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  • Writer's pictureRhiannon Handcock

Should you use Teaser Bulls for Dairy Cattle?

Updated: Aug 14, 2020

Putting a vasectomised or "teaser" ram with ewes has been shown to bring the onset of cycling forward and even synchronise ewes. For beef cattle with calves at foot, the presence of a bull can shorten the post-partum anoestrus interval (time from calving until cycling resumes) compared with cows that weren't exposed to a bull.

But what about dairy cattle?

I had a really good hunt for some studies on this topic and I have to say there is not much out there! For studies in dairy cattle, I could only find one from New Zealand and a handful from overseas. The general consensus leads to it not having the same effect as is seen in ewes or beef cows, so lets dive a little deeper!


Key Points

  1. Fenceline exposure to a bull after calving had no effect or a negative effect on reproductive performance and oestrus behaviour.

  2. Submission rate and 4-week in-calf rate may improve with teaser bulls running with non-cycling cows.

  3. Study design is a very important component when interpreting research findings.


Let's start with some of the overseas research.

The first study was completed in Utah using Holstein cows housed indoors.

This study had 3 treatments:

  1. No bull exposure (15 cows)

  2. Fenceline bull exposure twice a day (15 cows)

  3. Continuous fenceline bull exposure (15 cows)

This study found that cows that had no contact with the bull had a slightly shorter time from calving until they began cycling (as measured by progesterone concentration in blood) compared with both of the bull exposure treatments.

Figure 1. Interval in days from calving until the first increase in progesterone concentration for cows with no bull exposure, bull exposure twice per day or constant bull exposure through a fenceline. Modified from Shipka & Ellis 1999

There were no differences among the 3 treatments in reproductive performance (conception rate, days from calving until 1st service or days from calving until pregnancy).

Importantly, there was only 15 cows in each treatment, which is not quite enough cows to detect differences in reproductive performance.


Another study by the same authors used the same 3 treatments and had 15 cows per treatment.

This study measured the expression of oestrus behaviour (or heats) in cows that had no bull exposure, fenceline bull exposure twice a day or continuous fenceline bull exposure.

Cows in all 3 treatments had similar intervals from calving until first ovulation (measured by progesterone), and first oestrus behaviour. They also had similar oestrus behaviour scores.

Figure 2. Oestrus behaviour score at first or second ovulation after calving for cows with no bull exposure, bull exposure twice per day or constant bull exposure through a fenceline. Modified from Shipka & Ellis 1998

Oestrus behaviour scores in this study were used to measure the intensity of the expression of oestrus behaviour. A score of 0 meant ovulation, but no visible oestrus and a score of 4 meant ovulation was accompanied by primary signs of oestrus in 2 or more consecutive detection periods.

Now, both of these studies used an entire bull not a teaser/vasectomised bull and the exposure to the bull was through a fenceline. In NZ, if we are going to use a teaser bull he will usually be running with the cows, not on the other side of the fence!

Despite this difference in management, these 2 studies provide some evidence that in dairy cows the presence of a bull is unlikely to stimulate cows to begin cycling earlier than if there was no bull around.

The NZ study was presented at South Island Dairy Event (SIDE) in 2008.

The study was done in the 2007/08 season on 7 farms in Canterbury with around 2000 cows used.

At the start of mating, cows that had not been identified as on heat (with tail paint) were split into 2 groups;

  1. run with teaser bulls (treatment)

  2. no bull exposure (control)

Now, this is where I have a couple of doubts with this study... let me explain.

The "control" cows were all run with the rest of the milking herd, so, although there were even numbers of control and treatment cows the actual size of the mobs were different.

For example, trial farm 5 had 1350 cows total and 368 of them were non-cycling and used for the experiment. From the non-cycling cows one half were allocated to the treatment mob with bull exposure (184 non-cycling cows + 2 bulls = 186 in the mob) and the other half were run with the rest of the milking herd (184 non-cycling cows + 982 cycling cows = 1,166 in the mob).

By having 2 very different mob sizes the question the study was asking was "what is the effect of having a teaser bull in a small mob of non-cycling cows compared with having a small mob of non-cycling cows within a large herd of cycling cows?"

It does not answer the question "what is the effect of having a teaser bull running with non-cycling cows compared with not using a teaser bull?"

Scientifically it would have been more correct to have 2 approximately even sized groups of 184 for the control group and 184 cows + 2 bulls for the treatment group.

Additionally, once the cows were put up for artificial insemination (AI) they joined the herd of milkers and control cows. This means as the experiment progressed, the big mob (controls) was getting bigger and the small mob (treatment) was getting smaller. This would have made the bull's job easier as time went on!

The main results from this study were:

The 21-day submission rate (SR) was 7% higher for cows with the teaser bulls compared with the control group (purple vs blue below). The 4-week in-calf rate (ICR) was also higher (5.9%) for cows with the teaser bulls compared with the control group.

Figure 3. Submission rate, in-calf rates and non-pregnancy rates for cows that were run with a teaser bull or no bulls for 21 days from the start of mating. Modified from Norton 2008.

Both groups had similar 7-week ICR and final non-pregnancy rate (NPR) to each other.

From this study it appears that having a teaser bull running with cows that are yet to visually show oestrus may help in identifying heats when they do occur. This may help improve submission rates, which is a key factor in improving the 6-week in-calf rate.

However, this study definitely needs to be interpreted carefully due to the experimental design issues I outlined before. I would really like to see this study replicated with even mob sizes and a measure of when cows ovulated (progesterone in blood or even milk would be great!) to test if having teaser bulls helped to bring on oestrus/ovulation (as is seen in beef and sheep) or if it is just another heat detection aid in the tool-box.

What did you think about this week's post? Do you use teaser bulls on your farm currently? Or have you used them before?
Let us know your thoughts on our Facebook page ( or in the comments below.


Papers Used:



haha thanks! I wonder if we try touch base to ask? perhaps the 3 and 6 week results didn't show what they wanted it to? ;-)


Rhiannon Handcock
Rhiannon Handcock

Hi Samantha, I agree, 4 and 7 week in-calf rate is an odd measure to use instead of 3 and 6 week. I have just had another scan over the paper and they don't mention why they used these in-calf rates. The study was done in the 07/08 season, not sure if that plays a part? Sorry that wasn't much help!



Is there any info why they reported on 4 and 7 week ICR in the NZ study?? seems very odd!

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