The Intricacies of Bull Fertility

Co-Authored by Samantha Roberts, Department of Animal Science Graduate Teaching Assistant

Bull fertility is influenced by various factors. In a Breeding Soundness Evaluation (BSE), key aspects such as physical soundness, scrotal circumference, and sperm quality are assessed to estimate fertility potential. Sperm quality itself can be impacted by processes like spermatogenesis and sperm maturation, as well as interactions with the testis, epididymis, and the proteins and other molecules present in seminal plasma. Additionally, during the processing of sperm for artificial insemination (AI), proper semen handling is crucial for maintaining sperm quality. This is because sperm must undergo freezing and thawing, which can significantly impact their viability if not managed correctly. In females, the vaginal, cervical, uterine, and oviductal environments significantly influence sperm transport, hyperactivation, capacitation, and the interaction between sperm and egg. Overall, bull fertility and sperm quality are complex and rely on a series of coordinated events that must occur successfully for a viable pregnancy to be achieved.

We can describe an inseminate with optimum fertility as having enough of all the attributes that it needs to go through all these processes described above. Some of these attributes are known; however, several of these desirable characteristics are either unknown or we don’t know what is enough for that characteristic. For example, for sperm motility, we have identified that a threshold of 30% is enough to reach optimum fertility and going beyond that have been reported to have little or no correlation with improved fertility. Nevertheless, semen below 30% will display subfertility. Similarly, we have identified that 70% normal morphology will yield optimum fertility, where below that threshold we start observing subfertility.

The amount of sperm in the straw is another characteristic that have been described to influence optimum fertility. Sperm dosage in A.I. straws have been proven to have an impact on pregnancy rates. If one increases the insemination dosage of the A.I. straws, pregnancy rates will increase until it eventually reaches a plateau. This plateau is dependent on the maximum (optimum) fertility of the female population (Fig. 1). One study, conducted by Den Daas and others (1998), found that bulls reach their maximum fertility at different insemination dosages (1 million to 10 million viable sperm per straw). Zoca and others (2020) found in a study that there is no variation in pregnancy rates when using A.I. straws that have 10 million to 40 million sperm cells in them (conventional commercially available sperm dosage variation). These variations in fertility can be due to sperm traits, and their relevance for fertility are divided into compensable and uncompensable traits as originally described by Dr. Saacke in 1994 (Fig 1). Compensable traits refer to the sperm’s inability to reach the site of fertilization and fertilize the oocyte. An example of this trait is low motility, when the insemination dose is increased, more motile sperm is inseminated, thus more sperm with ability to reach the site of fertilization and initiate fertilization in inseminated leading to increase in fertility (for example bulls A and B in Fig. 1). An uncompensable trait is where sperm initiates fertilization, but it is unable to support embryo development. This will impact the maximum fertility of a bull, being a cause of subfertility. Therefore, increasing the insemination dose does not increase fertility because both population of sperm (“the good” and “the bad”) are increased. Sperm with ability to develop a viable embryo and those without the ability to develop a viable embryo increase in the same rate. This means that while more sperm are present, the proportion of those capable of developing a viable embryo remains unchanged (for example bulls C and D in Fig. 1). An example of this trait is damage to the sperm DNA chromatin.

Although several characteristics are known to impact bull fertility, such as motility, morphology, DNA integrity, acrosome integrity, proteins, miRNAs, and others, we only have specific thresholds for a few of those characteristics. Performing a through BSE is still the best way to estimate bull fertility and decrease the chances of using a subfertile bull during your breeding season. Similarly, buying semen for reputable places will increase the chances of a successful AI program.

Figure 1. Illustration of change in bull fertility relative to insemination dose and optimum female fertility. Bull fertility increases as the number of viable sperm inseminated increases until it reaches the optimum female fertility or male maximum fertility (Adapted from Salisbury and VanDemark, 1961; Sullivan and Elliott, 1968; Den Daas et al., 1998).