Photoperiodism in pigs

The routine castration of male piglets, which is performed in many countries to avoid boar taint in meat, is a cause for great concern in terms of animal welfare. Castrates also have reduced feed efficiency and more fat than do entire males. More of the ingested energy therefore goes to fat tissue than to muscle tissue. The European wild boar is a seasonal short-day breeder and although domestic pigs breed all year round there are indications that they remain responsive to photoperiod. Since boar taint is closely associated with male sexual maturation, the use of artificial light regimens to delay puberty could be a non-invasive method of reducing boar taint in entire males. Therefore, a series of experiments were performed in order to study photoperiodism in young pigs. In two experiments, matched winter-born siblings of crossbred males were allocated after weaning to either one of two light-sealed rooms with high-intensity light regimens or a conventional stable environment. In the first study, groups subjected to an 'artificial autumn' treatment and an 'artificial spring' treatment were compared with a 'natural spring' group. Animals in the 'artificial autumn' group were less sexually mature at the time of slaughter than were those of the 'natural spring' group, which probably was due to the abrupt and large increase in photoperiod for the 'artificial autumn' group. At the start of the second experiment, animals were transferred from 6.5 h of natural light to 12 h of artificial light. Thereafter, the 'artificial winter' group were exposed to short days, whereas the 'artificial summer' group had long days and both were compared with a 'natural spring' group. At slaughter, the 'artificial summer' group showed immature spermatogenesis and less boar taint than did the 'artificial winter' group. An interesting observation was that all three groups which were exposed to an abrupt shift in photoperiod at the start of the experiment ('artificial autumn', 'winter' and 'summer') showed a deviation from the normally parallel secretion of testicular steroids. In these pigs, testosterone increased while oestrone-sulphate remained low. In contrast to wild pigs, a photoperiod-induced seasonal pattern in prolactin secretion has not been demonstrated for domestic pigs. Although both experiments showed tendencies of higher prolactin concentrations during long days, the results of these studies are not conclusive. Together, the two studies show that photoperiod influences the timing of puberty in boars. Short days stimulate puberty and long days delay sexual maturation and reduce boar taint. However, the pre-weaning photoperiod can influence the response to the subsequent photoperiod. In mammals, seasonal changes in photoperiod are mediated through the pineal hormone melatonin. Melatonin secretion is typically high during the dark hours and negligible during the day, and thus reflects the period of darkness. In pigs, there have been considerable difficulties to demonstrate a typical melatonin profile, which has led to speculation that impaired melatonin production accounts for the lack of a seasonal breeding pattern in domestic pigs. Two assays for human melatonin were compared in plasma from pigs that had been exposed to either standard stable lighting or artificial short days or long days. According to conventional laboratory evaluating procedures both assays performed well, but only one assay measured a typical circadian plasma melatonin profile entrained by the photoperiod for all individuals. The measured increase in melatonin secretion during the dark phase was low compared with that in other species. However, the variation in melatonin levels between animals was high and there were indications of a genetic background to this. To investigate whether the pineal melatonin synthesis has been impaired by domestication, European wild boars of both sexes and domestic gilts were cannulated and sampled for 48 h in four seasons. A circadian melatonin profile entrained by the photoperiod of the season was observed in all animals and there was no difference was seen between wild and domestic pigs. To further study the great variation between animals in the increase of nocturnal melatonin, blood from 48 piglets and their parents was sampled during the day and night. Although the sampling method occasionally appeared to cause overestimated values, differences between litters in nocturnal melatonin concentrations strengthen the hypothesis that the observed variation between animals of nocturnal melatonin secretion in pigs is genetically based