Sperm population structure in high and low field fertility rams

The aim of the present study was to investigate whether differences in field fertility of rams are reflected in differences in sperm morphometric and kinematic population structures. The association between sperm morphometric and kinematic subpopulations was also investigated. Ejaculates from 8 adult rams, 4 with high and 4 with low field fertility, were collected weekly using an artificial vagina over 6 consecutive weeks. Analyses of sperm motility using computer-assisted sperm analysis (CASA) and sperm nuclear morphometry using computer-assisted sperm morphometry-fluorescence were performed. Clustering procedures using the kinematic and morphometric data from high and low field fertility rams resulted in the classification of spermatozoa in three kinematic and three morphometric sperm subpopulations. The distribution of subpopulations between rams of high and low field fertility was significantly different (P < 0.05), with higher percentages of spermatozoa exhibiting fast and linear movements and those with large and long nuclei in the high fertility group. However, these subpopulations were not correlated. Logistic regression analyses were also performed to evaluate the relative utility of sperm subpopulations to classify rams in high and low field fertility. Total progressive sperm motility and the proportion of large and long spermatozoa were identified as the most consistent indicators of fertility. It was concluded that high and low fertility rams had clear differences in morphometric and kinematic sperm subpopulations, and that the most consistent indicators of fertility were the total progressive motility and the proportion of spermatozoa with large and long head present in the ejaculate.