Weight study reveals intriguing metabolic differences between male and female racehorses

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Racing in Japan. Photo: Goki CC BY-SA 3.0 via Wikimedia Commons
Racing in Japan. Photo: Goki CC BY-SA 3.0 via Wikimedia Commons

Seasonal fluctuations seen in the weight of thousands of Thoroughbred racehorses points to possible differences in energy balance mechanisms between the sexes, according to researchers.

Japanese researchers Yuji and Toshiyuki Takahashi said both wild and domesticated horses adapted to environmental conditions through seasonal fluctuations in their metabolic rate.

The seasonal change of metabolic rates of domesticated horses in pastures is well documented, the pair noted in a paper just published in the journal BMC Veterinary Research.

However, there have been few investigations on seasonal body-weight change of domesticated horses housed in stables, who are provided with a constant energy intake throughout the year.

Seasonal changes in the body weight of racehorses during their athletic career is known to a lesser extent, they said, because their body weights were not measured in most countries, except for some Asian countries, including Japan.

The Japan Racing Association records body-weight data for all horses competing in races.

The pair set out to explore body-weight changes in Thoroughbred racehorses that competed in flat races held by the association between January 1, 2002, and December 31, 2014.

In Japan, there is no off-season for racing, with about 65 flat races held every week.

The researchers acquired 640,431 body-weight measurements for race starts, of which they used 632,540, being the horses aged 2 to 7.

The data included horses who competed in several races over their careers. The average was six starts, with the number ranging from 1 to 84 races.

Breaking down the numbers, there were 377,301 males, 19,100 geldings, and 236,139 females.

The researchers found that the body weight of stallions and geldings peaked in autumn and winter, and reached their lowest in summer.

In contrast, the body weight of mares peaked in autumn and reached their low point in spring.

The body-weight changes in geldings were similar to those of the intact male horses, although they showed broader peaks in autumn and winter.

The horses gained around 30kg during their careers, the pair reported. Most of the increase in body weight was observed when all sexes approached five years of age. The rate of body weight gain was smaller after that.

“These results indicate that the body weight of a Thoroughbred racehorse fluctuates seasonally,” they concluded, “and that there may be sex differences in energy balance mechanisms.”

The results suggested that the physiological development of Thoroughbred racehorses was completed just before they reached five.

So, what were the weights? Over the course of their athletic careers, the body weights changed as follows: stallions, from 461 ± 0.7 kg to 493 ± 0.6 kg; geldings, from 458 ± 2.5 kg to 484 ± 1.7 kg; and females, from 442 ± 0.7 kg to 472 ± 2.4 kg.

The magnitudes of the seasonal fluctuations were 7kg for stallions, 8kg for geldings,and 6kg for mares.

This compares with 22kg annual fluctuations in the body weight of Przewalski’s horses kept in semi-natural conditions, which is equivalent to 7% of their average body weight.

“This suggests that the horses in the present study were fed more consistently than horses housed under semi-natural conditions.”

Discussing their findings, the researchers said the body-weight variations occurred despite a relatively constant diet throughout the year.

Although body weight increased up to about seven years of age, most of the increase occurred before the age of about five.

The average body weights of the racehorses at the completion of physiological development were about 490kg for stallions, 480kg for geldings and 465kg for females.

They said seasonal environmental factors such as temperature or changes in daily exposure to light may explain the seasonal changes in metabolic rate.

“The seasonal fluctuations in body weight may have important implications for equine clinicians, racehorse trainers, or both,” the pair said.

“Our previous study shows that racehorses that are heavy at race time are at higher risk of superficial digital flexor tendon injury compared with horses that weigh less.

“Further, increases in body weight might affect the results of the submaximal exercise test. Therefore, equine clinicians and racehorse staff should adjust the amount of feed according to seasonal body weight changes such as the reduction of feeding amount in autumn and winter in males and geldings, and in autumn in females.”

Seasonal fluctuations in body weight during growth of Thoroughbred racehorses during their athletic career
Yuji Takahashi and Toshiyuki Takahashi.
BMC Veterinary Research2017 13:257 https://doi.org/10.1186/s12917-017-1184-3

The study, published under a Creative Commons License, can be read here

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