The value of bull breeding soundness examinations (BSE) is well known. Approximately one in five bulls do not meet the minimum BSE standards established by the Society for Theriogenology. Bulls not meeting these minimum standards are seldom infertile; they are usually subfertile and will have lower conception rates than those who meet these standards.

Subfertility reduces reproductive efficiency and will cost in fewer cows getting pregnant early in the breeding season. Reproductive efficiency is the most important economic trait for cattle producers and therefore should be a priority when selecting bulls and replacement heifers. Higher fertile bulls will help cattle producers improve or maintain high reproductive efficiency within the herd.

Bull fertility can vary throughout the year and is affected by the environment, injury, stress, disease and genetics. During the shorter days of winter, testosterone levels decrease. This can decrease sperm production and cause some sperm abnormalities. Extreme cold and stressful weather patterns can also negatively impact sperm quality.

The heat of summer causes hypoxia within the testicles, negatively affecting bull fertility. These external influences are seen in the structure of the sperm cell. We refer to this as sperm morphology. A BSE should be performed as close to the breeding season as possible for the most valid results and to know the status of the bull at the time it is being used.

In an article to veterinarians titled Guidelines for using the Bull Breeding Soundness Evaluation Form by P.J. Chenoweth et.al., it reads, “Veterinarians are encouraged to work with their clients to accept higher standards for bulls than the ‘minimum acceptable’ standards employed in this BSE system.” The minimal acceptable standards established by the Society for Theriogenology use a minimum of 70% normal morphology and 30% forward progressive motility sperm in the ejaculate to be classified as satisfactory. Improvements on this system can help veterinarians to better understand possible causes of poor semen quality, create a prognosis and estimated time frame for possible recovery, and help bull producers select bulls for better semen quality.

Improving on this system and creating higher standards requires a complete differential spermiogram (CDS). A CDS breaks down and lists each of about 24 morphological abnormalities and records the percent of each. A CDS is a must to really understand the potential fertility of many bulls. A more complete picture of the bull’s sperm production is created through a CDS. A significant benefit comes when we utilize the potential influences each type of morphological abnormality can have on fertility.

For example, detached heads and distal midpiece reflexes have very little negative effects on fertility until they reach levels of 30% to 40 % in the ejaculate. On the other hand, proximal cytoplasmic droplets, nuclear vacuoles and pyriform heads will begin to negatively impact fertility around 15% to 20% of the ejaculate. Without a CDS, this information is missing and some subfertile bulls will be used while other bulls will receive an unsatisfactory classification yet would perform well. Using a CDS, veterinarians can better help producers make informed breeding management decisions through selecting correct bulls for optimal herd reproductive performance.

The genetic influences on bull fertility is another area of increased value that should be considered when evaluating BSEs and especially the CDS. Several studies have shown that breed plays a significant role in the percent of bulls passing a BSE evaluation. The variation between breeds and family lines indicates that we can influence bull fertility through genetic selection.

Multiple studies have shown the phylotypic qualities of sperm are influenced by genetics. We can improve the reproductive qualities of a bull through selection the same way we are currently improving growth, feed efficiency, carcass and maternal traits. Some sperm morphological abnormalities, when seen in significant numbers, have a direct genetic cause.

Many morphological abnormalities appear to be epigenetic; this is when the combination of stress or environment and a genetic trait allows abnormalities to form more easily. Diadem defects and distal midpiece reflex are two examples. In some bulls, sperm with distal midpiece reflex is more likely to be seen during late winter due to lower testosterone or mild stressors, and diadem defects are more common during the heat of summer.

Genetic selection is difficult when trying to compare different bulls. There is a lack of consistency between evaluators and the equipment used to assess semen quality. Several questions arise to create a system for comparison of sperm phenotypic traits: what age should we test bulls, what time of year, how do we account for environmental factors?

While these are legitimate concerns, it is easy to start with any single herd where the bulls being evaluated are under the same management and environmental conditions. When testing bulls, it is vital to place an emphasis on the quality of equipment and methodologies of evaluation. We need to identify every morphological abnormality and have a goal of culling the bottom performing bulls. This will add real value for potential buyers.

When evaluating semen and creating a CDS, it becomes vital to use a high-quality phase contrast or Differential interference contrast (DIC) microscope with very good optics and resolution. The evaluator needs training and experience to do a good job. A CDS takes a lot more effort and time to compile than the simple separation of sperm into normal, head, midpiece and tail morphologies.

While most cattlemen and veterinarians are not utilizing a CDS during routine BSEs, there are more and more veterinarians doing them. Utilizing a CDS will help bull producers add value and improve bull genetics and will help cattlemen everywhere improve the reproductive performance of their cattle herds.

The feasibility of owning a high-quality phase contrast microscope, due to the expenses, may be difficult for many veterinarians. However, bull semen can be easily collected and preserved in a small quantity of 10% neutral buffered formalin and shipped for evaluation by an experienced veterinarian who has a quality microscope, or to an andrology lab. This may also have the advantage of minimizing normal human variabilities commonly occurring with today’s systems.