Q: When is a stallion most fertile?
A: Fertility in stallions is highest from six years of age until they are somewhere between 12-15 years of age. After this fertility tapers off through their late teens and drops again more dramatically once they are over 20.
It should be noted, however, that this does vary greatly among individual stallions. Nonetheless it is recommended that if you want to breed with an older stallion, you should use a younger mare and if you want to breed with an older mare, you should use a younger stallion.