Researchers at Penn State University have discovered a new genetic defect in Holstein cattle. A new haplotype potentially affecting calf well-being was discovered, that may result in calves that are weak and unable to stand at birth or shortly after.

This condition, currently known as calf recumbency, has found to be genetically more complex than other known faults. Some affected calves in the study were able to recover, but most did not survive beyond 6 weeks of age.

Given recumbency has similar symptoms to that of many other calf diseases and issues, it’s not surprising that it has taken this long for the connection to be made to a genetic default. Unlike many other recessive traits, recumbency is not always lethal, and the symptoms are not the same in every animal.

A test has now been developed for this recessive gene, and currently existing AI bulls are being tested.

The tested bulls have been grouped into three categories: Free, Carrier or Affected.

The bulls listed as “Free” are tested free from calf recumbency.

Those labelled “Carrier” have tested heterozygous for calf recumbency, meaning they carry one copy of the effected gene, which they don’t display themselves but can pass on to their offspring.

Those labelled “Affected” are considered homozygous for calf recumbency, meaning they carry two copies of the effected gene which they will always pass to their offspring. The offspring will then either be a ‘Carrier’ if they do not also get an affected gene from the other parent, or they themselves will be classed as ‘affected’ if they inherit two of the recumbency genes (one from each parent).

Note that this deviates from the historic understanding of genetics where a homozygous designation (Affected) is traditionally lethal. At this point, it is believed the affected individuals may have recovered from calf recumbency, but the reason is not yet fully understood. In the study, some calves that tested as Affected showed little to no symptoms, which is highly unusual for a recessive trait.

The impact this will have on artificial breeding in the dairy industry remains to be seen. Some breeding companies have already tested a good number of their currently standing bulls, and results are mixed as to how many are carriers and affected animals. One American company has found around 10% of their currently available bulls are carriers, with around 1% being classed as affected.

Currently, testing has not been commercialised, and it is not practical to undertake widescale testing of herds. However, the hope is that one day testing for Recumbency will become a mainstream part of genetic testing of females, and farmers will then be in a position to select against the trait when making their regular breeding decisions.