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.

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

“…Regarding reproduction, the first ovarian activity designated by the levels of progesterone was obviously earlier in LS fillies from both Hokkaido and Miyazaki. The present results in conjunction with similar findings of previous studies [ 24 , 38 ] suggested that artificial light supplementation was able to stimulate prolactin secretion and the hypothalamic-pituitary-gonadal axis, resulting in enhancement of gonadal function and acceleration of growth in winter Hokkaido yearlings to levels similar to those of Miyazaki yearlings.…”

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

“…The only important point concerning the measurement of estradiol by PATHFAST for use in the equine clinic is the detection limit of the assay. The range of the basal level of circulating estradiol during the estrous cycle [1,2,3,4,5] and the developmental stage [23,24,25,26,27] in mares is less than 20 pg/ml, and the detection limit of estradiol by PATHFAST is 20–2000 pg/ml. Extraction of serum by ether or other chemicals [28] will be required for measuring the basal estradiol level in mares using this estradiol PATHFAST.…”

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

“…Extended photoperiod (EP) treatment activates the ovarian function earlier in broodmares, and it is widely used to advance the ovulation time in husbandry [ 15 ], because horses are typical long-day seasonal breeders. It has recently been reported that an EP treatment was applied to yearling horses and that it promoted growth rates, increased muscle mass, and advanced activation of gonadal function and coat molting [ 10 , 13 , 17 ]. The aim of the present study was to investigate whether EP treatment could successfully advance the annual molting pattern and gonadal function in Thoroughbred weanlings.…”

“…The present study demonstrated for first time that EP treatment promotes the change from winter to summer coats (molting of winter coats) in weanlings. The EP treatment advanced early elevation of circulating progesterone in fillies, indicating that EP treatment also activates the gonadal function and ovulation in weanlings as well as broodmares and yearlings [ 10 , 13 , 15 , 17 ].…”

“…This is also true in Thoroughbred fillies born in Japan in the northern hemisphere [ 5 ]. Our previous papers also reported that molting of the winter hair coat and gonadal function were stimulated by long-day EP treatment in Thoroughbred yearlings reared in Japan [ 10 , 13 , 17 ]. In our previous studies, prolactin in both sexes correlated positively with the day length.…”