One in every 100 sets of twin calves born in Ireland is sired by two separate bulls, ICBF geneticists have discovered.

The research, published in the Animal Genetics Journal, has finally put hard facts on what many farmers have seen in their own herds.

Heteropaternal superfecundation (HS) occurs when one or more of the cow’s eggs are fertilised by two or more males

Matthew McClure, Jennifer McClure and John McCarthy set about studying how often twins can be sired by different bulls after they were contacted by a farmer who was convinced his twin calves did not share the same sire.

John Cantillon notified the ICBF team that one of the calves looked like an Angus cross and the other a Hereford cross.

The dairy farmer had both an Angus and a Hereford stock bull on his farm.

“This type of birth, which is called heteropaternal superfecundation, or twins with different sires, in cattle has been noted before in popular press articles, including the Irish Farmers Journal, but it had never been reported in the scientific literature,” said Matthew McClure.

Heteropaternal superfecundation (HS) occurs when one or more of the cow’s eggs are fertilised by two or more males during the same reproductive cycle.

It is usually only noticed when the calves look physically different to each other, as in the case of differing Hereford and Angus colourings and markings.

“With ICBF having recently passed the landmark one million genotyped Irish cattle, we decided to look into the national HS rate and the identical twin rate, using twins where both calves and the parents were genotyped,” McClure added.

On average, 2.14m cattle are born in Ireland every year and 36,600 sets of twins are born, which equates to 1.7% of all calves born.

The trio studied almost 366,000 twin calves recorded on the ICBF database between 2008 and 2017, excluding any embryo transfer calves.

Different sires

The geneticists found that around 1% of all twins were HS twins with two different sires, while the rate of identical twin calves is 5%.

“Before genomics, farmers were only able to report HS cases when the two sires were different breeds, but with genomics we can also identify HS twins when the sires are of the same breed,” McClure told the Irish Farmers Journal.

“At ICBF this national rate is beneficial to help explain why twin cattle can have different genomic breeding values (EBI or €uroStar), as 95% of twins are not identical twins and in fact 1% have different sires,” he added.

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