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  • Title: Three quick questions to consider before you stop AI

  • Category:

  • Date: Thursday, 25 May 2017


AI mating is a busy time on farm, but it speeds by quickly and before you know it it’s coming to an end. The natural mating period follows on most farms, with service bulls ready and waiting.

As the season unfolds, your farm’s plan may need adjusting. Mating is no different.

Before you decide to stop AI mating, 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 synchrony returns to consider in my plan?

You can use the guidelines below to estimate your numbers using your herd information for 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

Although the numbers do vary between seasons and herds, as a rule of thumb for farms with an average 50% conception rate we estimate that it takes close to five (5) inseminations (insems) to breed each replacement heifer and have her complete a first lactation in the herd.  Talk to your vet or advisor about the number that is best for you to use.


Total replacement semen insems used divided by 5 = estimated expected heifers in the herd.

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.

(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 52%).

1. Calculate your expected number of pregnant cows:

Total insems X conception rate (52% is national average) = 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:

1200 x 50% = 600 estimated cows pregnant

1000 – 600 = 400 estimated cows non-pregnant

400 / 15 =27 estimated bulls required on farm (without spares).



New Zealand bull power recommendations 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.

So that’s one bull per 15 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 more information about your local recommendations on bull numbers and management talk to your vet.

Contact your local LIC sales representative to discuss AI options that will help you reach your goals.

Disclaimer: Any advice, tasks or suggestions given on this website (“advice”) are of a general nature only and may not be suitable for your individual herd requirements.  We recommend that you discuss your individual herd requirements with your veterinary and farm advisory professionals. Any results from the advice given on this website may vary and LIC gives no warranty that the intended outcome will be achieved.

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