Seasonal changes in plasma concentrations of prolactin, LH, FSH, and testosterone in young adult bulls

Abstract

Seasonal changes in several reproductive parameters were studied in young adult Holstein bulls throughout a 1-year period. Every 2 weeks, blood plasma pools were prepared for each bull from jugular blood samples drawn at 20-min intervals for 6 h. These bi-weekly pools were assayed for prolactin (PRL), luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone (T). Selected individual samples were assayed for LH and T. Scrotal circumference and testicular firmness were measured once per week. Concentrations of PRL were elevated ( P<0.05) during the summer months and reached a peak of 17.1±2.2 ng/ml in July, which represented a six to sevenfold increase over the lowest months of November, December and January. Levels of LH (3.6±0.6 ng/ml) were highest ( P<0.05) during winter. There were no differences in mean monthly T concentrations(3.9±0.3 ng/ml) although a small elevation ( P<0.05) was noted in winter when the data were grouped by season. The concentrations of LH and T ( r=0.46, P<0.01) and LH and FSH ( r=0.34, P<0.01) were positively correlated. The number and height of LH and T peaks and area under the 6-h profiles were not different when PRL was increasing (spring), at peak (summer) or near basal levels (fall). Concentrations of FSH were highest ( P<0.05) in fall, a time when PRL was lowest. Differences ( P<0.01) among bulls were observed for all hormones measured and interactions between bull and season were seen in the concentrations of LH and FSH, suggesting that seasonal changes in the levels of these hormones did not occur at the same time for all bulls. Scrotal circumference was lowest and testicular firmness was highest during the summer months ( P<0.05). During the period of increasing FSH (August–October), a positive correlation ( r=0.70, P<0.01) was observed between the concentration of this hormone and scrotal circumference 6 weeks later. While day-length and average daily temperature were positively correlated with PRL concentration and testicular firmness, they were negatively correlated with LH concentration and scrotal circumference. These data suggest that, like PRL, circulating levels of LH, FSH and perhaps T in bulls are influenced by season.