Is My Mare Too Old to Breed? The Older Mare Work-Up

Is a mare of 12, 15 or 18 too old to breed?

Mares cycle and conceive into their twenties. What changes with age is not the ovary so much as the uterus. Every foaling exposes the lining to the bacteria of the lower tract, and every year takes a little from the muscle tone and the lymphatic drainage that push fluid and bacteria out again. From about ten years, and earlier in a mare with several foals, the uterus clears less well, and that is what shows up as fluid after covering, a short return to estrus, an early loss, or simply an empty scan.

So the honest answer to "is she too old" is: probably not, but she needs a better work-up than a young mare, and she needs it before the season is spent on repeat covers.

What age does to the uterus

Three things, and they can be told apart:

• Delayed clearance. The uterus holds fluid after covering instead of emptying within about 48 hours. This is treatable in the cycle: lavage and oxytocin, timed on the scan. More in fluid in the mare's uterus after breeding.
• Degeneration of the lining. Fibrosis and cystic glands accumulate. A biopsy grades this as a Kenney score, and the score tells you what the uterus can still carry. A IIB uterus will not become a I again; the point of knowing is to decide how much to invest, and to treat everything else that is treatable.
• Chronic, hidden infection. This is the one that is missed, and the one that can be put right. It builds up over the years; in the 2025 Copenhagen thesis on subfertile mares, the mean age rose with the biopsy grade, from about 8 years in the healthiest group to about 15 years in the group with chronic degenerative changes (unpublished student thesis, 15 mares).

The older maiden mare

A special case, and the mare breeders write about most: bred for the first time at 12 or 14, often with frozen semen, fluid after covering, a tight cervix, flushed every cycle, and sent home with a clean culture and cytology and no pregnancy. Maiden status does not protect the uterus; age still lowers its clearance, and frozen semen provokes a stronger inflammation than fresh. She deserves the same work-up as a barren mare with foals.

Why the swab lets the older mare down

A negative swab in an older mare that stays empty is the most misleading result in breeding. Culture from a swab found 34 % of infected mares in a direct comparison; culture from a biopsy found 82 % (Nielsen, Theriogenology 2005). In chronically infected mares the bacteria sit in foci 300 to 500 micrometers below the epithelium (Petersen et al., Clinical Theriogenology 2009), and part of them are dormant, in a state where they do not grow in culture. After activation with a culture medium the organism was detectable in 64 % of mares, after saline in 8 % (Petersen et al., Veterinary Microbiology 2015).

That is exactly the older mare's infection: acquired with a foaling years ago, never cleared, quiet, and invisible to the routine check.

What the older problem mares in the series looked like

At Kildangan Stud in Ireland, 19 Thoroughbred problem mares with an average age of 12.7 years (range 6 to 21), barren for 1.4 years and 6.3 cycles on average, were activated with bActivate and treated on the culture result. 84 % were activation-positive, and 89 % of the 18 mares that were covered became pregnant, at 1.1 covers per mare (Petersen et al., Journal of Equine Veterinary Science 2018; internal clinical data, no placebo group).

At Hagyard in Kentucky, 64 Thoroughbred problem mares barren for at least three cycles were examined the same way: 47 % culture-positive after activation, 83 % pregnant, 70 % of those with a known outcome foaled, again without a placebo group (Petersen and Bojesen, Clinical Theriogenology 2014). One veterinarian using bActivate has reported a mare that conceived at 24 and foaled at 25.

The work-up for an older mare, in order

1. Timing and semen first. Ovulation scans against the cover, and the stallion's results with other mares. Cheap to check and often the answer in a young mare; rarely the whole answer in an older one.
2. Scan for fluid before and 12 to 24 hours after covering.
3. Biopsy. Culture, cytology and histology from one sample: it finds more infections than the swab and it gives the Kenney score.
4. Activation culture if the sample is clean and she has failed two or more cycles, or if she has fluid, a previous endometritis, three or more foals, or is ten or older. The vet instills 10 ml of bActivate in early estrus, cultures again after 48 hours, treats on the antibiogram, and covers her in the following cycle. bActivate is a diagnostic aid, not a treatment.
5. Then decide. With a clean uterus and a I or IIA biopsy, breed her. With a IIB and a cleared infection, breed her with realistic expectations. With a III, consider embryo transfer.

Frequently asked questions

At what age do mares stop getting in foal?
There is no fixed age. Mares conceive into their twenties. Pregnancy and foaling rates fall from about ten years because the uterus clears fluid and bacteria less well, not because the mare stops cycling.

Should an older mare be bred with frozen semen?
It can work, but frozen semen provokes a stronger post-breeding inflammation, so an older mare should be scanned and, if needed, lavaged within 24 hours of insemination. If she holds fluid every cycle, have the uterus examined before spending another dose.

What does a IIB biopsy score mean for an older mare?
Moderate chronic changes in the lining. She can still carry a foal, with a lower chance than a I or IIA. The score does not change, but any infection that is found alongside it can be treated, and that is what improves her odds.

Is bActivate for old mares?
It is for problem mares of any age, and older mares are the largest group among them. It does not treat age; it finds the dormant infection that age lets accumulate.

Sources

• Nielsen JM. Theriogenology 2005;64:510-518.
• Petersen MR et al. Clinical Theriogenology 2009;1:393-409.
• Petersen MR et al. Veterinary Microbiology 2015;179:119-125.
• Petersen MR, Rosenbrock A, Osborne M, Bojesen AM. High prevalence of subclinical endometritis in problem mares: effect of activation and treatment on fertility. Journal of Equine Veterinary Science 2018;66:117 (Kildangan).
• Petersen MR, Bojesen AM. Clinical Theriogenology 2014;6(3):313-314 (Hagyard).
• Hjenner EK, Ravn KK. University of Copenhagen DVM thesis, 2025 (unpublished).

Written by the Bojesen & Petersen Biotech ApS team. Medical oversight: Prof. Anders Miki Bojesen DVM PhD (University of Copenhagen) and Dr. Morten Rønn Petersen DVM PhD Dipl. ACT.