Merck Manual

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Pet Owner Version

Infertility in Horses


Patricia L. Sertich

, VMD, DACT, Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania

Reviewed/Revised Sep 2019 | Modified Oct 2022

Proper ovulation in females and ejaculation of fertile and normal sperm by males are regulated through a sequence of events in the brain, nervous system, and sexual organs. For optimal results, ovulation and deposition of semen into the female genital tract must be closely synchronized. Failure of any single functional event in either sex leads to infertility or sterility. The ultimate result of infertility is the failure to produce offspring. In females, infertility may be due to the absence of the estrous cycle, abnormal ovulation, failure to conceive, failure of normal fetal development, or prenatal death. Major infertility problems in males are caused by disturbances in the production, transport, or storage of sperm; loss of libido; and partial or complete inability to mate. Most major infertility problems are complex; several factors, singly or in combination, can cause failure to produce offspring.

Your veterinarian will first attempt to determine whether it is the mare or the stallion that is infertile. Infertility can be diagnosed through laboratory tests, semen evaluation, and ultrasonography. Infertility is seldom accompanied by obvious signs of illness or infection. Often, infertility is caused by the mare not being bred with sufficient amounts of good quality semen at the correct time during her estrous cycle.

Infertility may be helped by the administration of hormones that act directly on the ovaries or regulate their functions, or act to help maintain pregnancy. Antibiotics are used for treatment of infection of the reproductive tract. The selection of the antibiotic is based on tests that determine the nature of the bacteria or infectious agent. Unfortunately, there are few medications that can cause a stallion to make better quality semen. However, good intensive management of both the stallion and the mare can help in producing offspring.

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