Before you decide to finish artificial insemination (AI), consider these three quick questions:
1. Am I likely to get enough replacement heifer calves next year?
2. Do I have enough service bulls on hand to meet demand and minimise my herd’s final empty rate?
3. Do I have returns from timed AI to consider
in my plan?
Use the guidelines below to estimate your numbers using your herd information for the mating season to date. If your expectations are not being met, you can adjust your plan by considering options such as extending AI joining to:
• generate more replacements
• reduce bull-power requirements
• cover returns to timed AI programmes, or
• compact next year’s calving.
Estimating heifer replacement numbers
Although replacement numbers can vary seasonally and among herds, as a general guideline, for farms with an average 50% conception rate, it typically requires approximately five inseminations to breed each replacement heifer and have her complete a first lactation in the herd.
Consult your vet or advisor about the number that is best for you to use.
Calculation: total replacement semen inseminations used ÷5 = estimated expected heifers in the herd.
The 5 inseminations account for potential losses from conception through the first lactation, as well as some discretionary culling of cows for non-reproductive reasons, including those that may already be pregnant from AI.
Estimating bull power requirements
Follow the steps below to estimate the number of bulls required on your farm. Insert your own figures for herd size, inseminations and conception rate. (If you are unsure of your herd’s conception rate, check with your rural professional to determine the figure you should use. The New Zealand national average is approximately 52%.)
1. Calculate your expected number of pregnant cows:
Total insems X conception rate = estimated pregnancies.
2. Subtract the pregnant cows from the total herd size to get non-pregnant cows.
3. 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:
1. 1200 x 50% = 600 estimated cows pregnant
2. 1000 – 600 = 400 estimated cows non-pregnant
3. 400 / 15 =27 estimated bulls required on farm (without spares).
Assumptions
New Zealand bull power recommendations allow for:
• one healthy fertile two-year-old bull per 30 non-pregnant mixed age cows, and
• two teams of bulls, rotated every 24 to 48 hours. 1:15 available on farm for mixed age cows.
And remember, you’ll need more bull power in the field if you have days with returns to synchrony treatments.
For more information about your local recommendations on bull numbers and management talk to your vet.
Contact your District Manager to explore AI options that can assist you in achieving your goals.
