Use of Melatonin as a Feed Additive

Abstract

Melatonin is a molecule that plays an active role in reducing many stress factors in plants and has important functions in the growth, development and reproduction of plants. It has many physiological functions that directly affect feed consumption, feed efficiency, energy metabolism and immune system in animal organisms. In addition, its anti-inflammatory, antioxidant, anticancer and antiapoptotic effects are also known. While melatonin has an antioxidative effect at low doses, it can exert a prooxidant effect at high doses. It has been suggested that when melatonin is used as a silage additive, it increases the total acid content of the silage and significantly improves the silage fermentation quality by lowering the pH level and butyric acid. Although it has positive effects on mammary gland involution and general health in ruminants, its effects on yield parameters have not been proven. Broilers and layers are expected high productivity and performance, in this regard, they are faced with stress factors such as intensive feeding and housing conditions. Considering its positive effects on stress factors, health and productivity, melatonin is a promising feed additive. Effects of melatonin additive or supplements on animal productivity and health should be revealed in further studies.

1. Introduction

Melatonin is recognized as an active oxygen scavenger, which can inhibit peroxidation, effectively scavenge reactive oxygen radicals, delay the wilting of plants, and alleviate salt, drought, heavy metal, cold, pathogens and other adversities [1]. Melatonin, a derivative of tryptophan, has a low molecular weight and an indole ring structure and is an evolutionarily conserved pleiotropic molecule ubiquitous in living organisms. Melatonin is a molecule that plays an active role in reducing many stress factors in plants and has important functions in the growth, development and reproduction of plants [2]. It is accepted that melatonin can regulate vegetative growth and flowering processes such as rooting, photosynthetic yield and biomass yield, and plays a potential regulator role in the formation and maturation of fruits and seeds [3].

Melatonin is a substance secreted by the pineal gland in the darkness and can regulate biological rhythms in many physiological systems in animals, including the behavioral, cardiovascular, reproductive, immune, excretory, thermoregulatory and neuroendocrine systems [4]. Melatonin has many physiological functions that directly affects feed consumption, feed efficiency, energy metabolism and immune system in animals. Studies on the effects of melatonin on animals have shown different results [5, 6]. In a study on the use of melatonin as a silage additive, Li et al. stated that it significantly improved the quality of the silage by increased volatile fatty acid levels and decreased pH. In addition to these positive effects, melatonin had curative effects on silage fermentation by increasing microbial diversity [1].

The use of melatonin as an additive is not common. Because, melatonin has not been studied sufficiently in animals as a feed additive and its effects have not been adequately explained. In this chapter, the effects of melatonin as a feed additive especially in ruminants, broiler and layer chickens on production, yield and animal health were reviewed in order to popularize the use of melatonin as an additive and supplement. Also, the effects and functions of melatonin on plants were tried to be explained.

2. Synthesis and functional properties of melatonin in animals

Melatonin was isolated from the pituitary gland in 1958. A lot of research has been conducted about the effects and usage areas of melatonin. It was discovered as a skin lightening molecule that acts on frog and fish melanocytes, and found to be an important hormone rhythmically secreted by the brain’s pineal gland [7, 8]. The relationship between the pineal gland and light has caused it to be called the third eye. Melatonin can be synthesized in almost every living creatures, including many vertebrates and invertebrates, bacteria, protozoa, plants and fungi. Melatonin has immunostimulatory and cytoprotective agent functions that regulate the sleep-wake cycle. Also, it activates T and B lymphocytes, monocytes and stimulates the reproduction of thymocyte cells and the release of cytokines. In addition to these, its anti-inflammatory, antioxidant, anticancer and antiapoptotic effects are also known [9].

Apart from the pineal gland, melatonin is also secreted from the ovary, lens of the eye, bone marrow cells, gall bladder and gastrointestinal tract. However, the level of circulating melatonin reflects the production of melatonin in the pineal gland. While there is no difference in terms of human and animal health in matters such as the mode of action, release and chemical structure of melatonin, its usage areas vary. The antioxidant properties of melatonin and its effect on sleep disorders are at the forefront on humans. But, it has also been reported to have protective effects in neuronal degeneration and neuroprotective properties in oxidative stress-induced neuronal apoptosis [10, 11].

Light exposure level is the most effective factor that determines the rate of melatonin secretion. In general, light decreases melatonin production, while darkness increases it. The starting material of melatonin synthesis is tryptophan, an indole amino acid taken from plasma [12]. Tryptophan is hydroxylated to 5-hydroxytryptophan in pinealocytes by the enzyme tryptophanhydroxylase. 5-hydroxytryptophan is decarboxylated to 5-hydroxytryptamine (seratonin) by aromatic-l-amino acid decarboxylase. Seratonin is converted to N-acetylseratonin by N-acetyltransferase (NAT) enzyme, and this is converted to melatonin (N-acetyl-5-methoxytryptamine) by the effect of hydroxyindole-o-methyl transferase (HOMT). It has been determined that NAT and HOMT activities, which enable the conversion of serotonin to melatonin, are higher at night. It has been demonstrated by immunohistochemical methods that the enzymes required for melatonin synthesis are also present in the suprachiasmatic nucleus, retina and small intestine, apart from pinealocytes [13, 14].

