When, Where, and How Much Frozen-Thawed Semen is Best?

When, Where, and How Much Frozen-Thawed Semen is Best?

Mares had higher pregnancy rates per cycle as sperm numbers and number of inseminations per cycle increased.

Photo: Courtesy Sandro Barbacini, DVM

In artificial insemination (AI) circles, it’s generally thought that the more sperm in frozen-thawed semen you use to inseminate a mare, the better your chances of getting her pregnant. Likewise, the more times you inseminate the mare during her estrous cycle, the more likely she’ll be to get in foal.

There’s been limited published data, however, on the actual impact of sperm numbers and insemination timing and location within the uterus. To that end, a group of researchers performed a retrospective study to determine the effects of sperm numbers, the number of AI per cycle, timing of AI, and site of AI on pregnancy rates of frozen-thawed semen. David Scofield, DVM, Dipl. ACT, of Select Breeders Services, in Chesapeake City, Maryland, presented their findings at the 2016 American Association of Equine Practitioners Convention, held Dec. 3-7 in Orlando, Florida.

Veterinarians can choose from several strategies when it comes to breeding with frozen-thawed semen. They can inseminate the mare multiple times throughout her estrous cycle using a fixed time AI protocol, or they can use ultrasound to determine insemination timing post-ovulation. Veterinarians can inseminate the mare into her uterine body or by a deep uterine horn technique with a variable number or sperm and/or sperm doses.

Taking all this into account, Scofield’s team looked at data from 10 veterinary clinics (seven in the United States and three in Europe). This data was comprised of mares inseminated on 1,169 cycles with frozen semen from 168 stallions. The researchers recorded each mare’s age, breed, and reproductive status; total sperm; and AI timing and location.

Most of these mares (96%) were inseminated with either four or eight straws (each straw contains 100-125 million sperm), said Scofield; these were the mares the team included in their analysis. There was no statistical difference in pregnancy rates between different types of mares (maiden, donor, barren, foaling), he said.

Scofield listed their findings:

  • Mares that received one insemination of 400 million sperm had a 35% per cycle conception rate;
  • Mares that received two inseminations of 400 million sperm each had a 42% per cycle conception rate;
  • Mares that received one insemination of 800 million sperm had a 48% per cycle conception rate;
  • Mares that received two inseminations of 800 million sperm each had a 57% per cycle conception rate; and
  • Site of semen deposition (body vs. deep horn) had no significant effect on pregnancy rates.

“Mares had higher pregnancy rates per cycle as sperm numbers and number of AI per cycle increased,” he said, recommending breeders freeze stallion’s semen in doses of 800 million sperm, with at least 30% post-thaw motility, to allow the mare owner and veterinarian to decide whether to administer a full or half dose, pre- and/or post-ovulation, and in the site of their choice.

Scofield cautioned that various other mare- and stallion-dependent factors do still affect fertility. “Ask about sperm quality and numbers to guide your management of the mare,” he said.

About the Author

Alexandra Beckstett, The Horse Managing Editor

Alexandra Beckstett, Managing Editor of The Horse and a native of Houston, Texas, is a lifelong horse owner who has shown successfully on the national hunter/jumper circuit and dabbled in hunter breeding. After graduating from Duke University, she joined Blood-Horse Publications as Assistant Editor of its book division, Eclipse Press, before joining The Horse.

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