Genes, not nutrition, determine adult size

A panel of renowned nutrition and veterinary experts gathered for the annual Thoroughbred Breeders’ Association seminar at Tattersalls and this year it focused on nutrition and breeding.

Growth rate, not adult size, can be affected by a myriad of factors including the environment and nutrition – that was the key message delivered by Dr Joe Pagan from Kentucky Equine Research.

“You’re not going to fool Mother Nature,” said Pagan. “You can’t get a horse to grow bigger than they’re genetically predisposed.

“However, thoroughbreds are expected to be competitive athletes by the age of two, at which point they will have reached around 85 per cent of their mature body weight, and 95 per cent of their mature height.”

The extensive weighing and measuring of thousands of foals and yearlings across the US, Britain, India and Australia showed foals gained 50 per cent of their total weight gain in the first six months of life, and 75 per cent in the first year.

The data also showed yearlings typically underwent a growth spurt in the spring their yearling season, regardless of which month they were born in.  However, foals born early in the season grew at a slower rate between day seven and day 30 than their later counterparts.

“The foals do catch up, but this type of compensatory growth pattern is not good,” said Dr Pagan.  “If horses grow too fast they can succumb to developmental problems.  Optimal growth is the rate which results in desirable body size at a specific age with the least amount of developmental problems.  So the question is, if we push foals to grow into a large yearling, will it be rewarded?”

The answer is yes according to Dr Pagan, who provided data showing that, of thousands of yearlings weighed and measured before going through the Keeneland sales ring, on average those who sold for above the session median were significantly heavier – while those who failed to sell were significantly lighter.