Serum concentrations of melatonin during scotophase and photophase in 3, 4, 5 and 6 month old gilts and barrows

One-hundred-twenty prepubertal crossbred gilts (Hampshire×Duroc)×(Yorkshire×Landrace) were removed from the nursery at 68.7±0.4 days of age and 23.6±0.9 kg body weight and relocated to a conventional grower–finisher unit. In addition, 60 barrows of similar genetics were relocated from the nursery at 71.0±0.5 days of age and 27.4±0.5 kg body weight to the same building. Twelve mature anestrous ewes that weighed 77.0±2.4 kg were assigned randomly to one of four pens of equal dimensions among the pens containing pigs. Ewes were included in this study to serve as positive controls since their secretory profiles of melatonin are well characterized. All pigs were bled by jugular venipuncture at approximately 3, 4, 5 and 6 months of age. At each age in the pigs and the mature ewes, a single sample was obtained during photophase and scotophase. Illumination intensity during the period of incandescent lighting averaged 220 lx. Blood collection was initiated approximately 4 h after sunrise and 3.5–4 h after sunset. The proportion of animals that exhibited a nocturnal rise in melatonin (MEL) was similar (P>0.05) between gilts and barrows, but was higher (P<0.002) in ewes than in pigs at each age examined. A greater proportion (P=0.007) of 3 month old barrows had a nocturnal rise of MEL than any other age of barrow. Similarly, there was a tendency (P=0.06) for more 3 month old gilts to exhibit a nocturnal increase in serum MEL than 4, 5 or 6 month old gilts.