Managing your working bulls

1 October 2019

Bull side on: Supplied by CRV Ambreed

When the bulls go in after artificial insemination has finished, there’s a sigh of relief on farms across the country. Stop the hard work going down the drain by focusing on the bull mating period for best results.

Get the right number of bulls

When bulls are run with the herd after artificial breeding (AB), at least one bull is required for every 30 cows still not in calf. But at the end of AB, you don’t know how many cows  are already in calf and how many will return.

If the herd is managed during mating in separate herds, estimate the bulls required for each herd at any one time. Always round up numbers to the nearest bull and never run fewer than two bulls with any herd or mob.

If you used a synchrony, don’t forget to at least double the ratio of bulls for a six-day period when these cows are due back on heat (18 to 24 days after first synchronised insemination). Or consider reintroducing AB to cover this period.

To estimate the number of bulls required, visit

Bulls arriving on-farm

Bulls should arrive properly identified and with details of their movement history and health status of the herd they’ve been in. Ensure NAIT records are completed within 48 hours of arrival.

On arrival, the bulls should be held separately from the  main herd for at least seven days and monitored for any signs of ill health. If you have any concerns about the bulls’ health, contact your veterinarian before you mix the bulls with the herd.

Roster bull rotations

Bulls need breaks to keep them keen and interested, and for maintaining semen viability. You need additional bulls to allow for regular bull rotations. Swap bull teams in the milking herd daily, if possible, especially over a synchro-return period.

  • A ‘half-resting, half-working’ bull rotation policy will require double the number of
  • As mating progresses, fewer bulls will be needed as the number of non-pregnant cows decreases. If in doubt seek advice.

Focus on healthy, happy bulls

Replace bulls that become inactive or unhealthy. High temperatures caused by fever or heat stress affect semen production and increase the number of abnormal semen. Even slight increases in body temperature (one to two degrees Celsius) can cause major disturbances in semen production.

Once a bull recovers from fever or sickness, it can take two months for normal fertility to be regained. Therefore, he won’t be useable for the remainder of the mating period.

Watch out for signs of a broken penis, a penile haematoma, low libido and hip or back injuries. These conditions will prevent affected bulls getting cows pregnant. If dominant bulls are affected, they’ll also prevent other bulls in the mob mounting.

Key Points

  1. Monitor bulls for lameness each day, immediately removing lame bulls and replacing with healthy
  2. Regularly observe bulls to ensure they’re serving correctly, immediately removing bulls that aren’t and replacing them with more capable bulls.
  3. Consider reintroducing or extending AB if bull numbers are ever an issue. Talk to your AB company for advice.

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