After melatonin is synthesized in the pineal gland, it quickly passes into the capillaries without being stored in the organism. Due to its high lipophilic effect, it can be distributed to many biological tissues and fluids in the organism. Approximately 70% of plasma is transported bound to albumin. While melatonin can be metabolized in the kidney, this process generally takes place in the liver. Melatonin is converted to 6-hydroxymelatonin in the liver; this, in turn, binds to sulfate and glucuronic acid through the kidneys and is excreted in the urine [15].

Morphological, biochemical and molecular studies in both animals and humans in recent years have shown that oxidative stress plays a primary role in the development of degenerative changes in cells and tissues in our body. The highest degree of oxidative damage usually occurs in organs such as the brain, heart, and skeletal muscle. Melatonin inhibits free radicals from their pyrrole rings and interacts with them, reducing their activity. It also shows its effect by inducing the production of antioxidants. Melatonin has the ability to scavenge all free radicals formed in the cell. Thus, an increase is observed in the expression of genes encoding antioxidants, while genes that cause an increase in free radicals are suppressed metabolites such as melatonin also have very protective effects against oxidative stress. Lipid peroxidation, which occurs as a result of oxidative damage and accumulation of free radicals in cells, causes deterioration in cell membranes. As a result of damage, signal transmission and activation of signal pathways in cells are affected and various metabolic functions become ineffective. Melatonin prevents this lipid peroxidation and minimizes cell damage. Melatonin also neutralizes radicals caused by nitrogen and prevents nitric oxide formed [16].

3. Effects of melatonin in plants

Under extreme stress conditions, the natural defense mechanisms of plants do not provide adequate protection; In this case, exogenous biostimulants can be used to improve plant stress tolerance [17]. Recent studies have indicated that plant growth regulators manage stress mechanisms. Among these regulators, melatonin (N-acetyl-5-methoxytryptamine) is a functional natural antioxidant widely used among plants [18]. While melatonin plays an important role in plant growth and development, it promotes root and hypocotyl growth and increases the biomass of plants with its auxin-like functions [19]. Studies on the functions of melatonin in plants have revealed that melatonin plays a very important role in plant growth and development under abiotic stress conditions. Melatonin is known to increase plant tolerance under salinity stress, improve photosynthesis capacity to maintain plant ionic balance (Na+/K+ ratio), protect chlorophyll and carotenoids, and reduce photorespiration [1]. The transcriptome analysis results showed that melatonin particularly affected the pathways of plant hormone signal transduction and biosynthesis of secondary metabolites.

Melatonin is a powerful antioxidant and has the ability to purify reactive oxygen species, reactive nitrogen species and various chemical pollutants. It has been suggested that melatonin detoxifies the oxidative stress caused by excess cadmium in tomatoes by stimulating antioxidant enzymatic activity [20]. In another study, it was reported that antioxidants suppress H₂O₂ production, reduce malondialdehyde and regulate various physiological processes. In addition, exogenous application of melatonin improves the chlorophyll content and photosynthesis capacity of various plant species under salt stress by decreasing the production of reactive oxygen species and increasing the soluble protein content [21].

4. Use of melatonin as silage and feed additive in ruminants

Fresh roughage is fermented to preserve its nutrient content for a long time by ensiling. The purpose of silage additives is to control the fermentation products by ensuring domination of lactic acid bacteria during fermentation and to preserve the nutrients in the feed as much as possible. In recent years, the popularity of silage additives has increased and has found a wide range of uses. Many additives have been studied for many years to support the fermentation process. Additives improve feed consumption, feed efficiency and performance in animals with their positive effects on silage quality together with an efficient ensiling [22]. Many silage additives are produced biotechnologically. Among them, bacterial inoculants and enzymes are used to provide fast and effective silage fermentation. The purpose of using melatonin as an additive is to control silage fermentation, form the desired end products and obtain appropriate quality silage. Melatonin has antioxidant and bacteriostasis properties as a natural preservative. With these properties, it can be considered as a silage additive. There is no literature on the use of melatonin as an additive in corn or mazie silages used in dairy cattle feeding. However, Stylosanthes guianensis (stylo), one of the hot season legume forage plants, was ensiled with the addition of melatonin at different rates (5, 10, 20 mg/kg), and the rate of 5 mg/kg increased the lactic acid and total acid level, decreasing the pH value and butyric acid content. It was revealed that the silage fermentation quality was significantly improved. With its antioxidation effect, it inhibited unwanted bacteria and managed to protect the silage [23]. Melatonin is a promising silage additive as it improves the silage properties of one of the legume feeds, which is difficult to ensilage due to its high buffer capacity and low sugar content. It affected the silage microbiota and metabolism of the stylo plant [23]. There is insufficient literature on its use as a silage additive in various plants. The effects of melatonin on the quality of silage feeds, which are most commonly used in ruminant feeding, should be investigated by comparing them with other additives.

