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  • Writer's pictureRhiannon Handcock

Why am I getting so many bull calves?

Updated: Aug 22, 2021

Cattle usually have one calf in each pregnancy, so is it the flip of a coin as to whether a cow produces a heifer or a bull calf? Or is there something else going on?

In this summary we are taking a closer look into heifer-to-bull ratios to investigate why we sometimes get so many bull calves!

To do this we have combined the results of multiple studies to give you a good overview on why some herds or years may result in more bull calves than heifer calves, or vice-versa.


Key Points

  1. The sex of the previous calf was related to the sex of the current calf.

  2. Cows that had higher BCS at calving were more likely to have a heifer in their next calving.

  3. Cows may be more likely to have bull calves if the weather is better at mating.


Firstly, what determines if a calf is a bull or a heifer?

A cow will pass on one X chromosome to the calf and a bull will pass on either one X chromosome or one Y chromosome to the calf. This means that the calf will either have XX (two X chromosomes) or XY (one X and one Y chromosome). An XX calf is a heifer and an XY calf is a bull. Usually, a bull's semen will be made up of half X chromosome carrying sperm and half Y chromosome carrying sperm, so we would say there is a 50% chance of an XX (heifer calf) and a 50% chance of an XY (bull calf).

There are a few factors related to influencing the sex of the calf.

These include:

Random chance

Now if you have been farming for any amount of time you will know that it is not always half heifers and half bulls, some years you will get more heifers than bulls and some years you will get less heifers than bulls.

One of these reasons is simply random chance. If cow A is having a heifer, she doesn't tell cow B to have a bull to keep the numbers even! Sometimes both cow A and cow B have heifers and other times they both have bulls. We can calculate the chance that these events happen. From these two cows:

  • the chance of two heifers is 25% (50% chance of a heifer x 50% chance of a heifer)

  • the chance of two bulls is also 25% (50% chance of a bull x 50% chance of a bull)

  • the chance of one heifer and one bull is 50% (50% chance of a bull x 50% chance of a heifer PLUS 50% chance of a heifer x 50% chance of a bull)

Now let's take this to the herd level, if we have a 500 cow dairy farm we would expect that we would have 250 heifers and 250 bulls. But we might have 249 heifers and 251 bulls or 248 heifers and 252 bulls... again we can calculate the chance of any of these combinations occurring. When we do this we get a pretty picture of how many heifers we expect in a 500 cow herd.

The graph below shows us what we expect if we have 1,000 dairy farms with 500 cows each. The tip of the graph shows that most farms will have near 250 heifers. In fact we would expect 377 out of the 1,000 farms to have between 245 and 255 heifer calves born. We would also expect that 311 lucky farms to have more than 255 heifers and 311 unlucky farms to have less than 245 heifers. Again this is random chance, and is similar to rolling a dice, or flipping a coin.

Figure 1. Estimated number of heifer calves born for 1,000 dairy farms that had 500 cows calving per year

Based on this example, nearly all the farms will have between 216 and 284 heifers born in one year which works out between 43% and 57% heifers born. However, there is a very very very very small chance that a farm will have 400 bulls and only 100 heifers (80% bulls, 20% heifers) or even 500 bulls and zero heifers.

For the first study (Roche et al. 2006) the average percentage of bulls born was 52% and heifers born was 48%. This is close to 50:50, but slightly favours bull calves. Taken over a 400 cow herd this would mean 208 bulls born and 192 heifers born (if we assume no twins).

The average percentage of bulls born is referred to as the Secondary Sex Ratio or SSR, whereas the average percentage of bulls conceived is referred to as the Primary Sex Ratio or PSR. It is very difficult to estimate the PSR, so the SSR is much more commonly recorded and reported.

Sex of the previous calf

The first one was the sex of the previous calf. The authors found that if a cow had a bull calf in one year, she was more likely to have a bull calf the next year. Potentially there may be something going on in the cow that makes her more likely to either conceive a bull calf or make her more likely to carry a bull calf to term.

Body Condition Score (BCS)

Another factor identified was cow body condition score (BCS) at the previous calving. The cows with higher BCS at the previous calving were more likely to have heifer calves than cows with lower BCS. Another study using the same dataset (Roche et al. 2006) put this down to cows with higher BCS at calving being more likely to experience a large drop in BCS from calving to mating compared with cows that were lower in BCS at calving. Therefore it seemed more likely that it was the change in BCS from calving to conception that was affecting the chance of having a heifer or a bull calf.

There is a hypothesis called the "Trivers-Willard hypothesis", that in mammals, the condition of the female around conception affects the probability of having a male or female offspring. In other words, cows with greater BCS near mating would be more likely to have bull calves and cows with lower BCS near mating would be more likely to have heifer calves. This second study found exactly that, a bull calf was more likely to be born from a cow that did not lose any BCS between calving and mating when compared with cows that lost BCS.


Climate factors, such as rainfall, temperature, humidity, sunlight hours, and evaporation rate, were averaged for the week before conception for each cow.

The below graph is illustrating what is called the "odds ratio" for each of the climate factors. What this figure shows is that if the blue circle higher than 1 (above the dashed line) then we would say that the factor was associated with more bull calves being born (i.e. less heifers). If the blue circle is lower than 1 (below the dashed line) then we would say that the factor was associated with less bull calves being born (i.e. more heifers). If the blue circle was very near to 1 (the dashed line) then we would say that the factor had little to no association with a bull or heifer calf being born.

We can see in the graph that a bull calf was more likely to be born following higher daily maximum or minimum temperature and greater evaporation around the time of mating.

Although the circles themselves are higher than 1 for sunlight hours and radiation, the error bars (blue lines on the graph) cover both above and below the dashed line. This means that with more sunlight hours or radiation, there was no change in the chance of the cow having a bull or a heifer calf.

Rainfall ranged from zero to 55 mm/day and was grouped into 11 categories. The authors found no association between rainfall the week before conception and the ratio of heifers to bulls born.

Potential reasons why cows may be more likely to have bull calves when the weather is better may go back to the "Trivers-Willard hypothesis" we mentioned before. In spring, having higher daily temperatures (both the daily high and daily low) would be conditions that are favourable for good grass growth. So having good grass growth would lead to cows potentially being better fed (better condition) leading to an increased chance of bull calves.

However, neither of these studies specifically measured and tested the grass growth in relation to having more bulls or heifer calves born, although it sounds like a sensible explanation.

Both of the studies were in dairy cattle, it is likely that there are similar influences on the sex of the calf in beef cattle as well, however I couldn't find much when I searched the studies.

There were also factors identified that had little to no effect on the sex of the calf.

These included:

  1. Breed of the cow

  2. The year of conception

  3. Lactation number (or parity) at conception

It appears that there are some cow and weather related things that influence whether cows were more likely to give birth to a bull or heifer calf.

I don't think these results are enough to make any changes to your farm system in a way to favour more heifer calves though! Perhaps it is more of an interesting finding to keep in mind at calving time if you get a big run of bull calves?

Something that can be done to change the percentage of heifer calves born would be to use sexed semen - we can cover that topic in a future post for those interested.

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LIC has also written a post on a similar topic, and done a fun YouTube video. I will link them here if you want more information:


Full Papers:

Study 1

Study 2


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