By W.L. Flowers, North Carolina State University Department of Animal Science
Last month in “Weaning-to-estrus profiles and breeding with ‘young semen” article, we discussed the importance of trying to use semen as soon as possible after collection for artificial insemination and data were presented with regards to adopting a “Last-in, First-out” program for managing insemination doses delivered to the farm. This generated quite a few questions concerning the relationships between common semen quality and fertility estimates since “test samples” are retained and evaluated daily at most boar studs. The idea is that as long as the test sample had acceptable quality, then the semen age really shouldn’t be that much of a concern with regards to fertility. This is true but only up to a certain point.
Data presented in Figure 1 are from a study in which changes in motility and several measures of the fertilizing potential of spermatozoa were measured over time for semen extended with a seven-day extender. Each day a sample was removed from the “test bottles” in the studs and motility, acrosin activity and the ability of sperm to bind ova in vitro were evaluated over time. One of the first things that sperm have to do during fertilization is to navigate their way through the cumulus cells which surround the ova. To do this, they use the forward motion generated by the movement of their tail and an enzyme called acrosin that is located within their acrosomal membrane that chemically digests the cumulus cells. The underlying theme is that both of these are critically important during the early stages of fertilization.
Figure 1: Changes in motility, acrosin activity, and a number of spermatozoa binding ova over time for boar semen extended with a seven-day extender.
The general pattern that is shown in Figure 1 is that sperm motility (black line) and acrosin activity (blue line) decreased gradually and at the same rate during the first three days of storage. As a result, the number of sperm that penetrated the cumulus cells and bound the ova was high (red line). However, after Day 3, acrosin activity began to decrease at a much faster rate than motility; in fact, motility remained over 70% during the entire seven-day period. Changes in the ability of sperm to bind ova were similar to those observed for acrosin activity decreasing at a much faster rate than motility. The main reason for this is that, as mentioned previously, both motility and acrosin activity are equal partners regarding a sperm cell’s ability to penetrate the ova. Consequently, both would need to be maintained at a high level during storage for optimal sperm fertility.
This divergence between estimates of semen quality probably occurs to some extent in all extenders and for all boars. What really isn’t understood very well is what type of factors influence the time course over which these changes take place. In this particular study, it occurred after three to four days in a seven-day extender. However, since it has been demonstrated previously that various combinations of boars and extenders can differ regarding their fertility, it is reasonable to assume that with other combinations of boars and extenders this same relationship may progress at a different rate, either faster or slower. Consequently, in the absence of this information, inseminating extended semen as quickly as possible after collection is a wise practice.