Cessation of milking initiates the dry period in dairy cattle, but milk production continues and begins to accumulate in the mammary. With accumulation, milk leaks may occur. In this case, the mammary becomes open to infection. Cows are at high risk of developing intramammary infections due to udder enlargement and altered immune functions during the transition period. When the mammary gland is completely involved, it becomes more resistant to infections. Therefore, it is beneficial to suppress milk yield and accelerate the involution process before the dry period [24]. The melatonin hormone is physiologically secreted at nights in cows. It has been determined that there is a higher rate of melatonin in milk in the morning. While prolonged exposure to sunlight is beneficial for lactating animals, it should be the opposite for animals in dry period. Exposure to sunlight for a short time or administration of melatonin during late lactation may accelerate mammary gland involution by reducing milk yield before the dry period. Several studies have found that the addition of melatonin to feed reduces blood prolactin levels. For example, the addition of melatonin at a dose of 4 mg/kg BW decreased the prolactin level in prepubertal heifers [25]. It has been reported that prolactin level decreased with the addition of melatonin to the rations of cows in the late lactation period for 8 weeks. However, melatonin mixed into the ration did not affect milk yield [26]. It has been reported that the application of melatonin in the form of implants without mixing with the feed did not affect the milk yield of the cows in the postpartum period [27].

The effect of melatonin feeding on prolactin hormone in prepartum heifers and cows was not as effective as “short day photoperiod” (SDPP, 16 s dark-8 s light) application [24]. The positive effects of melatonin supplementation in prepartum period on milk production in postpartum period are not certain. This may also be related to short trial duration or insufficient number of cows. Lacasse et al. applied “long day photoperiod” (LDPP, 16 s light-8 s dark) to cows starting 8 weeks before calving and added 25 mg melatonin to feed. It was stated that the milk yield of the cows treated with SDPP in the early lactation period was higher than those treated with LDPP + melatonin [28]. This situation can be explained by the source of the melatonin used. Because not all sources of melatonin may have the same effect. SDPP application in the dry period has a positive effect on feed utilization as well as postpartum milk yield. In the studies, adding melatonin instead of applying SDPP in the dry period did not show the same effects. More studies are needed on the relationship between melatonin feeding and performance in dairy cattle. Milk yield parameters in sheep and goats depend on melatonin and prolactin concentrations as much as dairy cows. Using melatonin as a feed additive can reduce the stress caused by injection and implant applications. The use of exogenous melatonin as a subcutaneous implant together with naturally produced endogenous melatonin under SDPP conditions had no effect on lactation performances in different breeds of sheep with different levels of milk production level [29, 30]. This situation can be associated with the stress created in animals.

Rumen fermentation is an issue to be considered when using melatonin as a feed additive or oral preparation. Digestive enzymes and microorganisms in the rumen can metabolize melatonin. If melatonin is involved in rumen fermentation, its bioavailability may be significantly reduced. Therefore, melatonin should be tried in different forms (preserved or by-pass) and by adding it to the rations at different levels. Because it has been reported that the protein, fat and dry matter of milk increased in cows given melatonin in rumen protected from [31]. These researchers also emphasized that milk lactose level decreased with the addition of melatonin. The effect of preserved melatonin on nutrient digestibility or nutrient availability of cows should be considered as reasons for these results. Melatonin is an environmentally friendly molecule that is not toxic in the organism and its preserved forms are quite useful. For the treatment of mastitis, it may be recommended to use protected form of melatonin instead of antibiotics. Thus, the treatment cost would decrease, and the milk quality would increase. It has been determined that melatonin has an effect on some carcass parameters in beef cattle. In heifers given melatonin (4 mg/100 kg body weight) daily for 59 days, rib and longissimus muscle adiposity increased, carcass protein deposition decreased, but body weight gain was not affected [32].

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 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].

6. Conclusions and recommendations

Feed additives are frequently studied and new products are introduced to the market in order to keep the function animal metabolism healthy. A wide variety of additives are used in order to protect animal health at the maximum level, to obtain higher efficiency and direct animal products according to consumer and market demands. Melatonin has positive effects, especially on the productivity of poultry. However, different melatonin sources should be tested on animals in different life periods. The effects of melatonin, which is used as a feed additive and as an implant, on ruminant animals are not certain. As a result of a small number of studies in ruminant animals, it has been found that only daylight and lighting duration are sometimes more effective than melatonin feeding. Apart from melatonin secreted by animals at night, the effects of melatonin added to feed should be examined in more detailed studies. The effects of melatonin as a silage additive, should be investigated on the most commonly ensiling forages used in ruminant feeding. Benefits of melatonin on silage quality by comparing it with other additives that affect silage fermentation should be proven. In conclusion, melatonin is a promising feed additive whose antioxidant and immune system support properties have been proven in many studies in animals.