Melatonin in Livestock Production: Bridging Chronobiology, Health, and Sustainable Outcomes.

Melatonin (N-acetyl-5-methoxytryptamine) is a phylogenetically ancient hormone synthesized primarily in the pineal gland in response to light-dark cycles, where it regulates circadian rhythms, but it is also produced in peripheral tissues, including the gastrointestinal tract. In livestock, melatonin extends beyond chronobiology to modulate immune function, reproduction, metabolism, and adaptation to environmental stressors such as heat, transport, and high production intensity. This review consolidates current knowledge on melatonin's application in animal nutrition, tracing from its discovery and biosynthesis to its receptor-mediated signaling (MT1 and MT2). The core analysis examines melatonin's dietary sources, including cereal grains, legumes, oilseeds, and forages, and supplementation strategies across livestock species. Key physiological benefits include potent antioxidant properties (scavenging reactive oxygen and nitrogen species while upregulating superoxide dismutase, glutathione peroxidase, and catalase) and immunomodulatory capacity (increasing IL-2 and interferon-gamma, decreasing TNF-α and IL-6 via nuclear factor kappa-B pathway inhibition). A dedicated synthesis of effects on livestock performance reveals benefits for growth performance, reproductive health (estrus synchronization, sperm quality, and conception rates), and stress mitigation across species. The review provides species-specific dosage recommendations (e.g., 5-15 mg/d for pigs, 1-2 mg/d for poultry, 30-50 mg/d for cattle) and delivery methods (feed, water, and implants). By integrating fundamental mechanisms with applied nutritional outcomes, this work highlights melatonin's significant potential as a multifunctional nutraceutical to enhance productivity, health, welfare, and sustainability in modern livestock production systems.