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What affects bull fertility?

Lorna Pillar BVM BVS MRCVS, Molecare Veterinary Services

Bull fertility is something which is very often overlooked when observing a herd’s fertility performance. Although bulls are rarely completely infertile many can be sub-fertile which can have a huge impact on the productivity of a herd.

There are four main components to bull fertility:

  1. Quantity/quality of semen
  2. Physical fitness
  3. Ability to achieve intromission
  4. Infectious disease


Semen production:

There are a number of things which can affect the amount and quality of semen that a bull produces. Firstly, the scrotal circumference is important for the quantity of semen produced as it has a direct correlation to the volume of testicular tissue. Generally speaking, bulls should have a scrotal circumference of 32cm at 18 months and 34cm by 24 months of age (although this may vary with breed). Bulls can temporarily have a reduction in sperm production for a number of reasons including heat stress, fever, lameness and stress. The semen also has to be of a good quality and this can be assessed by looking at a sample under a microscope.

Physical fitness:

With physical fitness lameness is particularly important, as well as potentially causing a reduction in semen production it will also mean the bull will be reluctant to work. Some bulls may need regular foot trimming to ensure they remain sound.

Ability to achieve intromission:

Working bulls might develop a number of penile abnormalities and so it is important to continue to monitor bulls even if they have worked fine in previous years. The most common deformity is a corkscrew penis which can develop in older bulls. Other kinds of traumatic injury can result in penile ruptures and haematomas which may lead to the bull becoming permanently infertile.

Infectious disease:

The main infectious diseases which can affect bull fertility are BVD, IBR, Leptospirosis and campylobacter. It is important to know the infectious disease status of farms that you buy breeding bulls from and also know the risk of these diseases on your own farm. It is important to remember that these diseases all affect fertility of the cows in a herd as well. There are vaccines available for BVD, IBR and Leptospirosis.

For more information phone Molecare Veterinary Services on 01392 872934.

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