Comparison of growth and endocrine changes in Thoroughbred colts and fillies reared under different climate conditions

Abstract

Development and endocrine changes in Thoroughbreds colts and fillies were compared between those reared at two facilities of the Japan Racing Association, the Hidaka Training and Research Center (Hidaka) and Miyazaki Yearling Training Farm (Miyazaki). Thoroughbred colts and fillies born in Japan between 2003 and 2010 were used. Each colt group and filly group was divided into 2 groups, respectively, and raised in Hidaka or Miyazaki for 7 months from September at 1 year old to April at 2 years old. For the growth parameters, the body weight, height at withers, and girth and cannon circumferences were measured once a month. For parameters of endocrine function, circulating prolactin, luteinizing hormone (LH), follicle-stimulating hormone (FSH), insulin-like growth factor-1 (IGF-1), testosterone, progesterone, and estradiol-17β levels were measured. Regarding growth, the rate of increase over the 7-month period was significantly higher in both colts and fillies raised in Miyazaki than in Hidaka in all 4 parameters: body weight, height at withers, and girth and cannon circumferences. The endocrine changes of the colts and fillies born in 2007 were as follows. In colts, although circulating prolactin tended to be higher in colts reared in Hidaka from October to April, circulating LH, FSH, testosterone, estradiol-17β and IGF-1 tended to be higher in colts reared in Miyazaki than in Hidaka, suggesting that the gonadotropin-releasing hormone-LH/FSH system and the growth hormone-IGF-1 system were more active in colts reared in Miyazaki as compared with those reared in Hidaka. In fillies, circulating prolactin tended to be higher in fillies reared in Hidaka in February and March, but no significant difference was noted in the serum LH, FSH, IGF-1, or progesterone level between the 2 groups. Circulating estradiol-17β tended to be higher in fillies reared in Miyazaki than in Hidaka in October and November. Regarding ovarian function, the initial ovulation occurred by the end of March in 2 (16.7%) of 12 fillies reared in Hidaka and 7 (38.9%) of 18 fillies reared in Miyazaki, suggesting that the ovarian function was more active in fillies reared in Miyazaki as compared with those reared in Hidaka. Based on these findings, it was clarified that development of the body and gonads was faster in Miyazaki compared with Hidaka in both colts and fillies.

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

“…No effect of the EP treatment on growth and gonadal function was noted in colts reared in Miyazaki, and the reason for this is unclear at this point. In our previous observations for comparing the growth of yearlings reared under natural light between Miyazaki and Hidaka, growth was superior in both colts and fillies reared in Miyazaki [ 45 ]. Since there is a limit to growth, the light stimulation-induced promotion of growth may not have been perceivable for body weight, height at withers, girth, or cannon circumference in colts reared in Miyazaki.…”

“…In summary, it was clarified that the growth of Thoroughbred yearlings reared under natural light in Japanese climates is superior in Miyazaki (southern area) compared with Hidaka (northern area) [ 45 ]. However, it may be possible to promote growth in Hidaka so that it is close to that in Miyazaki by extending the hours of sunlight by EP treatment.…”

“…At production sites, extended photoperiod (EP) treatment is applied to broodmares to advance the breeding season and achieve conception and delivery earlier than in those under natural conditions. It has recently been reported that EP treatment was applied for rearing horses at Hidaka Training and Research Center (Hidaka), Japan Racing Association (JRA), and that it advanced activation of gonadal function and coat molting and increased the muscle mass [ 45 , 50 ]. In Japan, it is well known at production and rearing sites that southland horses grow faster than northland horses.…”

“…In Japan, it is well known at production and rearing sites that southland horses grow faster than northland horses. Comparison of the growth of Thoroughbred colts and fillies reared under natural light between Miyazaki Yearling Training Farm (Miyazaki), JRA, and Hidaka clearly showed that both Thoroughbred colts and fillies reared in Miyazaki grew faster than those reared in Hidaka [ 45 ]. In addition, colts and fillies reared in Miyazaki showed faster development of testicular function and earlier ovulation, respectively, than those reared in Hidaka, clarifying the earlier promotion of gonadal function in Miyazaki [ 45 ].…”

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.…”