This dictionary is meant for informational purposes only to help explain common terminology used in equine reproduction and is not meant to be cited as a reference source.

Artificial insemination – The process of breeding a mare by putting live spermatozoa (either using cooled or frozen semen) into the uterus.

Colostrum – The thick, yellowy milk present at birth for a foal that is rich in maternal antibodies and is essential for passive immunity to the newborn foal.

Cooled Semen – Semen from a stallion that has been collected and processed for breeding a mare. May be shipped overnight for mares located at a different breeding facility than the stallion.

Corpus Luteum (CL) – The progesterone-secreting mass on the ovary formed from a follicle that has ovulated.

Donor Mare – The valuable client owned mare that is bred to pass on her genetics either via carrying the pregnancy herself or through embryo transfer.

Embryo – The earliest stage of development after an egg is fertilized by a sperm cell and then may grow into a fetus.

Embryo transfer (ET) – Collection of fertilized ova from one female (donor mare) before they become implanted and transfer to another female (recipient mare) to complete the gestation.

Failure of passive transfer – When the newborn foal does not absorb the maternal antibodies in the mare’s colostrum from its gut into its bloodstream and does not get the passive immunity essential to its protection from viral and bacterial pathogens in the environment.

Fetus – The developing young while in the uterus after the embryonic period and after major anatomical structures have been outlined.
Follicle – The saclike structure on an ovary that holds the egg until ovulation.

ICSI (Intracytoplasmic Sperm Injection) – A form of in vitro fertilization where an egg is aspirated from the donor mare and is then injected with one spermatozoon to create an embryo which can then be transferred to a recipient mare to achieve a pregnancy.

Frozen Semen – Semen from a stallion that has been collected and processed in straws for breeding of a mare. Frozen semen is kept in liquid nitrogen until the mare is ready to be bred.

IgG – Immunoglobulin G, an antibody used to measure passive transfer in the foal

Mare – A female horse, 4 years or older.

Ovary – The female gonad, the round structures in which the ova are formed and which the sex hormones, estrogen and progesterone, are released.

Ovum – An egg, which after fertilization is capable of growing into an embryo.

Ovulation– The rupture and collapse of the follicle releasing the egg for fertilization.

Progesterone – A steroid sex hormone that is the primary hormone in maintaining pregnancy in the horse.

Recipient (Recip) Mare – The mare that is used to transfer an embryo into in order to carry the pregnancy of the donor mare and the stallion the donor mare was bred to. The recipient mare does not pass on any genetics to the foal she carries.

Regumate – The brand name of a common FDA approved form of altrenogest, which is often used to supplement pregnant mares that are producing lower than desired levels of progesterone.

Vitrification – Freezing of an embryo so that it may be stored in liquid nitrogen until a later date when it may be transferred.