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

Abstract

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.

Keywords: Thoroughbred yearlings; artificial light; climate; growth; reproductive function.

Fig. 1.

Changes of circulating total T4 (a, b), IGF-1 (c, d), PRL (e, f), and cortisol (g, h) concentrations (ng/ml) in colts (a, c, e, g) and fillies (b, d, f, h) from Hokkaido (—) and Miyazaki (- - -) under natural conditions from December to April. Values are expressed as the mean ± SEM. *Significant differences (P<0.05) between different climates in the same period in each sex.

Fig. 2.

Comparison of cannon bone circumferences between Hokkaido (□) and Miyazaki () colts (a) and fillies (b) under natural conditions from October to March. Values are expressed as the mean ± SEM. *Significant differences (P<0.05) between different climates in the same period in each sex.

Fig. 3.

Changes of circulating total T4 (a, b), IGF-1 (c, d), PRL (e, f), and cortisol (g, h) concentrations (ng/ml) in colts (a, c, e, g) and fillies (b, d, f, h) from Hokkaido in control (○) and light supplementation groups (●) from December to April. Values are expressed as the mean ± SEM. *Significant differences (P<0.05) between different treatment groups in the same period in each sex.

Fig. 4.

Comparison of increment percentages of body weight (BW) between the control (□) and light supplementation (■) groups of colts and fillies from Hokkaido (a, b), and Miyazaki (c, d) from November to March. Values are expressed as the mean ± SEM.

Fig. 5.

Hair coat condition scores of colts and fillies from Hokkaido and Miyazaki under natural (□, ) and light supplementation (■, ) conditions, respectively, in April. Values are expressed as the mean ± SEM. ^{a, b}Significant differences (P<0.05) between different climates in the same treatment group and period in each sex. ^{c, d}Significant differences (P<0.05) between different treatment groups in the same period in each place and sex.

Fig. 6.

Changes of circulating progesterone concentrations (ng/ml) from December to April in individual Hokkaido (a, b) and Miyazaki (c, d) fillies under natural (a, c) and light supplementation (b, d) conditions, respectively.

Fig. 7.

Changes of circulating total T4 concentrations (ng/ml) in colts (a) and fillies (b) from Hokkaido (—) and Miyazaki (- - -) under light supplementation from December to April. Values are expressed as the mean ± SEM. *Significant differences (P<0.05) between different climates in the same period in each sex.

Fig. 8.

Comparison of increment percentages of body weight (BW; a, b) and girth circumference (GC; c, d) between Hokkaido (□) and Miyazaki () colts (a, c) and fillies (b, d) under light supplementation from November to March. Values are expressed as the mean ± SEM. *Significant differences (P<0.05) between different climates in the same period in each sex.