The effect of daylength on the growth of lambs. 5. Skeletal long photoperiod

Abstract

ABSTRACTFour experiments were carried out, with a total of 98 Suffolk-cross lambs fed at a restricted level, to study the effects of skeletal long photoperiod (7 h light: 10 h dark: 1 h light: 6 h dark) compared with short photoperiod (8 h light: 16 h dark) on growth and blood levels of hormones. The experiments started when the lambs were from 2 to 10 months of age and continued for 9 to 14 weeks.In the first experiment, the weight gains of both females and castrated males were significantly increased by skeletal long photoperiod. Plasma prolactin was increased and cortisol decreased by skeletal long photoperiod.Experiment 2 included females and castrated males; prolactin was stimulated by skeletal long photoperiod and there was a significant positive correlation between live-weight gain and prolactin.The weight gains and carcass characteristics of the female lambs used in the third experiment were not affected by photoperiod. Live-weight gain was negatively related to plasma cortisol.In experiment 4, there were 12 intact males and 12 castrated males. The intact animals grew significantly faster; skeletal long photoperiod caused a depression in testosterone levels in plasma even though it tended to increase live-weight gain.When the data from all four experiments were pooled, it was found that skeletal long photoperiod gave a significant increase in live-weight gain in animals whose pineal gland was intact (40 per photoperiod; 138 v. 113 g/day); there was no effect on gut fill at slaughter and the increase in carcass weight (0·5 kg) was not significant.