Fertility of Yearling Bulls
Livestock Update, May 2005
Dr. W. Dee Whittier, Extension Veterinarian, Cattle VA-MD Regional College of Veterinary Medicine
The US beef cattle industry has developed a system wherein a large percentage of beef bulls are put into service at 15 to 18 months of age. While this adds the efficiency of keeping bulls for less time before they pay a return, it also presents some challenges. The fertility of the yearling bull may be somewhat lower than that of older bulls.
Puberty for bulls is not an immediate process. As young bulls go through puberty a number of changes must occur as they move from an infertile calf to a fully fertile bull. As the testicles develop and sperm production begins there is a period during which the sperm cells which are produced are not fully mature.
The Breeding Soundness Examination
The breeding soundness examination is a process, designed to be performed on the farm with portable equipment which allow a veterinarian to assess the potential fertility of a bull. The process is outlined by the American Society for Theriogenology based on all the accumulated research available.
The breeding soundness examination is based on a bull being examined in four different ways. Bulls must be assessed as being normal in terms of : 1) physical normality including overall health and the reproductive tract specifically; 2) scrotal circumference where minimum values for increasing ages up to 24 month are set; 3) examination of sperm cells for adequate motility (forward movement), and; 4) an examination of individual sperm cells for normality (morphology).
It is important to note that the breeding soundness examination does not assess some other important characteristics of bulls that can dramatically affect fertility. It does not assess mating ability, whether a bull can successfully mount a cow and complete the mating act. It also does not examine for libido or sex drive nor for the presence of venereal diseases.
In 1999 the Society for Theriogenology changed the protocol for deciding whether a bull passed the BSE. Prior to this time a bull was given up to 100 points for scrotal circumference, sperm motility and sperm morphology. By this system, a bull with excellent scrotal circumference could still pass the examination with some deficiencies in sperm qualities. The 1999 change required that all bulls pass each portion of the examination independently. Thus, each bull must have at least 30% sperm cell movement and at least 70% normal sperm cells as well as a large enough scrotal circumference.
The Process of Maturity in the Bull
As bulls go through the changes of puberty, the sperm production process matures as well. From the time that the first sperm cells are produced until the time that daily production of mostly normal cells occurs requires several months. Original sperm production results in very few cells, most of which are abnormal. As bull mature the number of sperm cells that are produced as well as the percentage of these cells that are normal.
A study was done at the University of Saskatchewan by Dr. Al Barth and reported in 1999. Bulls were followed during their time at a test station and were examined once a month at ages 11 through 15 months. Interestingly, only 57% of bulls had >greater than 70% normal sperm cells by 15 months. Table 1 below shows more detail of the Saskatchewan data.
Table 1. Reproductive characteristics of bulls in a bull test facility at increasing ages.
|Age||Mean Scrotal Circumference||% Bulls with Mature Sperm Patterns|
|12 Months||34.4 cm||30%|
|13 Months||35.2 cm||51%|
|14 Months||35.8 cm||48%|
|15 Months||36.3 cm||57%|
Take Home Lessons
Results of breeding soundness examinations performed on yearling bulls must be carefully interpreted. Semen is not collected on bulls completing Virginia BCIA bull tests although the rest of the breeding soundness examination is performed. Semen would have to be collected several weeks before the sale when bulls are younger. Because culling more than half of the bulls at bull test stations is not thought to be wise, bulls are sold instead with a guarantee to be breeders.
How did our industry get in a position where more than half of the bulls that we sell are not reproductively mature? It is certain that there has been a selection for more growth and size in modern bulls. According to those who remember the original Virginia bull test stations, it took nearly 20 years before the first bull completed the test with a weight of 1000 pounds. Now bulls finish with weights over 1400 pounds. Larger cattle will have later maturity. Perhaps this also is part of our challenge to get good pregnancy outcomes with the bull's yearling heifer sisters. However, because these bulls are so large we tend to have great expectations of them reproductively.
Following are some suggestions for dealing with the challenge of delayed maturation of sperm production in yearling bulls: