Different effects of an extended photoperiod treatment on growth, gonadal function, and condition of hair coats in Thoroughbred yearlings reared under different climate conditions

Effects of an extended photoperiod on body composition of young Thoroughbreds in training

The effects of an extended photoperiod (EP) on body composition of Thoroughbreds colts and fillies from December at one year old to April at two years old were investigated. Seventy-three Thoroughbreds reared and trained in Hidaka Training and Research Center, Japan Racing Association, Hokkaido were used. Forty-one horses were under the EP conditions from December 20 to April 15, and the 32 horses were under natural light alone as the control group. Body weight (BW), rump fat thickness (RFT), fat free mass (FFM) and percentage of fat (%F) were used as parameters of body composition. The present study revealed that BW and FFM increased with age in the EP group in colts. In fillies, BW increased with age in both the EP and the control group, however FFM increased with age only in the EP group. From December to April, only colts had a higher rate of increase in both BW and FFM in the EP group than in the control group. However, according to the mean rates of increase in FFM from January to March, the EP group was significantly higher than the control group in both sexes. Furthermore, monthly increase rate of FFM in March was significantly higher in the EP group than in the control group in both sexes. These results suggests that EP treatment to young Thoroughbreds in training at Hokkaido, which is shorter daylength in winter, accelerate the increase of FFM, representing muscle mass.

Effects of different winter climates in Japan on body composition of young Thoroughbreds in training

Changes in the body composition of 50 Thoroughbreds colts and fillies, born between 2004 and 2010, were compared between those reared at the Hidaka Training and Research Center (Hidaka), Hokkaido, which is extremely cold in winter, and those reared at the Miyazaki Yearling Training Farm (Miyazaki), Kyushu, which is mildly cold in winter. The horses were divided into two sex groups and reared and trained in Hidaka or Miyazaki for 7 months from October of one year of age to April of two years of age. Body weight (BW), rump fat thickness (RFT), fat-free mass (FFM), and percentage of fat (%F) were used as parameters of body composition. This study revealed that BW and FFM were higher, and %F was lower in colts than in fillies at both training sites. Among colts, Miyazaki colts tended to have higher FFM values than Hidaka colts, and %F was significantly lower in Miyazaki colts than in Hidaka colts. Furthermore, from October to April, Miyazaki horses had a higher rate of increase in BW than Hidaka horses in both sexes and a higher rate of increase in FFM in colts. The higher rate of increase in FFM in Miyazaki colts suggests that training young Thoroughbreds in winter under mildly cold climate is more effective, than severely cold climate, particularly in colts.

Comparison of physical body growth and metabolic and reproductive endocrine functions between north and south climates of Japan in trained Thoroughbred yearling horses

This study aimed to compare body growth, metabolic, and reproductive hormonal changes in trained Thoroughbred yearling horses under different climate conditions with and without light supplementation (LS). Thoroughbred yearlings raised at research centers of the Japan Racing Association in Hokkaido (north) or Miyazaki (south) were divided into control and LS groups. In the LS groups, 44 colts and 47 fillies from Hokkaido and 11 colts and 11 fillies from Miyazaki were exposed to LS with an extended photoperiod of 14.5 hr of daylight and 9.5 hr of darkness. One week before and once a month after LS, circulating total thyroxine (T4), insulin-like growth factor-I (IGF-1), prolactin (PRL), cortisol, and progesterone (P4) concentrations were measured by radioimmunoassay and fluoroimmunoassay, respectively. Growth parameters, including body weight, height, girth, and cannon bone circumferences, were measured monthly. Hair coat (HC) condition was scored. Under natural conditions, the T4 concentrations of Hokkaido yearlings tended to be higher, whereas the IGF-1 (colt) and PRL levels were significantly lower than those of yearlings in Miyazaki. Growth parameters and HC scores were lower in Hokkaido yearlings. With LS, the PRL and P4 concentrations in Hokkaido and Miyazaki were higher, and the first ovarian activity tended to be earlier than in the controls. Only LS Hokkaido yearlings showed significantly higher HC scores than the controls. Comparing the different climates among the LS yearlings, the levels of PRL and P4 and the HC scores in Hokkaido yearlings increased and reached levels similar to those in Miyazaki yearlings. The body weight and girth increment percentages of Hokkaido yearlings in January dramatically decreased and then eventually increased to levels similar to those of Miyazaki yearlings. This suggested that yearlings in naturally colder Hokkaido exhibit higher basal metabolism to maintain homeostasis. However, providing LS may help to improve growth and early development of reproductive function in Hokkaido yearlings to levels equal to those of Miyazaki horses.

PATHFAST, a novel chemiluminescent enzyme immunoassay for measuring estradiol in equine whole blood and serum

A novel chemiluminescent enzyme immunoassay system, PATHFAST, for the measurement of estradiol in horses was evaluated. The concentrations of estradiol in the whole blood and serum of mares were measured using PATHFAST and the estradiol concentrations measured by PATHFAST were compared with those measured by a time-resolved fluoro-immunoassay (FIA). To monitor physiological changes, serum estradiol concentrations in mares were measured using PATHFAST throughout the gestation period. The serum estradiol concentrations correlated highly with those in whole blood samples. The serum concentrations of estradiol measured by PATHFAST also correlated well with FIA. Circulating estradiol increased during mid-gestation and high levels of serum estradiol were maintained in late gestation, followed by an abrupt decline to term. These results demonstrate the utility of PATHFAST in equine clinics as an accurate diagnostic tool for the rapid assay of estradiol within 26 min using unextracted whole blood.

Promoting effects of an extended photoperiod treatment on the condition of hair coats and gonadal function in Thoroughbred weanlings

The effects of an extended photoperiod (EP) treatment (14.5 hr light, 9.5 hr dark) on Thoroughbred colts and fillies from December 25 at 7-9 months old to the following May at 12-14 months old on coat condition and gonadal functions were investigated. Coat condition was evaluated in April. The colts and fillies in the EP treatment group changed from winter to summer coats (molting of winter coats), whereas those in the control group did not. To determine the day of first ovulation, the plasma concentrations of progesterone were measured once a month in fillies. The day of first ovulation was advanced in the EP treatment fillies compared with the control fillies. The present study clearly demonstrated that the EP treatment advanced the molting of winter coats and advanced ovulation in fillies, even in weanlings.