Bull Fertility

The Importance Of The Bull In Herd Fertility

The influence of the bull on herd fertility in both dairy and beef herds is often overlooked with the focus being on cow infertility.  When cow management and infectious disease control are good then the limiting factor in herd fertility may be bull fertility rather than female factors.  Infertile bulls (incapable of achieving pregnancies) are rare but studies have shown 20% or more of unselected breeding bulls may be classed as subfertile and thus fail to perform optimally.  A fully fertile bull when run with breeding groups of 40-50 cows should be able to achieve average pregnancy rate to each service of 60% meaning in 9 weeks of breeding at least 94% of cows should be pregnant.  

Targets for efficient beef herd fertility should be 95% pregnancy rate in a 9-10 week mating period with 65% or more of cows calving in the first 3 weeks of the subsequent calving period. These figures cannot be achieved without a fully fertile bull stud and for this reason all farmers should consider routine examination of their stock bulls as an essential part of herd fertility management. The economic benefits of less barren cows are obvious but what is often overlooked is the financial benefit of calving more cows in the first 3 weeks of the calving period which will significantly increase average weaning weights and financial output. The practice of rotating bulls around breeding groups of 30-40 beef cows for extended mating periods may mask the presence of subfertile bulls on many farms but will lead to ongoing fertility inefficiency.

The Components Of Bull Fertility

For a bull to be fully fertile and perform to full potential each of the following components must be considered :

  • Libido (sex drive) and physical fitness
  • Ability to achieve intromission and deposit semen in the vagina
  • Production of sufficient quantities of high quality semen
  • Absence of disease that could be transmitted to cows during breeding eg Bovine Virus Diarrhoea (BVDV) or Campylobacter

Testicular Problems

There are many conditions that can effect the production and quality of semen produced from the testicles.  Scrotal circumference gives an indication of adequacy of testicular tissue volume and has a direct relationship with fertility. In addition to circumference it is important to look for bulls with good scrotal conformation. Bulls with tight, wedge shaped scrotums will be less able to regulate the temperature of their testicles and overheating can lead to reduction in semen quality. Bulls should achieve standards for scrotal circumference by certain ages and those that fail to reach these standards are likely to be subfertile due to failure of normal sperm production capacity.  Bulls with small testicles will at best produce small quantities of normal semen and at worst may be infertile due to hypoplasia of sperm producing cells. In general bulls should have scrotal circumference of at least 32cm at 18 months of age and 34cm by 24 months of age (individual breed society standards may vary).   Infection of testicles (orchitis) can cause infertility and will cause initial swelling and then degeneration (shrinking) of affected testicles.  Epididymitis causes swelling and blockage of the ducts that carry sperm form the testicles and may not be obvious unless careful palpation of the testicles is carried out.

Bulls can have temporary degeneration of sperm producing cells in their testicles for a variety of reasons including fever, lameness, stress, toxaemia and this can only be detected by collecting and examining a semen sample.  Semen can be collected on farm from most bulls by your vet by  electro-ejaculation .  The motility and percentage of normal sperm cells can be measured indicating the quality of the semen sample .

Infectious Diseases affecting Fertility

Bulls can carry and transmit various infectious diseases and much of this risk can be avoided by good health planning.  Bulls should always be tested clear of BVD virus and vaccinated before use.  Bulls from accredited herds may be sold naïve to BVD and are thus susceptible to BVD infection during transit/sale and when entering endemically infected herds.  Ideally all breeding bulls should be vaccinated for BVD  before sale and in herds where IBR and Leptospirosis are a risk then the bull should be vaccinated before mixing with the herd. It is easy to forget the bull during annual herd vaccination programmes but this can lead to temporary infertility if the bull is exposed to disease during the breeding period.  Use of hired or shared bulls carries the risk of introducing venereal Campylobacter infection which can have devastating consequences.