Mating is about to start, and with an average mating period of 11 weeks it is important that you take steps to ensure your bulls are working as well as they can. Across the practice last year, the average six week in-calf rate was 70%. If we assume 5 weeks of AB, that will mean approximately 40% of a herd will need to get in calf to a bull mating. With an average herd size of 310, that would leave 124 cows for the bulls and only 6 weeks (two cycles) to do it. 
 
The inCalf book provides some good guidance around bull power. 
 
This average herd should have four bulls in with the cows at any one time. Because we want bulls to be rested 50% of the time, that means at least eight bulls on the farm plus spares. For some people that might seem like a lot of bulls, but it lowers your farm’s risk of having too many empty cows. 
 
Last month we talked in depth about ways to reduce the risk of bulls bringing Mycoplasma onto your farm, but the essence of it is about the importance of getting a good history of each bull’s life up to that point. Bulls should ideally come from a single source, not have been mixed with stock from other farms, and not have been used for service previously.