Bull Management


5 min read

The right number of bulls Bull selection Before bulls arrive on farm Bulls arriving on farm Bull management during mating Reviewing bull performance

Bull management involves ensuring the right number of healthy bulls for your herd, minimising stress and reducing injury risks. A correct ratio depends on your herd's size and percentage of pregnant cows. Generally, one yearling bull per 20 heifers is advisable. Select bulls considering their health, age, size, and temperament. Keep bulls in good condition before and during mating periods, ensuring their diet and body score are appropriate. Regular checks and quarantine are essential to maintain a healthy herd. Lastly, monitor their mating performance by comparing expected and actual in-calf rates.

Good bull management means running adequate numbers of bulls with the herd, reducing their stress, and handling bulls to minimise the risk of injury to people and animals.

The right number of bulls

Having enough bulls when cows are likely to be on heat is important in ensuring good reproductive performance. The number of bulls required will depend on the number of cows or yearling heifers likely to come on heat while the bulls are with the group.

A minimum of 2 bulls should be with a mob at all times. The table below shows the minimum number of bulls required to run with the herd/mob at any one time. For a full bull rotation during mating, the total number of bulls required is double the numbers in the table.

Likely % of herd pregnant at start of bull mating
No. cows in the herd or mob 30% 40% 50% 60% 70% 80%
100 3 2 2 2 2 2
200 5 4 4 3 2 2
300 7 6 5 4 3 2
400 10 8 7 6 4 3
500 12 10 9 7 5 4
600 14 12 10 8 6 4

Bull power for yearling heifers

Run one yearling bull per 20 yearling heifers at all times to cover the poorer performance of yearling bulls. Ensure there are always at least two sexually active bulls running with each mob throughout the mating period. Bulls are typically run with yearling heifers on an all-in basis. You may want a few extra bulls around in case any need to be replaced. The ratio should be about one bull to 15-20 heifers.

Managing return heats after a synchrony treatment

If you are using heat synchrony, and returns will occur when bulls are running, you need to estimate the minimum number of bulls running with cattle during this period (using one bull per 10 non-pregnant cows). Alternatively, recommence heat detection and AB for 3 or 4 days, starting 19 days after the previously synchronised inseminations.

Bull selection

To make sure bulls are sexually mature and able to serve, bulls need to be well grown.  By the time a bull reaches 14–15 months, they should have achieved 50% of their mature weight. This should increase to 85% by two years of age.

To maintain the health of bulls and all other animals, ensure that bulls receive the same vaccination programme as the heifers and cows. Develop a drenching programme with your vet as well.

Considerations when choosing bulls

  • Select bulls from a bull rearer or leasing service with a reputation of growing and delivering healthy bulls. Query what disease exposure bulls may have had like Thieleria or BVD. Insist on bulls certified free of TB, BVD, IBR and EBL, and blood test negative for Johne’s Disease. If unsure about Johne’s, discuss with your vet.
  • Insist on bulls certified fully vaccinated for leptospirosis and BVD. They must have been vaccinated twice initially, 4 weeks apart and then boosted with a single shot annually for each of these diseases.
  • Use bulls that are no more than 3 years old. Older bulls can be temperamental, difficult to manage and are more likely to have injuries to the penis, back or legs. They increase the risk of injury to both the cattle and to the people working with them.
  • Choose virgin bulls whenever possible as they are less likely to introduce venereal diseases to the herd; but avoid using bulls that are less than 15 months old. If using non-virgin bulls, discuss testing for the venereal diseases, Trichomoniasis and Campylobacter with your vet.
  • Use bulls that are likely to minimise the number of calvings requiring assistance, especially with the heifers.
  • Select bulls ideally from the same mob. This will reduce fighting when they are with the herd. Otherwise the bulls need to arrive earlier to establish their social order well before mating start date.
  • Exclude fully horned bulls and those with deformed feet.
  • Select bulls of similar size to the cows or heifers to be mated. If bulls are substantially heavier than the cows or heifers (e.g. >100kg heavier) then injuries to both bulls and cows are more likely. Observe bulls serving tall cows; ensure they are able to serve correctly. Also observe larger bulls serving cows. If the cows collapse under the weight, find lighter bulls.

Before bulls arrive on farm

Bulls need to be kept in good body condition, particularly in the 3 months prior to their mating start date. Several weeks before the bulls will be used, make any required diet changes to ensure bulls are not too fat or too thin. They should be in body condition score 4.5 to 5.5 prior to mating. Body condition score bulls well before mating to give you time to make diet changes.

Consider veterinary examination of bulls at least a month before the bulls start work. Examinations range from a simple physical exam, to a serving ability test, or a full assessment of semen quality.

Bulls arriving on farm

Good bull management will ensure bulls are well adjusted to their environment before mating and have been through a biosecurity quarantine. Bulls should be moved to the farm between two to three months and ten days before they are required for work. Split the bulls into teams for rotating (half resting, half working) to reduce fighting. 

On arrival:

  • check for any injuries that may have occurred during transport
  • quarantine for 10 days and observe for any disease or walking defects
  • trim hooves if necessary
  • walk among them observing for any individuals showing aggression or ‘stalking’ behaviour, especially Jersey bulls – they may not be suitable to run with the milking herd.

Bull management during mating

When bulls are running with the herd, you can take several steps to increase bull activity and reduce health risks.

Steps to take

  • Regularly observe bulls serving to ensure they are serving correctly. Immediately remove bulls that are unable to serve properly and replace them with more capable bulls.
  • Monitor bulls for lameness each day. Remove lame bulls immediately and replace with healthy bulls. If bulls go lame or get sick they will need to be replaced for the rest of mating. Infections, antibiotic treatment, and elevated temperatures effect sperm production for 30+ days making them ineffective for use.
  • Do not allow bulls to enter the concrete milking yard with the milking herd as concrete can cause excessive hoof wear and lameness. To further reduce the risk of bull lameness and injury to bulls, cows and farm staff – train bulls to remain in the paddock when cows are brought to milkings. Identify bulls with reflective tape or some other means for easy location of bulls in the dark. It usually takes just two to three days to train bulls to hang back and let the cows go down the race.
  • In larger herds, there may be too many bulls hiding among too many cows to draft out in the paddock or race. The only alternative is to draft at the dairy shed. In this case, allow for extra bulls to replace those who go lame or stop cows moving on the race.
  • If applicable, ensure bulls do not gain access to concentrate rations. This can disrupt rumen function, causing sickness and reduced fertility.

Reviewing bull performance

In many herds, the period following the first 6 weeks of mating reflects the bull mating period. Herd reproductive performance during this time is an indicator of bull performance. By assessing performance at this time, you may be alerted that changes to bull management may be required.

  • Obtain the 6-week in-calf rate and the not-in-calf rate for your herd.
  • Identify the total weeks of mating (AB period plus bull mating period).
  • Look up the expected not-in-calf rate for your herd using the table below.
  • If the actual not-in-calf rate for your herd is higher than expected, it indicates that herd reproductive performance after week 6 of mating was unexpectedly low. If bulls were running with the herd for most of this time, poor bull performance is one possible cause.

Below is the expected not-in-calf rate based on a given 6-week in-calf rate and length of mating.

6-week in-calf rate Total weeks of mating (AB period plus bull mating period)
9 10 11 12
50% 34% 30% 27% 24%
55% 30% 26% 24% 23%
60% 26% 23% 21% 19%
65% 22% 19% 17% 16%
70% 19% 17% 15% 14%
75% 17% 15% 13% 12%
80% 13% 12% 11% 10%
Last updated: Sep 2023
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