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BMC genetics2020; 21(1); 41; doi: 10.1186/s12863-020-00847-1

The effects of inbreeding on covering success, gestation length and foal sex ratio in Australian thoroughbred horses.

Abstract: Horses produce only one foal from an eleven-month gestation period, making the maintenance of high reproductive rates essential. Genetic bottlenecks and inbreeding can increase the frequency of deleterious variants, resulting in reduced reproductive levels in a population. In this study we examined the influence of inbreeding levels on foaling rate, gestation length and secondary sex ratio in Australian Thoroughbred mares. We also investigated the genetic change in these traits throughout the history of the breed. Phenotypic data were obtained from 27,262 breeding records of Thoroughbred mares provided by three Australian stud farms. Inbreeding was estimated using the pedigree of each individual dating back to the foundation of the breed in the eighteenth century. While both gestation length and foaling rate were heritable, no measurable effect of inbreeding on either trait was found. However, we did find that the genetic value for both traits had decreased within recent generations. A number of environmental factors also had significant effects on foaling rate and gestation length. Secondary sex ratio had only an extremely small paternal heritable effect and was not susceptible to environmental influences. In contrast to racing performance, inbreeding had no measurable effect on foaling rate or gestation length in Australian Thoroughbred horses. This could be because the level of inbreeding in the population examined is not high enough to show a discernible effect on reproductive traits. Populations that experience higher levels of inbreeding due to use of artificial reproductive technologies or extremely small population sizes may show a more pronounced reduction in natural foaling rate or gestation length. It is also possible that the intensive management techniques used in the Thoroughbred population masks any negative effects of inbreeding. The decrease in the genetic value of foaling rate is likely to be because horses with unfavourable genetic potential have not yet been selected out of the population. The change in genetic value of gestation length may be due to selective breeding favouring horses with shorter pregnancies. We also found that prioritising the mating of older mares, and avoiding out of season mating could lead to an increased breeding success.
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Publication Date: 2020-04-08 PubMed ID: 32268877PubMed Central: PMC7140579DOI: 10.1186/s12863-020-00847-1Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research article focuses on the impact of inbreeding on reproductive success including foaling rate and gestation length, and also considers the secondary sex ratio in Australian Thoroughbred horses.

Objectives and Methodology

  • The objective of the study was to examine the effects of inbreeding on various reproductive aspects of Australian Thoroughbred mares. This investigation was prompted by the understanding that genetic bottlenecks and inbreeding can often result in an increase in harmful genetic variants, leading to reduced reproductive rates in a group of animals.
  • Phenotypic data was gathered from 27,262 breeding records provided by three stud farms in Australia, which allowed the researchers to estimate inbreeding based on the pedigree of each horse dating back to the 18th century, when the breed was founded.

Findings

  • The study found that while gestation length and foaling rate were heritable traits, no discernible impact of inbreeding on either was observed. This was contrary to their hypotheses about the harmful effects of inbreeding, especially the phenomenon called inbreeding depression where genetic diversity reduction could adversely impact the biological fitness of a population.
  • They did observe a decrease in the genetic value of these traits in more recent generations. This decrease suggests that horses with less favorable genetic potential haven’t been efficiently selected out of the population.
  • Another interesting result was that secondary sex ratio had minimal hereditary influence and was not affected by environmental factors.

Implications

  • The unexpected findings prompted the researchers to speculate that the level of inbreeding in the studied population may not be high enough to have an observable impact on reproductive parameters.
  • Furthermore, it is suggested that intensive management techniques used in the care of Thoroughbred horses could be obscuring any negative effects of inbreeding.
  • The projections based on these results suggest older mares should be prioritized for mating and that out-of-season mating should be avoided to increase reproductive success.

Future Research

  • The researchers recommend studying populations with higher levels of inbreeding, or those that rely heavily on artificial reproductive technologies, to differentiate if these factors could have a more pronounced influence on reproductive rates and gestation length.
  • The decrease in genetic values of reproductive traits raises questions about selective breeding practices, bringing up the need for more effective selection processes to maintain and enhance the genetic health of the population.

Cite This Article

APA
Todd ET, Hamilton NA, Velie BD, Thomson PC. (2020). The effects of inbreeding on covering success, gestation length and foal sex ratio in Australian thoroughbred horses. BMC Genet, 21(1), 41. https://doi.org/10.1186/s12863-020-00847-1

Publication

BMC genetics
ISSN: 1471-2156
NlmUniqueID: 100966978
Country: England
Language: English
Volume: 21
Issue: 1
Pages: 41

Researcher Affiliations

Todd, Evelyn T
  • School of Life and Environmental Sciences, The University of Sydney, Sydney, NSW, 2006, Australia. evelyn.todd@sydney.edu.au.
Hamilton, Natasha A
  • Racing Australia Equine Genetics Research Centre, Racing Australia, Sydney, NSW, 2000, Australia.
Velie, Brandon D
  • School of Life and Environmental Sciences, The University of Sydney, Sydney, NSW, 2006, Australia.
Thomson, Peter C
  • School of Life and Environmental Sciences, The University of Sydney, Sydney, NSW, 2006, Australia.

MeSH Terms

  • Animals
  • Breeding
  • Female
  • Genetic Fitness / genetics
  • Horses / genetics
  • Inbreeding
  • Pedigree
  • Pregnancy
  • Reproduction / genetics
  • Sex Ratio

Conflict of Interest Statement

NAH is supported by Racing Australia in the form of salary. All other authors declare that they have no competing interests.

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