The breeding stallion fertility


Fertility is a factor determining the productivity and economic performance of a stud farm. It must be rigorously evaluated at the level of the stallion and the mare, but also at the scale of the broodmare band and the farm in order to help the breeder make the most pertinent choices. Semen analysis is used to track changes in semen quality from purchase of the stallion through to its retirement.

How can fertility be improved?

A breeding method is chosen in accordance with the requirements of the breed and the semen analysis.

There are six main breeding methods:

  • pasture breeding: 90% of mares pregnant at the end of the season,
  • hand breeding: per cycle pregnancy rate 55%,
  • artificial insemination (AI) with fresh sperm: immediately after sperm collection, per cycle pregnancy rate 55%,
  • AI with chilled sperm: on the same day, sperm transported or not, per cycle pregnancy rate 50%,
  • AI with frozen sperm: per cycle pregnancy rate in the order of 45%,
  • AI with chilled semen: transported and used 24 hours after collection, per cycle pregnancy rate 30% (wide variations between stallions).

Whenever possible, the breeder must always select the technique enabling the highest pregnancy rate to be attained. Modern insemination techniques enabling semen to be transported are more practical for the breeder but have lower pregnancy rates … and besides, not all stallions are suitable.

Calculation methods and their meaning

For the stud farmer

The best way for the stud farmer to ascertain a stallion’s fertility is to calculate his per cycle pregnancy rate: this represents the percentage chance of it fertilizing a mare for each cycle bred.

It is calculated as follows: number of cycles resulting in a pregnancy* / total number of cycles bred** x 100

*The number of cycles bred resulting in a pregnancy including early pregnancies followed by an early embryo loss.
** the cumulative total of all the cycles bred (via covering or artificial insemination) using all the mares allocated to the stallion.

For the stallion, the per cycle pregnancy rate varies mainly as a function of:

  • semen quality
  • breeding method used.

A stallion is bred using AI with fresh sperm: his semen is available on-site, in the collection center and also all over France.

His per cycle pregnancy rate is:

  • On average 50% for all of the mares inseminated at the collection center (a one-in-two chance of the mare being fertilized on a given cycle)
  • On average 33% for all of the mares inseminated with semen that has been kept and transported (a one-in-three chance of the mare being fertilized on a given cycle).

WARNING: the per cycle pregnancy rate is only reliable if it is calculated on the basis of at least 20 recorded cycles.

Example: A stallion has achieved 10 positive results (early pregnancies), one cycle resulting in early resorption, and 17 negative results (negative pregnancy diagnosis). The result of a further 5 cycles that are still ongoing is unknown (no information available).

The per cycle pregnancy rate is: (10+1)/28×100= 39.3%

For the breeder

The best way for the breeder to ascertain a stallion’s fertility is to calculate the number of foals declared per stallion. This is the “apparent fertility” per stallion published by the French SIRE (Information System Relating to Equestrian Breeds).

It is calculated as follows: no. of births declared + no. of abortions declared / no. of mares covered x 100

This figure varies depending on:

  • the end of season pregnancy rate,
  • the rate of losses during pregnancy (early embryo loss 9% and abortion 9% on average),
  • the foal mortality rate (12% between 0 and 3 months on average).

Example 1: Taking into account the losses mentioned above, a stallion with an end of season pregnancy rate of 85% will have:

  • 70% of mares carrying to term,
  • an apparent fertility of approx. 62%

Example 2: A stallion producing 35 births and 3 declared abortions in 2013 having covered 60 mares in 2012 has an apparent fertility of (35+3)/60×100 = 63.4%.