What an Empty Season Costs: The Breeder's Calculation
A mare that does not get in foal costs more than the invoices that arrive for her. She costs a year of keep without a foal, a foal that comes a year later or not at all, and with every further cycle a new stud fee, new semen, new veterinary visits. This post does the sums honestly: what an empty season costs, what it costs to find the cause, and what the field data say. No promises, with sources.
What does a year without a foal cost?
The sum is different on every farm, so here are the items that belong in it rather than a ready-made total. Fill in your own numbers:
| Item | What it depends on | Your figure |
|---|---|---|
| Keeping the mare, 12 months | Grass or box, feed, farrier, worming, vaccinations | |
| Stud fee or semen, per attempt | Stallion, fresh or frozen semen, shipping | |
| Veterinarian per cycle | Follicle checks, insemination, pregnancy scan | |
| Transport and boarding | If the mare goes to the stallion or the clinic | |
| Number of attempts in the season | Usually two to four cycles before the season is over | |
| The foal that is missing | Sale value or breeding value, a year later or not at all |
Two things stand out once the table is filled in. First, the veterinary costs are rarely the largest item; the keep and the missing foal are. Second, every further cycle without a diagnosis costs the same as the one before, with one season less left.
Why does the cause so often go undetected?
Because the standard swab often misses the most common cause. Uterine inflammation without symptoms, subclinical endometritis, is the most common reason a mare stays empty, and in a direct comparison swab culture found only 34% of uterine infections while biopsy culture found 82% (Nielsen, Theriogenology 2005). A clean swab is therefore treacherous for the calculation: it costs little, it reassures, and it sends the mare into the next expensive cycle with nothing changed. More in why a clean swab does not rule out infection.
What does it cost to find the cause?
Prices differ by practice, laboratory and country, but the items are few:
- Swab with culture and cytology. The sampling by the veterinarian, usually together with an ultrasound scan, the laboratory culture and, if positive, the antibiogram.
- Activation culture. When the swab is clean and the mare still stays empty. One vial of bActivate costs 229 EUR per vial (about 265 USD at the current exchange rate, charged in EUR) from the manufacturer's online shop (September 2026); veterinarians order it through the trade. Add the veterinary work: instilling the medium, the swab 48 hours later, and the culture with antibiogram. bActivate is a diagnostic growth medium, not a medicine; it makes dormant bacteria visible in culture.
- Treatment, if something is found. For streptococci, the organism behind 79.7% of positive cultures in a German analysis of 28,887 samples, penicillin was effective in 99.5% of cases (Köhne et al., 2024). That is a standard antibiotic chosen from the antibiogram, not a reserve drug.
Set that against a single further cycle of stud fee, semen and veterinary work. On most farms the work-up costs less than the next attempt in the dark, and it is the only item on the list that changes the starting position.
What do the field data say?
At Hagyard Equine Medical Institute in Kentucky, 64 Thoroughbred problem mares that had been barren for at least three cycles were examined. 47% (30 of 64) were culture-positive after activation, 83% (53 of 64) became pregnant after targeted treatment and breeding, and 70% of the mares with a known outcome (32 of 46) delivered a live foal. The study had no placebo group; the figures describe the whole sequence of diagnosis and treatment, not the effect of a single product (Petersen and Bojesen, Clinical Theriogenology 2014). Every result is on the clinical evidence page. For comparison, the expected foaling rate for problem mares has historically been 15 to 50%, against 80 to 85% in the general population (Bosh et al., 2009).
For the calculation this means: in roughly every second problem mare, activation found an infection the standard route had missed. In the other half, the work-up ruled out a cause, and the search could move on without losing another cycle.
When is the work-up worth it?
Not after the first empty cycle, when timing or semen are the obvious explanation. But at the latest when several of these apply:
- Two or more well-timed coverings with proven semen, and the mare is empty.
- Pregnancy confirmed and lost before day 40.
- Fluid in the uterus on ultrasound.
- Clean swab, and the mare still does not get in foal.
- Ten years or older, or several foals.
Whoever clears these points in spring has the whole season for breeding. Whoever keeps trying until autumn has paid the sum above and starts next year in the same place. How to recognize a problem mare and in which order the veterinarian proceeds is in the problem mare: why she stays empty and what to check first.
What the calculation does not show
Three things cannot be entered in the table and still weigh: a foal a year earlier rather than a year later; a mare that does not become more chronic with every untreated season, because in chronically infected mares the bacteria sit 300 to 500 micrometers deep in the tissue and get harder to reach with time; and fewer antibiotics in the dark, because treatment follows an antibiogram rather than a hunch.
Frequently asked questions
Is bActivate expensive?
One vial is 229 EUR per vial (about 265 USD at the current exchange rate, charged in EUR) from the manufacturer's online shop, or through Hagyard Pharmacy and Midwest Veterinary Supply (September 2026), plus your veterinarian's fees. Compare that with your own figure for one more cycle of stud fee, semen and veterinary work.
Do I have to buy bActivate myself?
No. Your veterinarian orders it through the veterinary trade and instills it. What the vet needs is on the how to use page.
What if the activation culture is negative?
Then a bacterial uterine infection is unlikely, and the veterinarian looks elsewhere: cycle, semen, anatomy, hormones. The money has ruled out one cause before the next cycle was paid for.
Does the work-up guarantee a foal?
No. The Kentucky figures come from a field study without a placebo group. They show what happened to 64 problem mares after diagnosis and targeted treatment, not what will happen to your mare.
Sources
- Nielsen JM. Endometritis in the mare: a diagnostic study comparing cultures from swab and biopsy. Theriogenology 2005;64:510-518. PMID 15978661
- Petersen MR, Skive B, Christoffersen M, Lu K, Nielsen JM, Troedsson MHT, Bojesen AM. Activation of persistent Streptococcus equi subspecies zooepidemicus in mares with subclinical endometritis. Veterinary Microbiology 2015;179:119-125. DOI 10.1016/j.vetmic.2015.06.006
- Köhne M, Hegger A, Tönissen A, Heusinger A, Hader C, Görgens A, Sieme H. Frequency of potentially pathogenic bacterial and fungal isolates among 28,887 endometrial samples from mares. 2024.
- Petersen MR, Bojesen AM. Equine subclinical endometritis caused by dormant beta-hemolytic streptococci. Clinical Theriogenology 2014;6(3):313-314 (congress abstract, 64 problem mares at Hagyard Equine Medical Institute, no placebo group).
- Bosh KA, Powell D, Shelton B, Zent WW. Reproductive performance measures among Thoroughbred mares in central Kentucky, during the 2004 mating season. Equine Veterinary Journal 2009;41:883-888. PMID 20383985
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.
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