The selection of the stallion is commonly done on the basis of pedigree and performance, with little consideration being given to the individual's reproductive capability. The most reliable measure of fertility in a stallion is the number and percentage of mares he settled in a season. These figures are not always available. Additionally, previous history will not be possible in a rookie stallion. At this point I would like to address stallion anatomy and how it correlates to conception and fertility.
Most breeders only consider the first and last.
Additionally, the reproductive tract is supported within the pelvic cavity by the hammock like genital fold, and externally by the scrotum. These processes depend on and are coordinated by the neuroendocrine system. The process from the start of the creation of the sperm cell to final maturation in the epididimal tail is approximately 60 days. This means whatever a stallion ejaculates in February was begun in December. If an injury or infection, or any occurrence that would raise the temperature of the testes 2 degrees were to happen today, this stallion would still be fertile tomorrow, but would have the possibility of being infertile temporarily in 2 months.