THE CHRONOBIOLOGICAL MANIPULATION OF TIME OF CALVING AND BEHAVIOR OF DAIRY CATTLE

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Abstract
Thirty-two cows in their 8th month of gestation were exposed daily to either 12–13 h of continuous light or to various skeletal photoperiods consisting of a total of 8 h of light. Cows in the skeletal photoperiod light regimens received 6 h of light from 05:00 to 11:00 h and a 2-h light pulse between 15:00 and 17:00 h, 18:00 and 20:00 h or 21:00 and 23:00 h (pulse 15:00–17:00 h, pulse 18:00–20:00 h or pulse 21:00–23:00 h, respectively). Time of calving was recorded for all cows. Behavior of cows in the respective treatments was monitored and time spent eating, standing or lying down and frequency of urination and defecation recorded. The light regimen of pulse 18:00–20:00 h was influential in restricting (P < 0.004) the randomness of time of calving. All cows in the pulse 18:00–20:00 h light regimen calved within the normal working hours of the barn (mean time of calving 1315 h ± 1.5 h). The mean time of calving for the control, pulse 15:00–17:00 h and pulse 21:00–23:00 h were 14:23 h, 12:56 h and 11:30 h, respectively (± 8.6, ± 6.36 and ± 9.53, respectively). The eating profile of the cows in the pulse 18:00–20:00 h group showed an additional peak of eating associated with the skeletal photoperiod. It is concluded that a circadian rhythm of parturition having a photosensitive phase is present in the dairy cow. This photosensitive phase occurs between 13 and 15 h after subjective dawn. Key words: Chronobiology, skeletal photoperiod, calving, behavior, circadian rhythm, dairy cattle

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