Farmers may review and tweak their mating plan as the season progresses. One key time to do this arises towards the end of AI mating, when you can check off these three quick questions:
1. Do I usually have enough replacement heifers, or should I aim for more?
2. Are there enough service bulls on hand to meet demand and minimise my herd’s final empty rate?
3. Was there a synchrony programme and do I need to consider those returns in my planning?
The guidelines below may help you estimate your numbers using herd information from the mating season to date.
If things aren’t quite panning out as you’d envisaged, you can make adjustments to your plan such as extending AI mating to:
– generate more replacements,
– reduce bull-power requirements,
– cover returns to synchronies or
– compact next year’s calving.
Estimating heifer replacement numbers
Numbers can vary between herd situations but, as a rule of thumb, we estimate that it takes close to five (5) conventional semen inseminations to breed a replacement heifer and have her complete a first lactation in the herd.
Calculation for conventional replacement heifer semen use:
Total inseminations ÷ 5 = estimated number of heifers in the herd for the first lactation.
NOTE: The 5 insems figure allows for losses between the point of conception and the end of the first lactation as well as for some discretionary culling for reasons other than reproductive failure of cows that may be pregnant to AI.
Estimating bull power requirements
Use the steps below to estimate how many bulls could be required on your farm, inserting your own figures for herd size, inseminations and conception rate.
NOTE: If you don’t know your herd’s conception rate check with your rural professional which figure you should use. The New Zealand national average is around 54%.
Calculate your expected number of pregnant cows:
Total insems x conception rate (54% is national average) = estimated pregnancies.
Subtract the pregnant cows from the total herd size to get non-pregnant cows.
Total herd – pregnant cows = non-pregnant cows
Divide the number of non-pregnant cows by 15 to get the estimated number of bulls required on farm to meet natural mating period requirements at the ratios explained in the assumptions below.
For example, a 1000 cow herd that has had 1200 matings:
1200 x 54% = 648 estimated cows pregnant
1000 – 648 = 352 estimated cows non-pregnant
352 / 15 = 24 estimated bulls required on farm (minimum, without spares).
Assumptions:
Using NZ bull power recommendations as a reference, allow for:
– one healthy fertile two-year-old bull per 30 non-pregnant cows, and
– two teams of bulls, rotated every 24 to 48 hours.
That means one bull per fifteen non-pregnant cows on farm. And remember, you’ll need more bull power in the field if you have days with returns to synchrony treatments.
For local recommendations and advice, on bull numbers and management, talk to your vet or advisor to determine what’s best for your farm.
