**5. The importance of melatonin hormone in laying hens and broiler chickens**

The cycle of light and dark, which lasts for about 24 h, causes cyclical changes in birds. These cyclical, physiological, biochemical and behavioral effects are defined as circadian rhythm. The circadian rhythm in birds is highly synchronized by the pineal

gland, retina and hypothalamus. Melatonin, the major hormone of the pineal gland, plays a role in controlling circadian rhythms in poultry [33].

Light intensity and duration of lighting also play an important role in the secretion of hormones that play a role in growth, maturation, reproduction and circadian rhythm in poultry. Circadian rhythms are important biological features of broilers and laying hens and are synchronized by daylight. The melatonin hormone secreted from the pineal gland plays a major role in this synchronization. The secretion of melatonin from the pineal gland in the dark is a reflection of this biological clock [34].

Broilers cannot achieve daily synchronization without melatonin. Most of the melatonin in the bloodstream is secreted in the pineal gland, but a small amount of melatonin is also produced in the enterochromaffin cells of the small intestinal mucosa. Melatonin production in the gastrointestinal tract is particularly associated with the consumption of diets rich in tryptophan. Because tryptophan is a structural component of protein and a precursor of the hormones serotonin and melatonin, which play an important role in maintaining normal physiological processes in broilers such as tissue regeneration, feed consumption, growth performance, feed conversion ratio and immunity [35]. The highest serum melatonin level occurs at midnight and is lowest at noon. However, the secretion of this hormone by mucosal enteroendocrine cells is associated with feed consumption and feeding frequency rather than photoperiod [36]. Orally administered melatonin to monogastric animals has high intestinal absorption and high bioavailability.

Another important role of melatonin in the organism is to participate in the antioxidant system and to protect cells from the harmful effects of free radicals. Inadequate secretion of melatonin in broilers causes metabolic and physiological disorders leading to reproductive diseases and deaths, thus economic losses [37]. Drugs and other methods used for the treatment of metabolic diseases are expensive and can negatively affect efficiency. Public health may also be adversely affected by the use of drugs in poultry. For this reason, it is important to use feed additives such as melatonin, which have no side effects, in order to prevent metabolic diseases that may occur. Apart from its use as a feed additive, melatonin is found in different parts of various plants. Oats and sweet corn take the first place with their melatonin content. In addition, walnuts, tomatoes, grapes, hazelnuts, strawberries, cherries and sour cherries contain significant amounts of melatonin [38]. It is thought that feeds rich in melatonin will positively affect the metabolic and physiological functions of birds. Thus, animal welfare problems caused by intense and long-term lighting can be compensated in this way.

### **5.1 Effects of melatonin supplementation on performance and health in laying hens and broilers**

High productivity, rapid growth and intensive metabolism of broilers and layer hens are accompanied by excessive free radical formation. In addition, intense and prolonged light exposure may increase the risk of ascites and sudden death syndrome by preventing melatonin production [39]. Implementation of intermitted lighting programs in broiler farming resulted in decreased death rate and foot problems. Such lighting programs provide the opportunity to rest, less stress and high melatonin synthesis during the dark period [40]. Relić et al. added 30 mg/kg synthetic melatonin to broiler rations by applying continuous lighting for the first 2 weeks. At the end of the study, higher body weight gain in the group added melatonin to the feed was determined [35] . In another study where laying hens were supplemented with 10, 20 and

#### *Use of Melatonin as a Feed Additive DOI: http://dx.doi.org/10.5772/intechopen.105999*

30 mg melatonin per animal, it was reported that the optimal dose for the best egg production and quality was 10 mg. At the same time, positive effects of melatonin on egg weight, shell thickness, albumin height and haugh unit parameters were reported. The rate of ovulation also shows a positive correlation with the level of melatonin in the blood, but the use of 30 mg negatively affected egg production and quality [41].

