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

Abstract

One- to two-year-old Thoroughbred colts and fillies being reared in Miyazaki (warm climate) and Hidaka (cold climate), Japan, were administered extended photoperiod (EP) treatment between December 20 and the following April 10, and its effect on growth, endocrine changes, gonadal activation, and hair coat condition was investigated. In colts reared in Miyazaki, no effect of EP treatment was noted on the growth indices, including body weight (BW), height at withers (HW), girth, and cannon circumference (CC), whereas the BWs and CCs of fillies were significantly higher in the EP treatment group than the control. In Hidaka, the BWs and HWs of colts and HWs of fillies were significantly higher in the EP treatment group. Gonadal activation characterized by an increase in circulating hormone concentrations was earlier in the EP treatment group for fillies reared in Miyazaki [luteinizing hormone (LH), follicle-stimulating hormone (FSH), progesterone (P4), and estradiol-17β (E2)] and in colts (LH, testosterone, and E2) and fillies (LH, FSH, P4, and E2) reared in Hidaka. Regardless of sex and climate, prolactin was significantly higher in the EP treatment group, whereas insulin-like growth factor (IGF-I) was not. Initial ovulation occurred before April in more of the EP treatment group than the control regardless of the climate. Molting of the hair coat, examined in March, was advanced in the EP treatment group regardless of sex and climate. These results suggest that EP treatment may promote growth and gonadal activation in fillies reared in Miyazaki and in colts and fillies reared in Hidaka and that the effect may be mediated by prolactin.

Keywords: Thoroughbred colt and filly; climate; extended photoperiod treatment; growth; testis and ovary.

Fig. 1.

Comparison of the mean rates of increase of body weight, height at withers, and girth and cannon circumference between the control group (□) and extended photoperiod (EP) treatment group (■) over the four months from December at one year old to April at two years old in Thoroughbred colts (a) and fillies (b) reared at the Miyazaki Yearling Training Farm of the Japan Racing Association. Results are expressed as means ± SEM. *Significant difference between the two groups in each parameter at P<0.05.

Fig. 2.

Changes in the plasma concentrations of prolactin (a), LH (b), FSH (c), IGF-1 (d), testosterone (e), and estradiol-17β (f) in Thoroughbred colts in the extended photoperiod (EP) treatment group (●) and the control group (○) from November at one year old to April at two years old reared at the Miyazaki Yearling Training Farm of the Japan Racing Association. Results are expressed as means ± SEM. Months are indicated by their initial letter. *Significant difference between the two groups at the same sampling point at P<0.05.

Fig. 3.

Changes in the plasma concentrations of prolactin (a), LH (b), FSH (c), IGF-1 (d), progesterone (e), and estradiol-17β (f) in Thoroughbred fillies in the extended photoperiod (EP) treatment group (●) and the control group (○) from November at one year old to April at two years old reared at the Miyazaki Yearling Training Farm of the Japan Racing Association. Results are expressed as means ± SEM. Months are indicated by their initial letter. *Significant difference between the two groups at the same sampling point at P<0.05.

Fig. 4.

Changes in the plasma concentrations of progesterone from November at one year old to April at two years old in individual Thoroughbred fillies in the control (a) and extended photoperiod (EP) treatment groups (b) reared at the Miyazaki Yearling Training Farm of the Japan Racing Association. Months are indicated by their initial letter.

Fig. 5.

Comparison of the mean rates of increase of body weight, height at withers, girth, and cannon circumference between the control group (□) and extended photoperiod (EP) treatment group (■) over the four months from December at one year old to April at two years old in Thoroughbred colts (a) and fillies (b) reared at the Hidaka Training and Research Center of the Japan Racing Association. Results are expressed as means ± SEM. *Significant difference between the two groups in each parameter at P<0.05.

Fig. 6.

Changes in the plasma concentrations of prolactin (a), LH (b), FSH (c), IGF-1 (d), testosterone (e), and estradiol-17β (f) in Thoroughbred colts in the extended photoperiod (EP) treatment group (●) and the control group (○) from November at one year old to April at two years old reared at the Hidaka Training and Research Center of the Japan Racing Association. Results are expressed as means ± SEM. Months are indicated by their initial letter. *Significant difference between the two groups at the same sampling point at P<0.05.

Fig. 7.

Changes in the plasma concentrations of prolactin (a), LH (b), FSH (c), IGF-1 (d), progesterone (e), and estradiol-17β (f) in Thoroughbred fillies in the extended photoperiod (EP) treatment group (●) and the control group (○) from November at one year old to April at two years old reared at the Hidaka Training and Research Center of the Japan Racing Association. Results are expressed as means ± SEM. Months are indicated by their initial letter. *Significant difference between the two groups at the same sampling point at P<0.05.

Fig. 8.

Changes in the plasma concentrations of progesterone from November at one year old to April at two years old in individual Thoroughbred fillies in the control (a) and extended photoperiod (EP) treatment groups (b) reared at the Hidaka Training and Research Center of the Japan Racing Association. Months are indicated by their initial letter.

Fig. 9.

Comparison of the hair coat conditions of representative colts (control, A; extended photoperiod (EP)treatment, B) and fillies (control, C; EP treatment, D) reared at the Miyazaki Yearling Training Farm and of those of colts (control, E; EP treatment, F) and fillies (control, G; EP treatment, H) reared at the Hidaka Training and Research Center of the Japan Racing Association in March at two years old.

Fig. 10.

Comparison of scores for hair coat condition between the extended photoperiod (EP) treatment group (□) and control group (■) for horses reared at the Miyazaki Yearling Training Farm (colts, A; fillies, B) and Hidaka Training and Research Center of the Japan Racing Association (colts, C; fillies, D). Results are expressed as means ± SEM. *Significant difference at P<0.05.