By Michelle van der Spuy
For observant farmers who in the past considered differences in the appearance of twin calves as “suspicious”, modern science now confirms what has long been suspected – twin calves can indeed sometimes have different fathers.
When a farmer notices twin calves that look distinctly different – one might look like an Angus with a solid black coat, while the other has the distinctive white face of a Hereford – it’s likely due to a rare biological phenomenon called heteropaternal superfecundation.
The term may sound complicated, but the concept is fascinating: It’s when two calves, born as twins, have different fathers.
What is heteropaternal superfecundation?
This is when a female animal becomes pregnant with twins from different genetic fathers. In cattle, this occurs when a cow releases multiple eggs during a single oestrus cycle and these are fertilised by the semen of different bulls. This is when a cow is mated by at least two different bulls within a short period of time, while she has multiple eggs available for fertilisation.
Dr. Alan Fahey of University College Dublin explains it as follows: “This is nature’s way of maximising genetic diversity, although it rarely occurs in cattle compared to some other species. Unlike humans who typically release one egg per cycle, cattle sometimes release multiple eggs, creating the possibility of heteropaternal twins.”
How common is it?
Research conducted by the Irish Cattle Breeders’ Federation (ICBF) on more than 366 000 twin calves found heteropaternal superfecundation accounts for around 1% of all twin births in cattle. Given that twins themselves occur at a rate of 1% to 5% of all pregnancies in cattle, heteropaternal superfecundation is indeed a rare phenomenon.
Dr Margaret Kelleher of ICBF says before genomic testing became widespread, these cases often went unnoticed unless the calves had clearly different physical characteristics. “With modern DNA technology, we can now identify these cases, even when the sires are of the same breed.”
Notable case studies
One well-documented case was on a commercial beef cattle farm in County Clare, Ireland, where a farmer was surprised when twin calves were born with significant differences in appearance. Genomic testing confirmed they were half-siblings, having the same mother but different fathers – one calf was from artificial insemination (AI) three days before the cow accidentally ended up in a field with a neighbour’s bull.
In another study published in the journal Animal Reproduction, researchers documented a case of heteropaternal superfecundation in a Brazilian beef herd, where a cow had twins after being exposed to multiple bulls in a synchronised breeding programme. The calves showed clear phenotypic differences that were later confirmed by DNA analysis.
Probability in different breeds
Dairy cattle generally show higher twinning rates than beef cattle breeds, with Holsteins having the highest incidence of twin births at around 4% to 5%. As a result, heteropaternal superfecundation is more common in dairy herds, although it is rare across all breeds.
“We see higher twinning rates in Holsteins and other dairy cattle breeds,” says Dr Jonathan Keehan, a bovine reproduction specialist. “This is partly due to genetic selection for high milk production, which has indirectly selected for hormonal profiles that may increase multiple ovulations.”
Environmental factors
Several environmental conditions can influence the likelihood of heteropaternal superfecundation by affecting both multiple ovulation rates and breeding behaviour.
Nutrition: Cows that are well-fed are more likely to have multiple ovulations. Research shows cows on a high-energy diet around breeding time may experience increased rates of double ovulation, potentially paving the way for heteropaternal superfecundation if multiple matings occur.
Season: Seasonal variations influence reproductive cycles in cattle. Some studies indicate higher twinning rates during spring and early summer in temperate regions, possibly due to changes in daylight hours that affect hormonal regulation.
Management practices: Farms that follow a combination of AI by using “clean-up bulls” (natural service bulls put into service after AI) inadvertently create conditions where heteropaternal superfecundation is more likely. If a cow has multiple ovulations and is artificially inseminated and then mated with a grazing bull within 24 to 48 hours, there is the potential for heteropaternal twins.
Stress: While chronic stress generally reduces fertility, moderate stress around breeding time can sometimes increase the rate of multiple ovulations, although research on this is still limited.
Practical implications
For cattle producers, heteropaternal superfecundation has several practical implications. It complicates accurate pedigree accounting, which is essential for breeding programmes. It also affects genomic evaluations, as calves from the same pregnancy may have different genetic merits due to their different sires.
“Farmers using mixed-sire breeding strategies should be aware of this possibility,” says Kelleher. “Especially when making breeding decisions based on the performance of twins, it is worth considering that they may not be true brothers or sisters.”
* This article was written with the help of artificial intelligence from the following sources:
1. Irish Cattle Breeding Federation (ICBF). “Twins with Different Sires.” ICBF Research Reports, 2023.
2. Ferraz, P.A., et al. “A Rare Case of Heteropaternal Twin Calves After Natural Mating in Commercial Beef Cattle.” Animal Reproduction, 2020.
3. Berry, D.P., et al. “Factors affecting the incidence of heteropaternal superfecundation in dairy cattle.” Journal of Dairy Science, 2022.
4. McCarthy, J., et al. “Genomic confirmation of heteropaternal superfecundation in cattle.” Animal Genetics, 2021.
5. Kelleher, M., et al. “Environmental and genetic factors affecting twinning and heteropaternal superfecundation in Irish cattle.” Animals, 2023.
