Breeding Bull Care and Management

The bull may be thought of as “half of the herd”. It is therefore imperative that he is given due care and attention to keep him fit and healthy to fulfil his role.

Health and safety are paramount when handling bulls. Both dairy and beef breed bulls may be aggressive and so it is advisable to begin halter training from an early age. Also consider the use of a nose ring. The use of an appropriate race and crush is recommended. Ensure that facilities are large enough and strong enough to restrain a fully grown bull. Accidents can happen when least expected and preparation is key.


Bulls should be kept separately from other cattle. The ideal bull pen is within sight of other cattle, but strong enough to contain the bull. External gates should be lockable. Feeding and watering should be performed from outside the pen. There should be a refuge or emergency escape route within the pen. A crush or head-gate should be available for securing the bull. Never enter the pen when the bull is loose. Include a safety sign at the entrance to the building/pen.


Puberty usually occurs in bulls at around 11 months of age and is dependent on breed. Puberty is defined as the first time theanimal produces a fertile ejaculate of at least 50×106/ml concentration and 10% progressive motility. Scrotal circumference should be at least 25-27 cm, but this can vary with breed.

Behavioral changes may be noticed at puberty. The bull will begin exhibiting sexual interest around three weeks before pubertyand will attain mating ability approximately six weeks after reaching puberty. Libido is dependent upon the social position in the bull group and younger bulls are often lower in the hierarchy.


Diets adequate in protein, vitamins, minerals and energy can hasten the onset of puberty. However, high energy diets in post-pubertal bulls can be detrimental and may lead to reduced sperm production/quality and reduced libido. Deficiencies in zinc and selenium can impact fertility.

Body condition score (BCS) should ideally be 3-3.5 before mating. Bulls can lose up to 90 kg over an intensive breeding season.

Environment - thermoregulation

The testicular temperature should be between 2 and 6°C/35.6 and 42.8°F. Breeding for a pendulous scrotum can help to keep testicles away from the abdomen and thus cooler. The dartos muscle can respond to changes in temperature and change scrotal position. Thin, hairless scrotal skin is lacking in subcutaneous fat and testicular blood vessels allow heat exchange.

Causes of increased scrotal temperature may include insulation of the scrotal neck in bulls carrying excess condition, increased ambient environmental temperature, inflammation of the scrotum or testes, generalized pyrexia and prolonged recumbency, eg due to lameness.

Effects of increased scrotal temperature include decreased number of live spermatozoa in the ejaculate, decreased progressive motility and increased morphological abnormalities. The time from thermal insult to return to normal ejaculate is approximately 6 weeks.

Breeding season

A bull breeding soundness examination should be performed prior to each breeding season. It should include general physical examination, external and internal genitalia examination and semen assessment.

There is not a definitive ratio for bulls to cows, as it is dependent on the results of an individual’s breeding soundness exam. One rule of thumb is one cow/heifer per month of age of the bull up to 30 months old, ie an 18-month-old bull could run at a ratio of 1:18. Generally, young bulls should be run at around 1:20 and older bulls should be run at around 1:30.

Infectious diseases

Infectious organisms to be particularly mindful of when considering bull and herd health include bovine herpes virus 1 (BHV-1), Campylobacter foetus subsp. FoetusCampbylobacter jejuniLeptospira spp and Bovine viral diarrhea virus.

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