Effects of photointensity and photoperiod on milk yield and reproductive performance of Norwegian Red cattle

Relationships between light exposure, milk yield, and reproductive traits i n Norwegian Red cattle were studied using data from 1538 farms in southern Norway during the darkest part of the year (November 1 to February 28). Pro duction level was expressed as milk yield at first artificial insemination (AI) and 305-d milk yield. Reproductive performance was calculated using th e following dependent variables: age at first Al, age at first calving, day s open, calving interval, number of AI per cow, and nonreturn rate at 60 d. Days open and calving interval were both 4.0 d shorter, number of Al per co w was 0.07 less, nonreturn rate was 3.1% greater, and daily milk yield at f irst Al was 0.5 kg greater in herds that used dim illumination at night com pared with herds that used no illumination at night. Milk yield at first Al was 0.5 kg greater in herds with photoperiods >12 h. Photoperiods >12 h we re associated with a reduction in age at first Al by 4.8 d and age at first calving by 6.6 d. We concluded that dairy cattle kept at high latitudes during winter should be exposed to dim illumination at night and a minimum photoperiod of 12 h t o stimulate reproductive performance and milk yield.