Breeding a mare, especially one that is considered “older,” is a significant decision for horse owners. Many owners approach their veterinarians with concerns about their mare’s age and reproductive capabilities, particularly if her breeding history is unknown. Understanding the factors influencing fertility in older mares is crucial for making informed decisions.
Generally, healthy mares in good nutritional condition can be bred as early as two years old, though waiting until three is often recommended.1 Mares can remain productive well into their late teens or early to mid-twenties. However, fertility in older mares naturally declines with each passing year. A mare that has recently given birth has a better chance of conceiving again compared to a mare of similar age who has not conceived despite being bred multiple times. It’s also important to note that older mares who have never been bred before can be particularly challenging to get in foal.2
The Importance of a Breeding Soundness Exam (BSE)
The most effective way to assess a mare’s reproductive health and potential is through a Breeding Soundness Exam (BSE) conducted by a veterinarian. This examination typically involves rectal palpation combined with ultrasound to evaluate the uterus and monitor ovarian activity. As mares age, they may experience changes in their estrus cycles, such as a decrease in frequency or increased irregularity. Palpation can help identify the presence of follicles, cysts, and other ovarian structures, both normal and abnormal.
For mares with limited reproductive history, a uterine culture and biopsy can provide valuable insights. A uterine culture can detect subclinical infections that may impair fertility, while a biopsy can reveal microscopic details of the uterine lining (endometrium) and identify abnormalities not detectable by palpation.3
A comprehensive evaluation for breeding suitability should also include a thorough review of the mare’s general medical and management history, along with a detailed reproductive history.
General History should include:
- Age
- Purchase date
- Performance history
- Serologic tests
- Vaccination history
- Boarding facilities
- Feed
- Previous use
- Intended use
- Medical history
- Surgical history
- Disease problems
- Weight loss or gain
Reproductive History (if available) should include:
- Age at first heat
- Heat dates
- Interval between heats
- Length of heats
- Age first bred
- Breeding dates
- Foaling dates
- Date of last foaling
- Abnormal or assisted foalings
- Number of pregnancies
- Abnormal pregnancy
- Previous year’s breeding cycle pattern
- Number of breedings for conception
- Evidence of vaginal discharge
- Mothering ability
- Milk production
- Teasing method
- Breeding method (pasture, hand breeding or artificial insemination)4
Breeding Specifics for the Older Mare
Peak fertility in horses is typically observed between 6 and 7 years of age. Fertility starts to decline around 15 years old, making it harder to achieve pregnancy and increasing the risk of pregnancy loss. A young, reproductively sound mare has a 50% to 60% chance of conceiving in a given estrous cycle when bred to a fertile stallion. In contrast, an older mare might have a 30% to 40% chance or even less per cycle.3 Older mares may require more estrous cycles, on average, to become pregnant than their younger counterparts.
It is advisable to have a veterinarian perform a reproductive evaluation on an older mare before the breeding season commences. If an aged mare has no prior history of infertility, the examination might be limited to an ultrasound and a uterine culture. However, if the mare has been infertile for several years, the evaluation should be more extensive, including an assessment of the perineal anatomy (vulvar angle and muscle tone), a vaginal speculum examination, digital examination of the cervix, uterine cytology and culture, an endometrial biopsy, and an ultrasonographic evaluation of the reproductive tract.
Older mares can develop several clinical conditions that negatively impact fertility. These may include poor perineal conformation, a heightened susceptibility to uterine infections and persistent post-mating inflammation, increased uterine scarring and endometrial cyst formation, a higher incidence of ovulation failure, and other contributing factors.3
References
- Harman, A., DVM. Appropriate Breeding Age. Equisearch, Aug. 2011.
- McCue, P., DVM. Breeding the Older Mare. Colorado State University American College of Theriogenologists, Equine Reproduction Laboratory, 2007.
- O’Brien, A., DVM. How Old is Too Old to Breed a Mare? Horse Illustrated, 26 Feb. 2019.
- Troedsson, M.H.T., Breeding Soundness Examination of the Mare. USDA National Institute of Food and Agriculture Extension, Jan. 2020, 1-3.
