As we prepare for the breeding season or for those in the midst of the breeding season, it is important to think about how we can manage bull fertility and understand critical factors that can impact fertility. There are many factors that can affect sperm production; however, the main factors that can decrease sperm production are disease, fever, injury, and extreme environmental conditions. We must keep in mind that spermatogenesis, the production of sperm, is a 61-day process in bulls; therefore, it will take upwards of 60 days to have normal sperm again following an injury/insult. Therefore, it is important to monitor and identify if a bull has experienced frost bite, or any other injury to the scrotum/testis.

In order to ensure our bulls are prepared for the breeding season, bulls should be tested approximately 4 to 6 weeks prior to the breeding season. The test that is performed is the Breeding Soundness Examination (BSE). The American Society for Theriogenology has developed minimum guidelines for a bull to pass a BSE. A veterinarian will evaluate the bull on the following criteria: a physical examination, scrotal circumference measurement, and evaluate semen quality for motility and morphology. To successfully complete a BSE, a bull must have at least 30% sperm motility, 70% normal sperm morphology, and a minimum scrotal circumference based on age. Bulls meeting the preceding minimum requirements are classified as satisfactory potential breeders. If a bull does not pass one of these tests, he is either classified as a classification deferred (meaning it is recommended that the bull be tested again) or as an unsatisfactory potential breeder.

The physical examination portion of the BSE determines the bull’s physical capabilities of successfully breeding a cow. A bull must be able to see, smell, eat, and move normally to successfully breed cows. Sound feet and legs are very important because if they are unsound, this can result in the inability to travel and mount for mating. Evaluating bull body condition during the winter and prior to the breeding season is just as important as evaluating your cows. More information on meeting nutritional needs for your bulls can be found in a recent article, “Evaluating and Preparing Bulls in Advance of the Breeding Season”.

The scrotal circumference tells us the testicular mass and as it increases, so does the daily production of high-quality sperm. Scrotal circumference is also an important measure since it is directly related to the onset of puberty in the bull and his female offspring.

Semen quality includes ejaculate volume, sperm cell motility, and sperm cell morphology. Sperm motility is evaluating the percentage of spermatozoa in an ejaculate that has progressive (headfirst) movement. Sperm morphology is calculated by evaluating the percentages of normal spermatozoa and sperm with abnormalities, and sperm morphology can have larger impacts on pregnancy success. Research from Wiltbank and Parish (1986) reported that bulls with 80% or more normal sperm had greater pregnancy rates compared to other bulls. Therefore, selection of bulls with greater normal sperm can increase overall pregnancy rates in a herd. It is important to remember that substandard nutrition, extreme environmental temperature, and disease can reduce semen quality, and the quality of semen from a single bull may change over time.

One other factor that is not evaluated in a BSE that can impact bull fertility is known as libido. Libido refers to the desire to mate and has positive effects on pregnancy rates. Libido can be evaluated by closely watching a bull after introducing him to a cow herd. Is he more interested in detecting cows that are in estrus, or in finding food in the bunk or in the pasture? Also keep in mind that with younger bulls, observing them more frequently during the first 5-7 days and 7-day intervals is recommended to monitor mating activity and capabilities. Overall, bulls with diminished libido may require rest and recuperation during a competitive breeding season.

Another factor to consider is bull social behavior. Under natural service conditions, the social ranking of bulls within the herd hierarchy can influence reproductive performance. Dominance is expressed more strongly in older bulls (i.e. 3 to 4 years of age and older) and is more related to seniority than any other factor. We may see the effects of dominance having greater impacts when we have lower bull to female ratios and limited estrus activity within a herd. Keep in mind that dominant bulls may impregnate more cows and therefore, subordinate bulls may have limited reproductive performance and fewer calves. Conversely, if dominance is associated with low semen quality or low sex drive, then herd fertility may be compromised.

Overall, it is important to remember that over time, the semen quality of an individual bull will change, and periodically evaluating a bull’s mating ability and libido should be considered. Consult with your local veterinarian about performing breeding soundness exams in your herd bulls. In conclusion, herd bulls have a large influence on many aspects of an operation including profitability, calf crop, and genetic improvement. With 90% of beef cows in the United States bred by natural service, managing bulls to optimize breeding performance is important.

References
Wiltbank, J. N. and N. R. Parish. 1986. Pregnancy rate in cows and heifers bred to bulls selected for semen quality. Theriogenology. 25: 779-783.

Interviews with the authors of BeefWatch newsletter articles become available throughout the month of publication and are accessible at https://go.unl.edu/podcast.