Evaluation of breeding strategies to reduce the inbreeding rate in the Friesian horse population

The Friesian horse is the oldest native horse breed in the Netherlands and has grown to a worldwide popular breed with high social, cultural-historical and economic value. Although the current size of the active breeding population is quite large (approximately 100 breeding stallions and 4000 breeding mares each year), the Friesian horse has had limited genetic diversity and high inbreeding rates for several generations. Consequently, several breed-specific disorders have emerged, some of which are severely obstructing the animals’ welfare (e.g. dwarfism), and others even have fatal outcomes (e.g. hydrocephalus, the excessive accumulation of cerebrospinal fluid in the brain).

Current measures to reduce inbreeding

Two decades ago, the Royal Friesian Horse Studbook (KFPS) took measures to prevent further loss of genetic diversity and reduce the negative effects of inbreeding depression (the reduced biological fitness of animals in a population as a result of inbreeding). First, they implemented a mating quota with a maximum of 180 matings per sire per year for its first three breeding years. In addition, they started publishing individual kinships and reduced inbreeding rates below 1% per generation.

However, there were still concerns about inbreeding and its negative consequences. “It’s been 10 years since the last comprehensive pedigree analysis,” says Marije Steensma, PhD at WUR-ABG and first author of the paper, “so it was high time for an updated analysis of past inbreeding trends and the effect of the previously implemented strategies, as well as an evaluation of the effectiveness of possible future strategies.” Marije and her colleagues used pedigree analysis to reflect on inbreeding trends and their main determinants in the past, and evaluated the effectiveness of the current additional breeding strategies using stochastic simulations based on parameters derived from the true population.

Results and recommendations for the future

“What we found is that the main determinants of inbreeding rate in the Friesian horse breed are selection and the unequal contribution* of breeding sires to the number of offspring,” says Marije. “Although the current inbreeding rate in the Friesian horse population is not excessive, we advise implementing additional breeding strategies to ensure that the inbreeding rate can be kept below 0.5% per generation. Our simulations show that breeding strategies related to kinship levels are the most effective in reducing inbreeding rates.”

Based on their results, Marije and her colleagues recommend a novel approach named ‘stratified mating quotas’, where breeding sires are assigned a mating quota (restrict the number of matings per sire per year) based on their individual mean kinship compared to the population mean kinship. This approach means that breeding sires with a high kinship have a stricter mating quota compared to the general mating quota, which could also stimulate stallion keepers to select breeding stallions with lower individual mean kinship values and hence indirectly reduce the inbreeding rate.

Read the full publication for more detailed information about the results and recommendations.

*Marije: “Breeders tend to breed with the champions of the annual stallion show, resulting in a few popular sires contributing to a large number of offspring. For example, in 2022 the top 10 most popular sires delivered 35% of all the offspring born in that year.”