The present study demonstrates that the body weight of a Thoroughbred racehorse that consumes a relatively constant diet varied circannually. The present study shows that a horse’s sex is associated with the seasonal cycle of body weight. Furthermore, although body weight increased up to approximately 7 years of age, most of the increase occurred before the age of approximately 5 years.
Body weight change depends on energy balance, which is defined as the relationship between energy intake and energy expenditure [26, 27]. This applies to horses [28]. The energy intake of Przewalski horses, which were originally wild herbivores of Central Asia, peaks in autumn and reaches its nadir in late winter [4], whereas energy expenditure is particularly low in winter compared with that in spring and summer [5]. The energy balance of Przewalski horses is positive in autumn, indicating that energy intake is higher than expenditure, and is negative in spring, indicating lower energy intake compared with expenditure. The consequence of these seasonal energy balances would lead to a seasonal change of body weight, which is high in autumn and low in spring [3].
In the present study, the body weights of the Thoroughbred male and gelding racehorses that received sufficient annual nutrition showed seasonal variations that peaked in autumn and winter and reached their nadir in summer. Unlike the Przewalski horses, the racehorses should have received a constant amount of food throughout the year. However, a limitation of the present study is its observational design. Although it is recommended that racehorses should be fed diets according to the results of the equation described in the Methods section, the racehorses studied here were managed by individual stable staff and so we lacked detailed information of the actual energy intake of each horse during the study period. Although establishing the detailed energy intake information by precise feeding control would be ideal, investigating a large sample size of racehorses throughout their athletic career would be difficult. Indeed, because this study analyzed data for a very large number of horses over a period of 13 years, any local or short-term variations in feeding would be expected to be “averaged out” and unlikely to result in noticeable effects. To produce a systematic seasonable effect as we observed would require a consistent, widespread seasonable variation in feed provision that lasted throughout the study period. However, we are unaware of any stable staff changing the feeding protocol according to the season (personal communication). In addition, the annual fluctuation in the body weight of the horses in this study was within 8 kg, which is equivalent to less than 2% of their mean body weight. This compares with 22 kg fluctuations in the annual mean amplitude of the body weight of Przewalski horses in a semi-natural condition, equivalent to 7% of their mean body weight [3]. This suggests that the horses in the present study were fed more consistently than horses housed under semi-natural conditions.
Thus, the results of this study suggest that male and gelding racehorses were energetically abundant in autumn and winter, and deficient in summer. The assumption that there was little change in energy intake suggest that these results are due to the seasonal changes in energy expenditure (high in summer and low in winter); this is consistent with a previous report [8].
Seasonal environmental factors such as temperature or photoperiod may explain the seasonal changes in metabolic rate. The range in average monthly temperature over the year at both training centres was about 23 °C, with the highest temperatures in August and the lowest in January. The horses were stabled at the ambient temperature, so the high temperatures in summer may have increased their metabolic rate and the low temperatures in winter may have reduced it, which would be consistent with the results of previous studies [8, 29, 30]. Although a long photoperiod induces a higher metabolic rate than a shorter photoperiod in rats and cats [9, 31], there are no published studies, to our knowledge, on the direct relationship between photoperiod and energy expenditure by horses. Further studies are required to confirm this potential relationship.
In contrast to male and gelding horses, the body weight of female horses peaked in autumn and reached the nadir in spring, consistent with a previous report [11]. These findings suggest that female horses employ a mechanism that maintains energy balance that differs from that of male and gelding horses. The sexual cycle of female horses may provide an explanation. For example, the locomotor activity of female Thoroughbred horses is higher during breeding season [32]. The metabolic rate can be higher in breeding season, similar to behavioural changes, because Thoroughbred racehorses stabled in the training centres maintain their oestrous cycle during training [33]. Further studies are required to identify sex-specific mechanisms that regulate body weight.
The seasonal fluctuations in body weight may have important implications for equine clinicians, racehorse trainers, or both. 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 [20]. Further, increases in body weight might affect the results of the submaximal exercise test [21]. 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.
We show here that the body weight of Thoroughbred racehorses increased 25 to 30 kg during their athletic career. According to the trend component, most of the increase was observed until horses approaches approximately 5 years of age. To our knowledge, this is the first report that determined the time interval associated with the increase in body weight of Thoroughbred racehorses during their athletic career. This finding is reasonable because the last epiphysis closure in a cervical vertebra occurs at the age of 4–5 years [22], indicating that horses mature by the end of their fourth year. Moreover, racing performance or average racing speed peaks at approximately 4.5–5 years of age [34, 35]. Together, the present and previous studies indicate that the physiological development of Thoroughbred racehorses is complete just before 5 years of age. It would be interesting to investigate the association between body weight increase and anatomical development, such as the development of bones and joints. However, this would be difficult to achieve for a large sample size. The mean body weights of the Thoroughbred racehorses at the completion of physiological development were approximately 490 kg, 480 kg and 465 kg for males, geldings and females, respectively. We propose the use of these values as standards for Thoroughbred racehorses in Japan.
The remainder component is an indication of the residual data after subtracting the seasonal and trend components [24] and shows variations due to random features. However, there appeared to be a small element of seasonality in the remainder component for the young male horses, especially those aged 2–3 years (Fig. 1d). Another age-related factor was that there were fewer older horses than younger ones, especially among the females (Table 1). It is possible that different number between older and younger horses may have had different seasonal patterns according to ages. For these reasons, we divided the data into two groups for each sex, younger horses (2–4 years) and older horses (those over 5 years). However, the results showed almost the same seasonal pattern as the undivided datasets, with the body weight of the males and geldings reaching a peak in autumn and winter, and their nadir in summer whereas the body weight of the females peaked in autumn and was at its lowest in spring, although the divided data showed a slightly wider range than the combined data for all ages. These additional results suggest that the datasets were appropriately divided into seasonal, trend and remainder components.
We should take care to correctly interpret the data acquired from the analysis of body weight change that occurs each month or season. For example, overall average body weight in JRA may be lower in June compared with May, because 2-year-old horses with lighter body weights compared with older horses are admitted to the JRA in June. To avoid such a misleading analysis, we applied STL techniques to body weight data classified according to age, which is a useful technique in diverse disciplines [36, 37]. The STL technique is an effective tool for visualizing and clarifying time series events by dividing them into seasonal, trend and remainder components [24]. We used this technique here to identify the seasonal change associated with growth.
Due to the JRA system described in the Introduction section, the distribution of age and month relatively changed according to the season. Especially after August, the number of 2-year-old horses increased while the number of 3-year-old horses decreased. If there were seasons specific for any age or gender, racing system can have possible role on body weight change. For example, if autumn and winter season were for only 2-year-old horses, reduction of workload for over 3-year-old horses might cause overall increase of body weight. However, races from Maiden to Group One are held every week regardless of age and gender in JRA. Therefore, we speculate that JRA system did not affect our interpretation.