Characteristics and fertility of stallion semen

A study of stallion fertility was conducted on commercial studs in eastern Australia over 4 breeding seasons (1974/75-1977/78). Data from 47 stallions (66 stallion seasons) aged 2-26 years and representing 7 breeds and 1664 mares were used to relate seminal characteristics to fertility. Percentage pregnancies per service was the most sensitive measure of fertility with stallions on commercial studs. Breed and age of stallion, breeding season and the referral status of the stallion ('suspect' or 'normal') had significant (P less than 0.01) effects on fertility. The semen characteristics that were clearly associated with percentage pregnancies per service were: total volume, gel-free volume, sperm concentration, total numbers of spermatozoa and total number of live spermatozoa. Approximate threshold levels for these characteristics are presented which could provide a diagnostic basis for the classification of stallions as being suitable or doubtful for breeding purposes.