Stallion fertility: what you need to know


Before choosing a stallion for his mare, any breeder or future breeder establishes various criteria that will influence his final decision, and the stallion's fertility is an important element that directly conditions the success of a breeding project. There are many aspects to stallion fertility, and it is advisable to be aware of them so that you can make an informed choice.

reproduction étalon Source: IFCE.


I. Variation factors in stallion fertility.

II. The impact of breeding techniques.

III. Predicting the fertility of a stallion.

IV. Calculation of stallion fertility.

  1. Fertility by heat.
  2. The stallion's end-of-season fertility.
  3. Apparent fertility.



I. Variation factors in stallion fertility

A stallion's fertility is his ability to fertilise mares, and several factors will have an influence on fertility: the breeding technique chosen, the quality of the stallion's semen, and of course, the mare's fertility (article to come on mare fertility!).


Several factors influence the quality of the semen: 

  • the stallion's breed: on average, heavy breeds stallions have lower semen quality than blood stallions, which include sport horses.
  • age: stallions reach sexual maturity around the age of 5 (between the ages of 4 and 5, the size of their testicles increases, as does the quantity of sperm produced). Between 5 and 12 years of age, there is a slight increase, then a plateau until the age of 15. Between 15 and 20 years of age, semen quality declines.
  • individual variations: the stallion's physical and mental health have a direct impact on the quality of the semen produced.
  • the stallion's medication: certain treatments/medications can have an impact on semen quality
  • collection conditions: the greater the sexual excitement, the greater the volume of semen produced (which reduces sperm concentration).
  • frequency of collection: the quality of semen from a stallion collected regularly will on average be better than that from a stallion collected occasionally. However, it will take 10 days of consecutive collection for the sperm count to stabilise. 
  • the season: sperm production is higher during the breeding season than in winter. During the breeding season, the volume of ejaculate is 40% higher than in December, the volume of gel (sexual arousal) is also increased, as is seminal plasma (sperm concentration is therefore reduced).


II. The impact of breeding techniques

There are 6 different covering techniques (more information on each technique in our article on the different breeding techniques):

  • Frozen semen: the success rate is around 45% on average for insemination with 8 straws (400 million spermatozoa). With a single straw, the success rate falls to 25%.

Good to know: the mobility of sperm in the same ejaculate can vary from 19% to 63%, which is why it's a good idea to buy 8 straws to compensate for this heterogeneity. Buying a single straw increases the chances of finding sperm with very low motility.

  • transported chilled semen: the success rate is 50% (and if used 24 hours after collection, the rate is reduced to 30%).
  • fresh semen: the success rate is 55%.
  • Hand-breeding: the success rate is also 55%.
  • Natural breeding: the success rate is 90%.


III. Predicting the fertility of a stallion

The spermogram is an examination of the seminal characteristics of the stallion: the behaviour of the stallion, the quantity and quality of the spermatozoa and the activity of the adnexal glands are observed. 

Performing a spermogram on a stallion intended for reproduction will provide the stud farmer with information on a number of points:

  • sperm motility: healthy motility corresponds to 60/70% of sperm able to move in a straight line. Less than 50% motility is considered problematic.
  • the number of spermatozoa in an ejaculate: although there are variations depending on age, testicle size, season, etc., ideally you want a concentration of 10 to 20 billion spermatozoa per ejaculate.
  • testicular size, which changes with age but also within the same year
  • the structure of the spermatozoa: 50% of them must have a healthy structure
  • a blood test to test the stallion for EVA (equine viral arteritis), among other things
  • a bacteriological analysis

Please note that the spermogram is not used to judge a stallion's fertility, but to predict its suitability for use as a breeding stallion, by placing it within a reference population for each criterion.


IV. Calculation of stallion fertility

1. Fertility by heat

The best indicator of a stallion's fertility is the heat fertility calculation. This is the percentage chance a stallion has of fertilise a mare in each cycle used. The heat fertility calculation can be calculated at any time, but only becomes very accurate after 150 cycles. Calculating fertility by heat requires at least 20 cycles with known results.

According to the IFCE*:

Number of heats fertilised (1)
---------------------------------------------    X 100
Number of heats with known result (2)


1. This is the number of heats followed by a pregnancy, including early gestations followed by early embryonic loss.
2. This total does not include heats without information (i.e. heats used for which the result is unknown).


2. The stallion's end-of-season fertility

It is calculated at the end of the season or in the autumn. It is the percentage chance that the mare will be in foal at the end of the season.


According to the IFCE*, it is calculated as follows:

Number of mares in foal at the end of the season + No. of MWI(3) X (Fertility per heat)
---------------------------------------------------------------------------    X 100
Total number of mares covered 

3. MWI: mare without information.


Good to know:

A stallion can have very good heat fertility and poor end-of-season fertility. In reality, calculating heat fertility provides an indication of the stallion's fertility, while calculating end-of-season fertility provides information on the quality of the monitoring of the mares fertilised

For example: if a stallion has a heat fertility of 50%, he has impregnated 50 mares out of 100 during his first heat. If the 50 mares left empty after their first cycle are inseminated again, 25 of them will be in foal, which will increase the stallion's fertility rate to 75%.


3. Apparent fertility

Apparent fertility is calculated as follows:

Number of births reported + number of abortions reported
--------------------------------------------------------------------- X 100
Total number of mares covered




  • Spermogram results are a good indicator of the stallion's intrinsic fertility potential.
  • Fertility by heat is also interesting, but is already biased by the fertility of the mare, the breeding technique, the level of expertise of the inseminator, etc. 
  • Fertility by season and apparent fertility are very biased by the management of the mares and their own fertility.


*IFCE : Institut Français du Cheval et de l’Equitation.