Annual Changes in Day-length, Temperature, and Circulating Reproductive Hormones in Thoroughbred Stallions and Geldings

Abstract

Changes in follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, immunoreactive(ir)-inhibin, testosterone, estradiol-17β, and insulin-like growth factor (IGF)-I in Thoroughbred stallions along with changes in prolactin secretion in geldings were studied. The correlations of day-length with changes in the concentrations of these hormones were also studied. Five stallions and thirteen geldings were employed to draw blood samples in monthly basis and radioimmunoassay was performed to measure these hormones. All hormones showed a seasonal pattern, the levels being highest during the breeding season and lowest during the winter months. Most of the hormones were at their highest concentration during the month of April, the mid of spring in northern hemisphere. The concentration of circulating IGF-I also demonstrated seasonality, the peak lying on the month of April. The plasma concentration of prolactin also increased during the breeding season. This phenomenon was similar both in stallions and geldings although geldings had lower concentration than that of stallions. The changes in concentration of prolactin in stallions and geldings correlated more towards the day-length than towards the temperature. These results clearly indicate the seasonality of pituitary and gonadal hormones of Thoroughbred stallions, the activity being highest during the month of April and May of the breeding season.

Keywords: IGF-I; Thoroughbred stallion; gonadotropins; prolactin; seasonality.

Fig. 1.

Monthly changes in concentration of circulating LH (a), FSH (b), testosterone (c), ir-inhibin (d), estradiol-17β (e), and IGF-I (f) in stallions. Each point represents the mean ± SEM value of two years (n=5). Different alphabets reflect significant differences at p<0.05.

Fig. 2.

Monthly changes in circulating levels of prolactin in stallion (a; n=5), gelding (b; n=13) and the pattern of day-length (●), and temperature (○) (c) in Japan. Values are represented as mean ± SEM and different alphabets represent the significant differences at p<0.05.

Fig. 3.

Correlation of day-length (○) with LH (a), FSH (b), testosterone (c), ir-inhibin (d), estradiol-17β (e), prolactin in stallion (f) (●), IGF-I (g), and prolactin in gelding (h) (■). Correlation coefficient at p<0.05 in each case is represented by r-values in respective graphs.

Fig. 4.

Correlation of temperature (□) with LH (a), FSH (b), testosterone (c), ir-inhibin (d), estradiol-17β (e), prolactin in stallion (f) (■), IGF-I (g), and prolactin in gelding (h) (●). Correlation coefficient at p<0.05 in each case is represented by r-values in respective graphs.