As horse owners, the desire to continue a mare’s lineage or introduce new bloodlines often leads to the question of age and breeding suitability. Many owners find themselves consulting their equine veterinarians or trusted experts with concerns about breeding an older mare, especially if her reproductive history is unknown. This guide aims to provide a comprehensive overview of breeding considerations for mares of advanced age, ensuring informed decisions for both mare and foal.
Understanding Mare Fertility and Age
Generally, a healthy filly with proper nutrition can be bred as early as two years of age, though many breeders opt to wait until three years old. Mares can remain reproductively active into their late teens and early to mid-twenties. However, it’s crucial to understand that 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 multiple breeding attempts. It’s also important to note that a maiden mare, one who has never been bred before, may present challenges. Older maiden mares can be notoriously difficult to get in foal.
Essential Pre-Breeding Evaluation: The Breeding Soundness Exam (BSE)
The most critical step in assessing an older mare’s reproductive capacity is a comprehensive Breeding Soundness Exam (BSE) performed by a veterinarian. This examination typically includes rectal palpation and ultrasound, allowing the veterinarian to evaluate the health of the mare’s uterus and monitor her ovarian activity. As mares age, they may experience a decrease in the frequency or regularity of estrus (heat) cycles. Palpation can identify the presence of follicles, cysts, and other ovarian structures, both normal and abnormal.
Depending on the mare’s reproductive history, a uterine culture and biopsy may also be recommended. A uterine culture can detect subclinical uterine infections that could impair fertility. A uterine biopsy provides a microscopic view of the endometrium (uterine lining), potentially revealing abnormalities not detectable through palpation.
When evaluating any mare for breeding, a thorough understanding of her general medical and management history, alongside a detailed reproductive history, is paramount. A general history should encompass:
- Age
- Purchase date
- Performance history
- Serologic tests
- Vaccination history
- Boarding facilities
- Diet and nutrition
- Previous and intended use
- Medical and surgical history
- Past disease problems
- Any significant weight loss or gain
The reproductive history, if available, should include:
- Age at first heat
- Dates of heat cycles
- Interval between heats
- Duration of heats
- Age at first breeding
- Breeding dates
- Foaling dates
- Date of the last foaling
- Any abnormal or assisted foalings
- Number of pregnancies
- Any abnormal pregnancies
- The previous year’s breeding cycle pattern
- Number of breedings required for conception
- Evidence of vaginal discharge
- Mothering ability
- Milk production
- Teasing method used
- Breeding method employed (pasture, hand breeding, or artificial insemination)
Specific Considerations for Breeding Older Mares
Peak fertility in horses is generally observed around 6 to 7 years of age. Fertility begins to decline noticeably around 15 years of age, making older mares more challenging to get in foal and increasing the risk of pregnancy loss. A young, reproductively healthy mare has a 50-60% chance of conceiving during a given estrous cycle when bred to a fertile stallion. In contrast, an older mare might have a pregnancy rate of 30-40% or less per cycle. Aged mares may require more estrous cycles on average to achieve pregnancy compared to younger mares.
It is highly recommended that a veterinarian conduct a reproductive evaluation on an older mare before the commencement of the breeding season. If the 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 barren for several years, a more extensive evaluation is warranted. This should include an assessment of her perineal anatomy (the angle and muscular tone of the vulva), a vaginal speculum examination, digital examination of the cervix, cytology and culture of the uterus, an endometrial biopsy, and a comprehensive ultrasonographic evaluation of the reproductive tract.
Older mares are more susceptible to developing clinical issues that can negatively impact their fertility. These can include poor perineal conformation, a heightened predisposition 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.
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.
