“Family tree” for Cleveland Bay horses has helped cut inbreeding

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Cleveland Bay mare Scarholme Litha.
Cleveland Bay mare Scarholme Litha. © David Snooks

Scientists in Britain have created a “family tree” that has helped to safeguard the genetic diversity of England’s oldest established horse breed, the endangered Cleveland Bay.

Using the newly developed “Breed Conservation and Management System” (BCAS) that offers recommendations based on horses’ genetic backgrounds, breeders can use a simple red, amber, green system based on whether horses are related and should be bred.

It has helped to reduce the rate of inbreeding in Cleveland Bay horses from more than three percent per generation to less than 0.5% per generation.

In addition, the effective population size in the UK – which takes into account the breeding, unrelated adults in any population – has increased from 20 in 1994 to more than 140 in 2020.

The results of the work of over almost 20 years by researchers from the School of Animal, Rural and Environmental Sciences at Nottingham Trent University has been published in the journal Ecology and Evolution. The project focused on minimising relatedness amongst the rare breed by supporting breeders with historical information about individual horses’ kinship via their pedigree records. It also involved the University of Lincoln and the Cleveland Bay Horse Society.

The team says the system could be applied to other rare and endangered animal breeds to ensure their genetic status is maintained.

The consequences of poor breed management and inbreeding can lead to a variety of serious health concerns in individual animals, including problems with fertility.

Small populations of animals, as well as larger unmanaged groups, can be plagued by genetic regression often due to a lack of breeding support tools that are easy to use and can support breeders’ decision-making.

The researchers argue that as a result of this work the Cleveland Bay is now in a genetically sustainable position.

They say their population management tool could be applied to any captive species, from rare and endangered breeds, to animals in zoos and livestock.

The Cleveland Bay is a heritage British breed that originated three centuries ago in the northeast of England, where it was used both as a carriage horse and to work the land.

Since the end of the first world war, the breed has suffered a substantial decline in numbers because of modernised transport and mechanisation of agriculture.

By the 1950s it was close to extinction, with only four purebred stallions remaining and it is currently one of seven equine breeds listed as a priority by the Rare Breeds Survival Trust.

The Cleveland Bay Horse Society published its first studbook in 1885, containing retrospective pedigrees of animals dating back to 1723; this now yields a non-Thoroughbred studbook dating back almost 300 years and covering 38 generations.

“What we have created is like a family tree for Cleveland Bay horses,” said Nottingham Trent University researcher Professor Philippe Wilson.

“There are serious and growing concerns about genetic diversity in many breeds. Animals are bred for all sorts of commercial purposes and for particular traits and qualities, but once genes are gone they are gone forever.

“If we do not have enough pure-bred individuals, or if one male in a small population is breeding with many females, for instance, this can have a deleterious effect on genetic diversity.”

16 Years of breed management brings substantial improvement in population genetics of the endangered Cleveland Bay Horse. Andrew Dell, Mark Curry, Elena Hunter, Ruth Dalton, Kelly Yarnell, Gareth Starbuck, Philippe B. Wilson. Ecology and Evolution. https://doi.org/10.1002/ece3.8118

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