Determination of the relationship between sperm morphologic classifications and fertility in stallions: 66 cases (1987-1988).

The analysis of breeding records and sperm morphologic classifications from ejaculated semen during 99 stallion seasons, over a 2-year period, revealed a significant correlation (r = 0.34, P less than 0.01) between the percentage of morphologically normal sperm in ejaculates and the per cycle fertility estimate of the stallions studied. In addition, the percentage of sperm classified as having major defects (abnormal heads, proximal droplets, and abnormal midpieces) was significantly inversely correlated (r = -0.36, P less than 0.01) with the same fertility estimates. Multiple variable regression demonstrated that the variation in 2 morphologic features classified as major defects, abnormal heads, and proximal droplets, accounted for the largest amount of variation in fertility. It appears that in stallions, a large percentage of ejaculated sperm with major defects or other defects in combination with major defects is associated with a larger reduction in fertility than is associated with other defects.