Sperm dosage per artificial insemination (AI) continues to be a hot topic. More sperm is better, right? The answer is a mix of “no” (in some cases) and “it depends (in other cases).”
In 1961 Salisbury and VanDemark first suggested the relationship between sperm quantity and quality, when they proposed fertility increases with increasing sperm numbers inseminated up to a threshold level. After this threshold level has been attained, the female population becomes the limiting factor and increases in sperm numbers do not result in further increases in fertility. This can be seen in Figure 1, where bull B reaches a threshold value for optimal results at 15 millionsperm per dose. Further increases in sperm numbers for bull B will not result in further increases in fertility, because the limiting factor is now the fertility level of the female population. In contrast, bulls A and C do not reach a threshold value for optimal results even when 20 millionsperm per dose are used (Figure 1).
Figure 1 provides graphic evidence there are seminal traits which are “compensable” and others which are “uncompensable,” as originally described by Dr. R.G. Saacke of Virginia Tech. Seminal deficiencies, seen as reduced fertility when sperm numbers are below threshold, which can be overcome or minimized by increasing the sperm dosage would be considered compensable.
Compensable traits of semen quality are believed to be related to sperm viability, specifically to the ability of inseminated sperm to not only reach the egg, but also bind to and penetrate the outer covering of the egg. Therefore, if a semen sample contains a low percentage of viable sperm, this may be compensated for by increasing the number of sperm per dose.
In contrast, uncompensable traits are related to the incompetence of the fertilizing sperm to complete fertilization and sustain early embryonic development, resulting in suppressed fertility regardless of sperm dosage. As illustrated in Figure 1, the maximal fertility of bulls A and C is a function of uncompensable traits and the optimum fertility of the cow population.
What are examples of compensable and uncompensable seminal traits? There is evidence severely misshapen sperm do not traverse the female reproductive tract and compete for fertilization, perhaps due to impaired progressive motility. Consequently, severely misshapen sperm within an otherwise normal semen sample are considered a compensable seminal trait.
On the other hand, uncompensable traits are also associated with abnormal sperm shape, and to make matters more difficult, DNA integrity. In fact, poor DNA integrity has been implicated in cases of male subfertility for nearly 50 years.
So, what does all this really mean? For producers to achieve optimal fertility, AI organizations routinely adjust the AI dose when compensable deficiencies are known. On-going research is focused on discovering uncompensable seminal traits, as these result in depressed fertility regardless of sperm numbers per dose. Finally, resist nonsensical advertising focused on some magic number of sperm per dose for all bulls. More sperm is not always better.