Variability in relationships between semen quality and estimates of in vivo and in vitro fertility in boars

The present experiment was designed to characterize relationships between common semen quality and fertility estimates for three boars known to differ in farrowing rate, number of pigs born alive, and monospermic penetration rate. The approach chosen to accomplish this was to monitor semen quality from these boars and use their semen alternately for either artificial insemination or in vitro fertilization for 40 weeks. This strategy relied on the variability in semen quality parameters that normally occurs in an individual boar over time. When comparisons were made among boars, farrowing rates, numbers of pigs born alive, and monospermic penetration rates were significantly different, but progressive motility, normal head and tail morphology, and acrosome morphology were not. However, when comparisons were made among ejaculates within individual boars, there were significant effects of semen quality on both in vivo and in vitro fertility. For boar 3495, the proportion of spermatozoa exhibiting progressive motility and distribution of spermatozoa in a percoll gradient had a positive linear effect on number born alive and monospermic penetration rate, respectively. For boar 2901, quadratic equations best described changes in litter size as a function of progressive motility and normal acrosomes. In addition, monospermic penetration rate increased linearly as normal acrosomes and the proportion of spermatozoa recovered from a percoll gradient increased. For boar 4291, the relationship between progressive motility and number born alive and between normal acrosomes and number of pigs born alive were also quadratic. However, a significant linear relationship was present only between normal acrosomes and monospermic penetration rate. These results demonstrate that simply relying on the means of common semen quality estimates from some boars has limited value in terms of being used as a prospective indicator of their in vivo or in vitro fertility. In contrast, characterization of relationships between semen quality and fertility estimates is useful for estimating differences in the fertility of ejaculates from individual boars. However, both quantitative and qualitative differences in these relationships among boars are present and a given semen quality estimate that is a good predictor of in vivo or in vitro fertilization for one boar, may not be applicable for others.