Photoperiod is the duration of daily light exposure. Manipulating photoperiod length can increase milk production in dairy cattle. When manipulating the photoperiod, two common time divisions are used. The first one is a long-day photoperiod, 16 hours of light and 8 hours of dark. The second is short-day photoperiod, 8 hours of light and 16 hours of dark. Many studies have shown having different photoperiods for the different stages of a cow"s life cycle improve milk production, reproduction, feed efficiency, and heifer growth.
A goal of the industry has been to get heifers into the milking herd as soon as possible. To achieve this goal, heifers are fed high energy diets so they reach breeding size faster. Previous research has indicated that long day photoperiod can lead to leaner growth, greater mammary development, and lower the age to puberty by an average of one month. One study determined how photoperiod would affect various aspects of dairy heifer growth. Breeding and calving of the heifers in the long day photoperiod occurred earlier than heifers in a short day photoperiod. Even though long-day-photoperiod heifers had a lower body weight, they did not experience limited skeletal growth. Instead, they had lower body condition scores because they were using the energy that they consumed for skeletal growth. Feed intakes did not differ between the short-day-photoperiod and long-day-photoperiod groups, and long-day -photoperiod heifers spent less time at the feed bunk, which would suggest they were more feed efficient. The long-day-photoperiod heifers also had higher milk production throughout the first 5 DHI tests.
In one of the first studies looking at differences in the growth between heifers with supplemented lighting (16 hours) and natural lighting, heifers in the supplemented lighting group had a larger heart girth size of about 1.6 inches after the 16-week trial. These heifers also averaged 1.9 pounds of daily gain compared to the 1.7 pounds for the heifers in a natural lighting scheme.
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