Intra- and inter-stallion variation in sperm morphology and their relationship with fertility

Dismount semen samples were collected from Thoroughbred stallions during the breeding season to determine the magnitude and significance of the variation in sperm morphology within (intra-) and among (inter-) stallions. The degree of intra-stallion variation in sperm morphology during the breeding season was substantial (coefficient of variation: 20%; range 8-56%), but did not affect the day-to-day fertility of sperm samples taken from the same stallion (intra-stallion). This variability did not affect the pregnancy rate of the mares with which each stallion was mated. The degree of interstallion variation in sperm morphology (coefficient of variation: 38%; range 22-79%) was greater than the degree of intra-stallion variation, with percentage normal spermatozoa, abnormal heads, detached heads, general mid-piece abnormalities, coiled tails and premature germ cells all being significant predictors of fertility. These results indicate that certain morphological features of spermatozoa (percentage of normal spermatozoa, abnormal or detached heads, mid-piece abnormalities, coiled tails and premature germ cells) influence the success of pregnancy establishment and may be used to predict differences in fertility among stallions.