257. Effects of Melatonin Implants on Circulating Melatonin, Cortisol, and Total Antioxidant Capacity in Late Gestating Beef Cows.

Abstract Late gestation is a physiologically demanding period in livestock, as it is characterized by marked adjustments in endocrine function, increased metabolic activities, and potential oxidative stress. The aim of the study was to evaluate the effects melatonin treatment on circulating melatonin, cortisol, and total antioxidant capacity (TAOC) in late gestating beef cows. Twenty Fall-calving Angus beef cows average age 4were assigned to control, no melatonin implant (CON) and melatonin-implanted (MEL) groups. The implant group received a single Melatonin implant (Melawin®24mg) subcutaneously twice at 28-day intervals from the beginning of the third trimester (on August 5th and September 2nd, expected calving October-November, 2025). Blood samples were collected on Days 0, 14, 28, and 42 relative to implant insertion. Concentrations of serum melatonin, cortisol, and TAOC were analyzed using commercially available assay kits. Data were evaluated using a repeated-measures mixed model in SAS. Melatonin implantation significantly (P < 0.05) affected serum melatonin concentration, with implanted cows showing a higher least-squares (171.58 ± 9.42 vs. 134.74 ± 9.42 pg/mL) mean than CON. The effects of day (P = 0.0556) and the treatment-by-day interaction (p = 0.0651) approached significance, which suggests a possible time-related response. Cortisol concentration was not affected by treatment (P = 0.9598), but varied significantly by day (P < 0.0001), with the highest concentration observed on day 42. Similarly, TAOC was not affected by treatment (P = 0.8318), but increased significantly over time (P < 0.0001). Treatment-by-day interactions for cortisol and TAOC were not significant (P = 0.7297 and P = 0.2572, respectively). In conclusion, melatonin implantation increased circulating concentrations of melatonin, but did not significantly alter cortisol or TAOC during the study period.