to evaluate two hormonal protocols for oestrus and ovulation synchronisation for timed artificial insemination (TAI) and their effect on the embryo recovery rate (ERR) in horses. Eleven crossbreed mares were allocated in three groups, taking 3 different oestrus cycles per mare, during two breeding seasons, totalling 17 cycles followed per group, with 11 cycles in the first season and 6 cycles in the second season. Conventional follicular follow-up in the control group (CG; n = 17) was performed by transrectal palpation and ultrasound (US) examination; the ovulation was induced with 500 μg histrelin (intramuscularly: IM) when the diameter of the follicles was ≥ 35 mm, the uterine oedema was grade 3 and cervical opening had begun. Artificial insemination with fresh semen (250 × 10 6 sperm cells) was performed 24 h after ovulation inductor administration, and embryos were collected eight days after ovulation (D8). The experimental groups had a pro-gesterone-releasing intravaginal device (PRID) inserted for 9 (PRID9) or 11 days only between the CG and PRID11 (P < 0.05). It was concluded that the hormonal protocol using PRID for nine days combined with histrelin administration four days after the PRID’s removal showed promising results related to the ERR in mares, while reducing conventional follicular monitoring, the number of gynaecological examinations, animal management and visits to the properties. Because no statistical difference was detected between the PRID9 protocol and the CG, it could be used in large equine herds, reducing animal management and the number of visits by professionals. Effects such as early ovulations, the small size of preovulatory follicles at TAI, the interval between P4 removal and ovulation induction, and low ERR in treatment groups might be improved in future studies, aiming at the possibility using the method for commercial purposes under the acceptance of TAI protocols by veterinarians and breeders. Further studies about TAI in horses are recommended.