Pregnancy rates in mares with pre and postovulation versus only postovulation frozen semen inseminations

Mares may be bred by live cover or artificial insemination with fresh, cooled or frozen semen. Aim of this retrospective study was to compare pregnancy rates in mares after insemination with 2 doses of frozen-thawed semen (pre and postovulation) versus 1 dose (postovulation), in a commercial equine breeding program. Our hypothesis was that pregnancy rate will be similar. Reproductive records from mares bred with frozen semen at Colorado State University over a 3 year period (2017 - 2019) were evaluated retrospectively. Only first breeding cycle of the year was evaluated for any given mare. Mares were monitored by ultrasonography once daily when in estrus and administered 1 dose of 500 µg gonadotropin releasing hormone (GnRH) agonist (histrelin) at 8:00 pm when a dominant follicle of an appropriate size was present along with uterine edema. Mares were subsequently monitored 2 - 4 times daily to predict and subsequently confirm ovulation. Mares were bred by deep uterine horn insemination with either 2 doses of frozen-thawed semen, with 1 dose prior to anticipated ovulation plus a second dose after ovulation was detected, or only 1 dose of semen immediately after ovulation. Selection of the stallion, number of breeding doses allocated per cycle and number of straws per insemination were made by the owner of the mare and/or stallion. A mare was considered pregnant if an embryo was collected during a uterine lavage 8 days after ovulation, or if an embryonic vesicle was visible on 14 days postovulation via transrectal ultrasonography. Pregnancy rate was compared by Chi Square analysis. Records from a total of 156 estrous cycles were evaluated. Percentage of mares that ovulated within specific time frames after administration of histrelin were: ≤ 12 hours (8.3%), 13 - 24 hours (6.4%), 25 - 35 hours (2.6%), 36 - 40 hours (53.8%), 41 - 48 hours (19.2%), and > 48 hours (9.6%). Pregnancy rate of 54 mares inseminated prior to and after ovulation (46%) was not different (p = 0.143) from 102 mares inseminated with 1 dose of frozen and thawed semen immediately after ovulation (34%). There was also no difference in pregnancy rates based on mare age (3 - 10, 11 - 15 or 16 - 20 years) or reproductive status (maiden, barren, open or foaling). However, there was a difference (p = 0.049) in pregnancy rate between mares bred with semen of low progressive motility (n = 8; ≤ 15%) and high progressive motility (n = 25; > 60%). In summary, a numerical, but not statistical, difference in pregnancy rate was noted between mares bred with 2 doses of frozen semen (pre and postovulation) versus 1 dose of frozen semen, however, further studies are needed with more number in each group to confirm this finding. It is becoming increasingly common to breed mares with a single or partial dose of frozen semen, with veterinarians having little control over the quality and/or quantity of semen utilized.

Keywords: Equine, frozen semen, pregnancy, dose, motility

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.