Significant amounts of time and money are invested in preparing for and undertaking a successful AB mating period in dairy herds. However, to ensure your herd reproductive performance (particularly overall not-in-calf rate) reaches target, it is essential that the bull mating period is successful. Poor bull management can lead to an inability to capitalise on the investments of the AB period by leaving cows either not in calf, or calving very late the following season.

Firstly, it is essential to ensure that adequate numbers of disease-free, fertile, adequately grown bulls are available at the start of the mating period. The exact number of bulls required depends on how many cows are pregnant at the end of AB; look at the table below from the DairyNZ InCalf Book to check your bull numbers.

Secondly, bulls should be closely observed daily for successful mating activity. Bulls should also be monitored every day for any signs of disease or injury. Affected bulls should be immediately removed and replaced before being assessed and submitted for veterinary attention where necessary. Additionally, bulls that are observed to be lacking mating activity on successive days should also be examined to determine if there is an underlying reason. Remember that illness and injury may have resulted in sub-fertility or infertility for some days before the bull is observed to be abnormal. Depending on the cause, a bull’s fertility may not recover for up to 10 weeks or with some conditions the damage may be permanent. Research in Australia in 2016, where bulls were examined at the end of the natural mating period, found 40% had abnormalities that would severely affect fertility (primarily lameness)!

Keys To A Successful Bull Mating Period

1. Form bull teams soon after arrival, taking note of dominance/aggression between bulls. Do not mix individuals who are consistently aggressive toward one another.
2. Rotate bull teams through the herd regularly, allowing bulls a work-to-rest ratio of 1:1.
3. Avoid bulls spending excessive time on concrete surfaces or walking on raceways.
4. Observe bulls daily. Lameness, swelling in the penis/sheath region or lack of mating should result in removal from the herd and assessment for injury/disease.
5. When lameness or other injuries are detected in bulls, act quickly to ensure adequate bull power is maintained:
   • • Enquire with your bull supplier to see if they can supply replacements
     • Consider utilising bulls that have been with your heifers and may now be surplus to requirements in that mating group
     • Begin heat detection and AB again while a solution to your bull power issues is sourced.

Other resources that you might find helpful:

Talking Bull: Tops tips for managing the bulls during natural mating