Act now if submission rate is less than 80pc
The first three weeks of mating should now be over for many of you with spring calving herds, so the submission rate at 21 days will be known at this stage. This figure is calculated by dividing the number of cows inseminated in the first 21 days by the number of cows at the start of mating and multiplying by 100.
If the result shows a submission rate of less than 80pc, action should be taken. If it is more than 90pc, you can be satisfied that mating is going well and that you can give remaining animals more time, especially if they are calved less than 35 days.
After the intense period of AI most farmers opt to use bulls from around six to eight weeks of mating. But we need to make a quick assessment that everything is in order for this time.
Ensuring good in-calf rates during the natural mating period is a critical feature of a well-run dairy herd but is reliant on good bull management and having a sufficient number of bulls on hand. This can be one of the simplest areas to fall down on as any change in the size of the herd, length of AI period, submission rate and conception rate can substantially affect the number of bulls required.
To calculate the number of bulls that you need to run with your herd depends on the estimated number of non-pregnant cows you will have at the end of the AI period.
At least one bull is required for every 30 non-pregnant cows in the paddock with additional bulls required to allow for regular bull rotations and to replace bulls that become inactive due to lameness or other health issues.
Your estimated bull number is calculated using your three-week submission rate, an estimated conception rate and the number of weeks that AI has been used for. Accurate conception rates can only be calculated from pregnancy testing. It can vary from 60pc in the top herds to as low as 40pc if your herd fertility is poor, so 50pc is probably the best guestimate we can make.
For example, the number of bulls required in the paddock after six weeks of AI for 150 cows with a submission rate (SR) of 84pc and conception rate (CR) of 50pc is shown using the following calculation: