The Effect of Lamps with Different Spectral Properties on Prolactin Release in Prepubertal Bulls1

Concentrations of prolactin in serum increased 1.8- to 7.8-fold within 6 weeks after duration of daily light was shifted from 8 to 16 hr. In comparison with cool-white fluorescent light, this increment in concentration of prolactin was similar when a light source that simulates natural sunlight (Vita-Lite), or incandescent, high-pressure sodium, or mercury vapor lamps were used. Regardless of light source, increasing the duration of daily light from 8 to 16 hr during warmer months (Aug) resulted in a greater magnitude of increase in mean prolactin concentrations as compared with increases observed during cooler months (Feb). We conclude that in prepubertal bulls, secretion of prolactin increases similarly when duration of light from lamps with different spectral properties is increased from 8 to 16 hr daily.