Although it has been stated that melatonin and its metabolites promote follicle maturation and ovulation by scavenging free radicals [42], high-dose melatonin supplementation may compromise the beneficial effect on ovulation by suppressing the physiological function of the ovaries. In order to evaluate the controversial results on this subject more meaningfully, the effects of different doses of melatonin on ovarian functions should be revealed in future studies. In a study examining melatonin effects on growth and development revealed that femur and tibia bones of chickens supplemented with melatonin were stronger. However, it caused a decrease in egg shell resistance [43]. The bone-strengthening effects of melatonin supplementation in chickens may be beneficial for poultry, but poor eggshell is undesirable. Therefore, melatonin supplementation may be more appropriate for broilers rather than layers. Although it has positive effects on laying hens, the melatonin metabolism of these animals is not known exactly.

Melatonin has effects on energy metabolism of broilers. In a study, physical activity decreased in animals placed in chambers for 20 days and exposed to intermittent lighting (16 h light-8 h dark) and added 40 ppm melatonin to their feed, thus energy loss was reduced. This effect was not observed in animals that exposed continuous lighting (23 h light-1 h dark) [6]. It is expected that the conserved energy that spent for physical activity positively affect the rate of feed conversion in broilers. Normally broilers do not eat at night. If they are exposed to continuous lighting, their feed consumption is considered to reach their maximum. However, several studies have shown that intermittent lighting programs improve body weight gain and feed efficiency, as well as reduce leg problems and mortality in broilers [44, 45]. The immune system strengthening effect of the increased melatonin secretion during the dark period helps to lower the mortality rate in poultry. In addition, application of melatonin alleviates the harmful effects of continuous lighting on broilers.

Melatonin plays a role in the development and maturation of the immune system. Stimulation of cytokine production, which increases lymphocyte activity, is one of its effects on immunity. Melatonin also increases the lethal activity of T cells and the production of interleukin and interferon in monocytes [46]. Melatonin hormone is also effective in regeneration of intestinal cells in poultry [36]. In a histopathological study, it was revealed that lymphoid hyperplasia in the liver, spleen and bursa fabricus was induced in broiler chickens treated with the addition of melatonin to the diet [47]. This result can be explained by the immunostimulating effect of melatonin. Bursa fabricus, which is involved in lymphocyte production in chickens, is suggested to be the target organ of melatonin [48]. It has been reported that 10–40 mg/kg melatonin supplementation significantly alleviates hepatic degeneration, necrosis and biliary hyperplasia resulting from aflatoxin in chicks [49]. This hormone may also interact with the thermoregulation mechanism in poultry. It has been reported that the body temperature of the 14-21d old and 150 mg/kg melatonin supplemented broiler chicks decreased and the heat distribution was regulated [50]. Melatonin can prolong life span by protecting erythrocytes in the blood from oxidative stress. It also stimulates the production of immune system cells such as lymphocytes, monocytes and eosinophils [51]. However, in another study, melatonin supplementation in broiler diets did not make a significant difference on hematological parameters [35].

Tryptophan, a precursor of melatonin, was used as an additive, increased cellular and humoral immunity, stimulated melatonin synthesis, and increased immunity by peritoneal macrophages reported as a result of the immunomodulatory effect of melatonin [52]. The effect of tryptophan on immune function is mediated by melatonin receptors in tissues. Patil et al. showed that tryptophan is a precursor of melatonin and inhibits oxidative damage in broilers, and also improves the enzymatic effect of catalase and superoxide dismutase [53]. However, Wang et al. emphasized that 1.5 times increased tryptophan level in the diet of broiler chickens housed under stress conditions, feed efficiency increased and oxidative stress reduced [54].

The optimal lighting level and the most appropriate feeding method are still being discussed in the poultry industry, where it is expected to achieve high level yield and production. With this expectation, animals are faced with stress factors such as intensive feeding and poultry housing conditions. Considering the properties of reducing stress factors, antioxidant and positive effects on animal health, the use of melatonin as an additive in poultry has been approved in many studies [35, 52].
