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.
A breeding method is chosen in accordance with the requirements of the breed and the semen analysis.
There are six main breeding methods:
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.
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:
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:
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%
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:
Example 1: Taking into account the losses mentioned above, a stallion with an end of season pregnancy rate of 85% will have:
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%.