Veterinary bull breeding soundness evaluations are a great tool to select bulls meeting a minimum standard of parameters improving the reproductive performance of cattle herds. Bulls are classified as “Satisfactory”, “Differed”, or “Unsatisfactory.” In many aspects it is treated like a pass or fail. There are three main categories of the exam: the physical exam, including scrotal circumference (SC), sperm motility, and sperm morphology. If any one of these three areas do not meet minimum standards, then the bull can not be classified as satisfactory. What many producers may not realize is that it can also be used to place pressure on the genetic merits of bulls to help improve the gene pool. Important traits include early maturity, bull serving capacity, and a high percent of early pregnancy rates during the breeding season. This takes a general understanding and knowledge of the spermatogenesis, which is how the sperm cells are produced and how genetic and environmental influences play an important role.
There may be some confusion around the following statement: “It is important to understand that the minimum requirements for SC, sperm motility and sperm morphology were established to select ‘satisfactory’ potential breeders and not necessarily ‘superior’ breeders. Moreover, the thresholds were selected from a physiological perspective and were not intended to be used as tools for genetic selection.” (Peter J. Chenoweth, 2014) This statement is describing how and why the minimum standards were established and how they may potentially impact breeding and satisfactory pregnancy rates. It is not saying that these traits cannot be influenced through genetic selection. The result of breeding soundness exams, when used properly, can be used to apply genetic pressure improving the fertility of bulls and cattle herds. Scrotal Circumference would be a very good example. It is one of the three minimum standards created and has also been shown to be influenced by selecting for this trait in a way to improve or increase the scrotal circumference size and thus the serving capacity of the bull.
Sperm morphology is another important parameter obtained during the basic breeding soundness evaluation. Morphology is the structure of the cell, abnormalities can be identified, grouped, and named. Semen with a lower number of morphologically normal sperm will usually have a lower pregnancy rate than when compared with semen with a higher percent of normal cells. This is especially true when looking at a group of animals even though it may not occur every time when comparing two individuals. This is because many other parameters can and will affect fertility. It is also important to understand that both genetics and environmental influences affect the percent of normal sperm, therefore both genetics and outside influences must be considered. Bulls where the percent of normal sperm are widely different are more likely to see pregnancy differences, where two bulls with normal morphology percentages that differ only slightly may see little or no differences in pregnancy rates per cycle.
Cattle breeders who are raising bulls, especially those who collect semen and sell bulls to other bull producers need to pay close attention the percent of morphologically normal cells and put pressure on improving these important reproductive traits. Using this and selecting bulls who can achieve high morphologically normal sperm prior to 14 months of age will help to improve the percent of bulls that will “pass” the breeding soundness evaluation prior to bull sales and improve the fertility of the herds who purchase these bulls.