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In-Breeding Impact Is Insignificant

Friday, 23rd March 2007

Research by PhD student Kao Castle (supervised by Sydney University's Professor Frank Nicholas using data provided by the Australian Stud Book) has shown a relatively low rate of increase in thoroughbred in-breeding over the past 20 years: the co-efficient has risen from 12.9% to only 14.3%. In-breeding is the practice of breeding between relatives & the inbreeding co-efficient indicates the proportion of genes inherited from each parent that are identical by descent. Over generations in-breeding leads to a reduction in generic variability & increased sameness, which causes reduced health & fitness in the population, increased incidence of hereditary disease & decreases in reproductive performance. Michael Ford, Keeper Of The Australian Stud Book, commented: "The low increase rate suggests breeding practices over the last 20 years have not posed a threat to the generic well-being of the Australian thoroughbred." These practices include: use of shuttle stallions (averaging 64 each year); increased number of mares covered by a stallion (averaging 10 in 1986 & now 28); & more stallions covering over 100 mares (24 in 1997 & now 73). Castle believes the use of shuttle stallions has positively influenced the average in-breeding co-efficient, because breeders used overseas stock to diversify the generic background of their foals. She reported: "There is likely to be no significant effect on increasing the in-breeding co-efficient as long as breeders diversify with overseas born sires." The research also found that in 2000, 63% of all thoroughbred foals born in Australia had an overseas parent (compared with 56% in 1990). And between 1990-2005, foals born with at least 1 overseas parent "were on average less in-bred than their colonial equivalents".

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