Getting to the end of AB is a welcome milestone in the dairy calendar. Unfortunately, that doesn’t actually mean you can simply sit back and let the bulls take over. It can be easy to assume that the most dominant bull in a group has the highest libido and is the most fertile but that is often not the case. Variability in bull reproductive success has been demonstrated in various studies – one study of a natural mating period in bulls on pasture with 27 sires found that five bulls produced greater than half of all viable offspring, and 10 bulls did not sire any calves at all!
What does this mean for your bulls?
This means that everything you do during natural mating needs to focus on minimising the risk that the bull team is underperforming. Bulls teams should be created early and kept as stable as possible throughout mating. Let the bulls tussle for dominance before mating starts so they can get down to business once they are in with the herd.
Selecting bulls that are tall enough to service your cows is paramount. Usually, selecting two-year-old bulls is a good idea for your herd as yearling bulls are often too small and may have lower fertility. The InCalf Book recommends 1 yearling bull for every 15-20 heifers while mature bulls can be run at a rate of 1 per 25-30 non-pregnant animals. These recommendations do not account for synchrony programmes that will result in lots of returns coming on heat at once, so bull numbers may need to be increased to account for this.
Every breeding bull should be certified as having been tested for BVD (to guarantee it is not persistently infected (PI)) and vaccinated for BVD and lepto. Vaccination boosters should be given at least 2 weeks before bulls are put in with the herd. This means that if the bulls haven’t been previously vaccinated, or their vaccination history is unknown (i.e. you don’t have a written certificate from a vet) then you need to start the vaccination course ASAP.
In order to get cows pregnant, a bull needs to have a strong enough libido (desire to mate), be capable of mounting and servicing a cow, and be producing enough viable sperm to fertilise the egg (i.e. conception). Libido and mounting can be assessed by observing bulls with cows on heat, but in order to assess sperm numbers and quality we need to examine a semen sample under a microscope. Last season, 1 in 7 bulls that our bull testing team tested did not pass their pre-mating fertility exam. If your bulls haven’t been tested then get them booked in.
The table below reproduced from the InCalf book, shows how many bulls are needed, depending what proportion of the herd are in-calf at the end of the AB period. For example a 400-cow herd, with 60% cows pregnant to AB, needs to have 6 bulls in the herd, and 6 bulls resting – a total of 12 bulls.
Day-to-day bull management
Bulls need to be rested between periods of work, hence the need for half the bulls on farm to be working and half resting at any one time. If bulls become fatigued, they won’t inseminate cows effectively and if they service too many cows in a short period of time, their sperm stores become depleted, and they need time to build them up again. There are several options of how to run the bulls, but systems such as ‘3 days in, 3 days out’ or ‘day bulls and night bulls’ can work well, depending on the farming operation.
Lameness in bulls
Australian data found 36.5% of bulls were classified as high risk of reduced fertility at the end of the natural mating period – mostly due to lameness. Therefore, managing the bull team to prevent lameness is crucial to minimise risk of a poor result in the bull mating period.
Regular resting and preventing the bulls from walking on the races and yard will reduce the risk of lameness. Do everything you can to reduce the risk of bulls going lame because if a bull does go lame you have to assume the mating season is over for that bull – there isn’t time for his lameness and sperm supply to recover. If the dominant bull in a mob is lame it doesn’t mean he won’t prevent other bulls from mating cows. Therefore, as soon as a bull is lame he must be removed from the herd.
Getting plans and systems for your bulls sorted in advance will pay off once natural mating starts. If you need any help with any aspects of bull management or want to book in fertility testing or vaccinations pre-mating, then get in touch with your local clinic. We are here to help.