The pattern of melatonin secretion is rhythmic in the domestic pig and responds rapidly to changes in daylength

The aim of the study was to investigate the capability of pigs to respond t o abrupt changes in lighting conditions by means of alterations in circadia n melatonin profiles. Sixteen pre-pubertal crossbred male pigs weighing 40- 45 kg were housed in individual pens in four temperature- and lighting-cont rolled climate rooms (four pigs per room). In two rooms there was a light-d ark cycle of 16 L:8 D (Group A) and in two other rooms 8 L:16 D (Group B). Under both lighting regimens light intensity at pig eye-level was 220-240 1 x during the light phase and less than 7 1x (red light) during the dark pha se. The lighting regimens were changed after 2 wks to the opposite regimen and the change was repeated after a further 2 wks, so that animals ended up with the same light cycle with which they started. Blood was sampled at 2- hr intervals for 48 hr spanning each time of change in lighting. A further 24-hr sampling was performed at the end of the experiment (2 wks after the last change) in both groups and 1 wk after the change from short to long da y lighting in Group A. On 83/86 occasions, pigs exhibited a clear circadian rhythm in plasma melatonin under both lighting regimens. Pigs responded im mediately to the change from long to short day lighting by advancing melato nin secretion to the earlier lights-off time and some pigs were able to ext end secretion to the delayed lights-on time. For short to long day changeov er there was a small immediate response, with secretion pattern following t he previously entrained endogenous rhythm to within 3 hr of the previous li ghts-on time. After 1 wk commencement of secretion was delayed by up to 2 h r. while after 2 wks some pigs were able to delay commencement of secretion until lights-off or to cease at lights-on. It is concluded that the domest ic pig is able to commence adjustment to abrupt changes in photoperiod with in a 1-wk acclimatization by altering circadian melatonin secretion. The pr esent study suggests that it may be possible to use simplified lighting reg imens instead of stepwise changing lighting programs in commercial piggerie s to reduce the influence of season on production.