'So-called 3 x 3 inbreeding is negligible' Soundness in thoroughbreds BLOODSTOCK WORLD.

Byline: Rachel Pagones, Amy Bennett and Nancy Sexton

In the second part of a week-long series, we ask professionals for their considered opinion on whether racehorses are becoming more fragile. Today Patrick Cunningham, professor of animal genetics at Trinity College, Dublin, and Matthew Binns, professor of genetics at the Royal Veterinary College in London, respond to the question: Is the amount of inbreeding now present in the thoroughbred a contributor to increasing fragility?

Patrick Cunningham "We defined the long-term base for inbreeding 20 years ago.

We found out if you go right back, there is a 13 per cent increase in inbreeding in the population base over three centuries. The current rate of inbreeding would be quite low, in terms of what would be dangerous.

"The small number of males stallions in the population has not had a significant effect on the level of inbreeding. We looked specifically at this, and we found the dominant stallions produced a hardly detectable change in the breeding population. Even stallions producing 100 foals per year are doing very little compared to what's possible.

"If you look outside of horses, the top bulls have a million offspring. The short answer is that having 20 top stallions globally siring 100 foals per year is not really affecting inbreeding - neither would 50 top stallions each siring 150 foals per year.

"In Australia, Frank Nicholas, a professor at the University of New South Wales, looked at the effect of shuttle stallions. The assumption was they were decreasing variation in the breed. And they found the shuttle stallions were actually increasing variation in the population because they were bringing in new genes. This was about ten years ago, but the level of inbreeding would not have changed much in ten years.

"There is very little inbreeding in the modern thoroughbred. We took 10,569 mares from the Stud Book from 1961-1964, which is all of Volume 35, and traced the breeding history of each mare back in time to get her complete record - an average 12-year reproductive life. There were 85,000 ancestors, and the full inbreeding coefficient over five generations was less than one per cent.

"One per cent over five generations is actually quite small. So-called 3 x 3 inbreeding is negligible, and it is not what scientists mean when we talk about inbreeding. The problem with blogs and public discussions is the people who talk about inbreeding don't know what they're talking about.

"For instance, there are 16 million people in central Asia who carry the Y chromosome of Genghis Khan. Inbreeding is a statistic that can be measured from 0 to 100 per cent.

"Now, there is a difference between inbreeding and selective breeding. We are selecting pretty intensely for extreme performance.

Intensive selection carries risks in all species - we've been breeding turkeys that can't stand up, for instance, because their chests are so heavy. Holstein cows worldwide are suffering a decline in fertility because of very intensive selective breeding. If there is a decline in fitness, in soundness - and these are very vague terms - in the thoroughbred, it's most likely a consequence of continued very intensive selection. There is no free lunch in nature."

Matthew Binns: "We don't know. While the predicted levels of inbreeding in the thoroughbred are not particularly high when compared to those in some other domestic species, there is evidence from DNA studies that the actual levels of inbreeding may be much higher.

"It will be important to establish some baseline data on the current levels of DNA homozygosity duplication of one gene in an individual in the breed, and to monitor whether increasing stallion book sizes and the reduction in the number of stallions at stud result in changes to these levels in the future.

"An early general indication that inbreeding is reaching problematic levels in a species is a decrease in reproductive efficiency. There is little indication that the proportion of thoroughbred mares producing live foals has changed, but the many advances in veterinary reproductive care over the last 30-40 years could have masked any problems that did exist.

"From a historical perspective, it is clear that owners and trainers have bemoaned the increasing unsoundness of the thoroughbred for more than a hundred years. Recent important epidemiological studies have established the extent of the injury problems today, and provide a valid fixed point against which future injury rates can be assessed.

"There has been little or no selection for soundness in the breed - it has been common for individuals with soundness problems to enter the breeding population, if they have good race records or pedigrees. It is possible that the increased desire for speed and precocity is leading to the selection of lighter-boned individuals less able to withstand the rigours of training and racing.

"Several of the most recent popular and successful stallions had soundness problems, and although we do not fully understand the genetic contribution to fracture and tendon injuries, it would not be surprising if, together with desirable genes for racing performance, these individuals passed on some genes that increased fragility."

CAPTION(S):

Patrick Cunningham: "stallions producing 100 foals per year are doing little compared to what's possible" ; Matthew Binns: "little or no selection for soundness in the breed"