Using Frozen-Thawed Semen for Artificial Insemination in Horses

Breeding mares by artificial insemination can be accomplished using fresh semen, chilled semen, or frozen-thawed semen. Each option has its own method of use and average rate of pregnancy. Of these three techniques, using frozen-thawed semen to inseminate mares seems to be most tricky.

Frozen semen may be packaged in several different ways including plastic straws, glass vials, pellets, and plastic bags. In the U.S., the most commonly used packaging is plastic straws with a capacity of 0.5 ml. The number of sperm per straw can be from 50 to 800 million depending on the laboratory that prepared the semen, so up to ten straws may be needed for one insemination dose. Various laboratories may recommend different insemination doses, as no standard dose is currently recognized. The most effective insemination dose and the average rate of pregnancy are related to when the mare is inseminated, where the semen is placed, and how well an individual stallion’s semen has performed in the past.

Insemination with frozen-thawed semen should be done up to 12 hours before ovulation or within six to eight hours after ovulation for the best chance of fertilization. If a veterinarian is able to examine the mare every six hours after she has been given an ovulatory induction agent, the practitioner can determine whether ovulation has occurred and can inseminate the mare as soon as possible after it has taken place.

A technique known as deep-horn insemination has been shown to result in acceptable rates of pregnancy and may require fewer sperm to be used compared to depositing semen in the body of the uterus. In deep-horn insemination, semen is positioned at the tip of the uterine horn adjacent to the ripe follicle, with placement being done prior to or following ovulation. Rates of pregnancy using this method average 30 to 55% within a total range of 0 to 70%.

Frozen-thawed semen from some stallions performs better and produces a higher rate of pregnancies than that of other stallions. It is helpful to know the level of progressively motile sperm in frozen-thawed semen from a particular stallion. If an owner is considering a stallion whose progressively motile sperm level is known to be low, the owner might want to order fresh or chilled semen from the stallion if these preservation methods have been more successful. Another option would be to use a larger insemination dose from the chosen stallion, though this would involve a greater expense.

Aspiring breeders should check with a veterinarian who is experienced in artificial insemination and get recommendations on the choice of laboratory, method and placement of insemination, and use of an ovulatory induction agent.