Effect of photoperiod on milk production and prolactin of Holstein dairy cows.

Milk yield and composition were measured in 74 Holstein cows and first-calf heifers in two experiments comparing varying lengths of photoperiod. High pressure sodium vapor lamps provided light intensity at cow eye level of 254 +/- 26 lx during the day and 132 +/- 9 lx at night in a free stall barn open on two sides. The first experiment compared continuous light with 18 h light for 16 wk during the winter in animals previously exposed to continuous light. Photoperiod had no effect on milk yield or basal concentration of serum prolactin in either age group. Peak concentration of prolactin after injection of thyrotropin releasing hormone was not affected by photoperiod in heifers but was higher in cows exposed to 18 h light. The second experiment compared natural photoperiod (9 to 12 h light) with 18 h light for 9 wk during the winter in animals previously exposed to a natural photoperiod. Photoperiod had no effect on 4% fat-corrected milk in heifers whereas there was a significant interaction of photoperiod by week for 4% fat-corrected milk in cows. Basal concentration of serum prolactin was lower in cows than in heifers although there were no differences due to age or photoperiod after injection of thyrotropin releasing hormone. We conclude that multipara Holstein cows produce about 7% more 4% fat-corrected milk during winter months when exposed to 18 h light providing they were previously in a natural photoperiod environment. For reasons not understood, 18 h light failed to enhance milk yield of cows previously in a continuous light environment. Milk yield and composition in heifers were not affected by 18 h or continuous light.