title: Effects of an extended photoperiod on gonadal function and condition of hair coats in Thoroughbred colts and fillies.
abstract: The effects of an extended photoperiod (EP) in Thoroughbreds colts and fillies from winter at one year old to spring at two years old on the gonadal functions, coat condition, and endocrine changes were investigated. Sixty-two Thoroughbreds (31 colts and 31 fillies) reared in the Hidaka Training and Research Center (Hidaka), Japan Racing Association were used. Thirty of them (15 colts and 15 fillies) were reared under EP conditions from December 20 to April 10, and the remaining 32 horses were reared under natural light alone as a control group. For EP, a 100-watt white bulb was set near the ceilings of stalls, and lighting conditions of 14.5-hr light and 9.5-hr dark periods were established. Blood was collected from the jugular vein once a month from October at one year old to February at two years old in both colts and fillies, and then twice a month in colts and weekly in fillies after March, and the coat condition was evaluated in January and April in 56 horses. To investigate endocrine changes, the plasma concentrations of prolactin, luteinizing hormone (LH), follicle-stimulating hormone (FSH), immunoreactive (ir-) inhibin, testosterone, estradiol-17β and progesterone were measured. No significant difference was noted in the coat condition between the two groups in January, but they changed from winter to summer coats (molting of winter coats) in April in the EP group compared with the control group. Regarding endocrine changes, the plasma concentrations of prolactin, FSH, ir-inhibin and testosterone were significantly higher in the EP colts than in the control group from January to April. The plasma concentrations of LH tended to rise in the EP colts from January to April compared with the control group. In the EP fillies, the plasma concentrations of prolactin, LH, ir-inhibin, estradiol-17β and progesterone were significantly higher during January and April, but a significantly high level of FSH was noted in the control than EP group in January. The ovulation day was advanced in the EP fillies compared with the control group. The present study clearly demonstrated that EP treatment during rearing advanced the molting of winter coats in both colts and fillies. These results suggested to be due to the action of prolactin being increased by EP treatment. In addition, EP treatment stimulated the hypothalamus-pituitary-gonadal axis even in yearlings, and advanced ovulation in fillies. Since EP treatment-induced changes in the yearlings were within the physiological range, and the method is safe and simple, EP treatment may be an effective technique in horse husbandry.
authors.affiliation: The Horse Racing School, Japan Racing Association, Chiba 270-1431, Japan.
authors.forename: Hirokazu
authors.initials: H
authors.lastname: Kunii
authors.affiliation: Department of Clinical Veterinary Science, Obihiro University of Agriculture and Veterinary Medicine, Hokkaido 080-8555, Japan ; United Graduate School of Veterinary Sciences, Gifu University, Gifu 501-1193, Japan.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Nambo
authors.affiliation: Miho Training Center, Japan Racing Association, Ibaraki 300-0493, Japan.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Okano
authors.affiliation: Equine Research Institute, Japan Racing Association, Tochigi 320-0856, Japan.
authors.forename: Akira
authors.initials: A
authors.lastname: Matsui
authors.affiliation: Hidaka Training and Research Center, Japan Racing Association, Hokkaido 057-0171, Japan.
authors.forename: Mutsuki
authors.initials: M
authors.lastname: Ishimaru
authors.affiliation: Miho Training Center, Japan Racing Association, Ibaraki 300-0493, Japan.
authors.forename: Yo
authors.initials: Y
authors.lastname: Asai
authors.affiliation: Hidaka Training and Research Center, Japan Racing Association, Hokkaido 057-0171, Japan.
authors.forename: Fumio
authors.initials: F
authors.lastname: Sato
authors.affiliation: United Graduate School of Veterinary Sciences, Gifu University, Gifu 501-1193, Japan ; Laboratory of Veterinary Physiology, Cooperative Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Tokyo 183-8509, Japan.
authors.forename: Kazuki
authors.initials: K
authors.lastname: Fujii
authors.affiliation: United Graduate School of Veterinary Sciences, Gifu University, Gifu 501-1193, Japan ; Laboratory of Veterinary Physiology, Cooperative Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Tokyo 183-8509, Japan.
authors.forename: Kentaro
authors.initials: K
authors.lastname: Nagaoka
authors.affiliation: United Graduate School of Veterinary Sciences, Gifu University, Gifu 501-1193, Japan ; Laboratory of Veterinary Physiology, Cooperative Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Tokyo 183-8509, Japan.
authors.forename: Gen
authors.initials: G
authors.lastname: Watanabe
authors.affiliation: Laboratory of Veterinary Physiology, Cooperative Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Tokyo 183-8509, Japan ; Shadai Corporation, Hokkaido 059-1432, Japan.
authors.forename: Kazuyoshi
authors.initials: K
authors.lastname: Taya
keywords: Thoroughbred colt and filly; endocrine changes; extended photoperiod; gonadal function; molting of winter coats