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The light side of calf performance

Drew A. Vermeire, Ph.D., for Progressive Dairy Published on 06 November 2020

Many people know the length of the daily photoperiod signals chickens and other birds to start or stop laying eggs.

People are surprised to learn that photoperiod length causes sheep and horses to begin or end their reproductive cycles, and they are even more surprised to learn that photoperiod has profound effects on cattle, too.



For many years, it was known that beef cattle had poorer grading in late summer and fall compared to late winter and spring. More than 30 years ago, researchers discovered the reason was because the longer photoperiod during summer results in less fat being deposited in the carcass and thus, lower marbling scores and poorer grading. This discovery prompted other studies in dairy cows and calves that show an increase in milk production of 5.5 pounds per day in dairy cows and increased growth rates in calves. Calf producers can benefit from improved calf performance by using this information, which costs very little to implement.

Long days in summer, short days in winter

On the equator, days and nights are 12 hours long throughout the year. But in latitudes further north or south from the equator, days are longer in summer and shorter in winter, with the longest day occurring on June 21 and the shortest day occurring on Dec. 21. When day length is increasing and photoperiod exceeds 14 hours, chickens begin to lay eggs. For growing cattle, day length of 16 hours or more causes a change in growth patterns, which results in less fat being deposited and thus, more efficient growth.

We want calves to be in “summer mode”

The length of the daily photoperiod affects the calf’s hormonal status, which, in turn, affects the calf’s growth rate and feed conversion. How does it happen? To use computer terms, the calf has a “default setting” of “winter mode.” In winter mode, calves store a higher proportion of feed energy as body fat. In “summer mode,” calves stop storing as much body fat and grow more muscle and bone. Naturally, growing muscle and bone is more efficient than storing fat, and feed conversion is better. With feedlot beef cattle, summer mode means cattle grow faster more efficiently but with less marbling and a lower percent of the cattle grading Choice. For young, growing calves, marbling is not important for grading, but growth rate and feed conversion are economically very important for calf producers because these variables directly impact the cost of gain.

Light affects hormones and growth

For a calf, daybreak occurs when the sun comes up or the lights come on in the morning. Light is perceived by the retina of the eye, which sends a signal to the pineal gland. The pineal gland secretes a substance called melatonin when the lights are off. When lights are on, melatonin production is blocked and insulin-like growth factor (IGF-I) is increased, which causes the calf to grow faster and deposit less fat. When melatonin is secreted (or fed), growth rate is slower and more body fat is deposited than when melatonin is absent.

Fifteen hours after dawn is the photosensitive period to determine summer versus winter mode. If light is perceived (low melatonin) during the 14-16 hour post-dawn timeframe, hormonal changes occur that result in faster growth and improved feed efficiency. Protein turnover is reduced, which increases body muscling and reduces body fat. Feed intake may eventually increase, but intake is the response to increased growth rate – not the reason for it.


In two studies with Holstein calves, researchers found that calves exposed to long day length had faster gain than calves exposed to short day length. In one study, calves exposed to 16 hours of light gained 0.165 pounds per day faster than calves exposed to less than 16 hours of light. The increased gain in this study equals 23 pounds live weight over 140 days.

Warning: Calves must have dark along with light

For photoperiod effects to persist, there must be a dark phase. For calves to switch to summer mode, photoperiod must be at least 16 hours in length. But if calves are kept in light 24 hours per day, they switch back to the default winter mode. Therefore, keeping lights on for 16 hours and off for eight hours is the best method to improve calf performance. This means that 24-hour security lights eliminate the dark phase and also eliminate the benefit of the 16-8 light-dark photoperiod.

Making it work for you, sweet 16

The simplest way to implement this concept is to install a timer to turn on lights and turn them off 16 hours later. Lights could also have a photocell that would turn lights off when there is enough natural light for calves. To have the effect, calves don’t require a lot of light, just a 16-hour day length plus a dark phase. The timer should turn lights on when morning feeding occurs and turn lights off 16 hours later. For example, if calves are fed at 4 a.m., the timer should turn lights on at 4 a.m. and off at 8 p.m. (4 a.m. plus 16 hours equals 8 p.m.). Even if the effect on gain is only one-tenth of the gain reported in the research, an increase of 2.3 pounds live weight more than pays for a few lights and a timer, with some profit to spare. Remember, to benefit from the light, calves must also be in the dark.  end mark

PHOTO: Photo by Mike Dixon.

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Drew A. Vermeire Ph.D.
  • Drew A. Vermeire Ph.D.

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