Managing bulls at mating

The calving distribution should consist of  65% calves dropped in the first three weeks, followed by 20% and then 10% in the subsequent three-week periods.

With this level of reproductive efficiency, you will produce an even line of calves of roughly the same age making them easier to manage and market.

To achieve these objectives, consider the following aspects of bull management:

  • bull health management
  • body condition prior to mating
  • breeding soundness of bulls
  • effective use of bulls
  • cow condition.

Bull health management


Bulls are often set stocked in small bull paddocks for long periods of the year and can therefore be prone to worm infestations. Drench bulls annually, preferably in summer with the most effective drench to combat the inhibited larval stage of intestinal round worms.

Clostridial disease

Bulls should be vaccinated annually against clostridial diseases. Newly purchased bulls should be routinely vaccinated twice, a month apart on receival, unless you know the previous owner has an adequate vaccination program. The annual vaccination can be given at the same time as the annual drench.


Vibriosis is a venereal disease transmitted at service, and causes early foetal death.

Bulls should be vaccinated twice, four to six weeks apart, with the second vaccination 1 month before joining. A booster vaccination should be given each year.

Body condition prior to mating

Pasture forms the basis of a bull's diet, with supplements given only to overcome deficiencies in pasture quality or quantity.

The ideal condition for a bull prior to mating is condition score 3. Bulls in poor condition must be fed well before mating, and should be in good condition at least 2 months prior to mating. Semen is produced several weeks before it is used, so last minute feeding to improve body condition may be too late.

Over-fat bulls (score 4 or 5) should be let down gradually, well before joining. Over-fatness can interfere with the heat exchange function of the testicles resulting in infertility. Over-fat bulls also put more strain on their legs and spine predisposing them to breakdown during joining.

Measuring the bull's testicles

The size of bulls testicles are in direct proportion to their semen production, so generally speaking, the larger the testicles the more females he can potentially cover. 

There is good evidence to suggest that bulls with a scrotal circumference of less than 300mm will have small quantities of poor quality semen making them incapable of achieving normal reproductive rates.

The general recommendation is to buy or keep only bulls with a scrotal circumference of 340mm or better. Where heavy mating loads are practised, a scrotal size of 360mm or better is required.

If you have access to a good cattle veterinarian, it can be worthwhile asking the vet to check the testicles for soundness. This involves checking the epididymis for blockage or damage and at the same time the vet could test the bulls for service capacity.

Service capacity yard test

The service capacity test is a yard test where bulls are given access to females for restricted periods and their capacity to cover those females is assessed. The number of services a bull achieves allows the vet to rate the bulls as either high, medium or low serving capacity. The major advantages of this test are to eliminate bulls with:

  • penile abnormalities and structural problems
  • no libido
  • sub-optimal service capacity

Examining your bulls yearly

To get the best possible results each year, have your bulls examined annually and to check structural soundness for any signs of problems that may cause poor mobility.

Effectively using your bulls

Join bulls of highest serving capacity to heifers because it is vital that heifers get in calf at their first joining. If they calve early in their first season they tend to be early calvers for the rest of their lives. Cows that calve early will wean heavier calves on average than late calving cows.

It is important to know the serving capacity of each bull. If bulls with a high serving capacity are used, the pregnancy rate of heifers at their first heat will be up to 40% higher than if bulls with low serving capacity were used, and up to 20% higher than if bulls with medium serving capacity were used.

Bulls that are rated as sound for breeding and have a testicle circumference of 360mm or better, can be mated to 40 cows or more, and get 95% or more in calf over a 9-week mating period. These results are achievable provided normal precautions are observed particularly in the first 3 weeks of mating when the maximum mating load places the bulls under most stress.

In multiple-joining situations, run bulls of the same age together. In a group of mixed age bulls older bulls tend to dominate younger bulls in a mating situation. This can lead to reduced fertility and slower genetic improvement because most of the calves will be sired by the older bulls. It can also increase the likelihood of injury due to fighting.

If your bulls have not been examined prior to joining, for breeding soundness, and you are single bull joining, it is extremely important that regular checks are made throughout the joining period.

Bulls that are well grown at 15 months of age can be used lightly over 15 to 20 females (usually heifers), provided there is no large disparity in height in favour of the females.

Page last updated: 25 May 2020