Please note this website requires cookies in order to function correctly. They do not store any specific information about you personally.
You may delete and block all cookies from this site, but parts of the site may not work. To find out more about cookies on this website, see our Privacy Statement.

Choose your Country

Get Started

Three quick questions to consider before you stop AI

Three quick questions to consider before you stop AI

AI mating is a busy time on farm, 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.

A lot of things can change during this season so, before you finish AI, your breeding plan may need adjusting. 

Use these three quick questions to help you with your decisions:

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 inseminations  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 five = estimated expected heifers in the herd.

Note: The five inseminations 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 these steps to estimate how many bulls may be required on your farm. Insert 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 inseminations 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:

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 representative to discuss AI options that will help you reach your goals.