The bull team
There are many aspects to consider when using bulls for mating in the dairy herd.
How many bulls do you need?
Another way of looking at this is how effective has AI been?
Determined by many factors including the calving spread, number of anoestrus cows, conception to AI and duration of the AI period, these issues are all beyond the scope of this article but have a major bearing on how many cows are left to still conceive after the AI period.
Consider the following:
It would be uncommon to have less than 40% of the herd empty or more than 80% in calf after 6 weeks of AI so the bulls could be left with between 20% and 40% of the herd. A generally accepted bull ratio is the 3% plus one rule i.e. three bulls per 100 cows plus one spare. A 400 cow herd with 70% pregnant to AI after 6 weeks would have 120 cows to be served by the bulls. At 3% plus one, this works out at 5 bulls.
Previous experience, length of the AI period, number of non-cyclers etc may suggest that less than 70% of the herd are pregnant to AI and bull:cow ratio increased.
Compared with the cost of a high empty rate, bulls are relatively cheap, have a good resale value and are there to get cows in calf in a short space of time. Do not skimp on them!
Other things to consider are:
Regardless of your herd's status, bulls coming onto your property should have a negative BVD antigen test and be vaccinated against BVD, two doses 4 weeks apart, before they join the herd. You must see an official lab report or a letter from a vet confirming a negative test.
Source bulls in plenty of time and allow them to get settled. Trucking is stressful for bulls and can cause a temporary decline in fertility.
Bulls should be fertility-tested to ensure that they are mature enough (yearlings) and are not firing blanks, have poor libido or back/penis issues. See previous articles on bull fertility testing.
When keeping bulls from season to season, ensure they are fed well enough to promote healthy sperm production but not so well that they develop a large rumen paunch and get too heavy.
Avoid lameness at all costs as it can have a major effect on fertility. Bulls should be left in paddocks to minimise this risk. Lame bulls should be removed immediately and replaced. A dominant lame bull may prevent other bulls from working as well as not serving himself.
For yearling bulls use either higher bull:cow ratios or rotate bulls giving frequent rest periods.
Set up bull mating groups based on age, size and time in the herd to avoid dominance issues. Avoid introducing new bulls to the group.
