Metabolic and Endocrine Markers of Oregano Essential Oil Effects on Antibacterial Immunity, Reproductive Function, Nutritional Status, and Production Performance of Late-Phase Laying Hens

This study aimed to evaluate the efficacy of oregano essential oil (OEO) in improving the production performance, health, and welfare of late-phase laying hens raised under commercial farm conditions by analyzing its effect on performance metrics and metabolic and endocrine profiles. Daily performance data for approximately 7884 Hy-Line Brown layers divided into two commercial flocks, one consisting of 96-week-old hens (<i>n</i> = 3849) and the other of 79-week-old hens (<i>n</i> = 4035), were recorded before (Pre-OEO Tx), during (OEO Tx-Week) and one week (Post-OEO Tx Week) following the week of water supplementation with commercial oregano essential oil (5%) of <i>Origanum heracleoticum</i> containing carvacrol (79.75%) as the main component (300 mL of product/1000 L of water). The results show a significant improvement in hen-day egg production (HDEP) during treatment (<i>p</i> < 0.05), a significant decrease in daily mortality one week after the cessation of treatment, mainly in the youngest hens (<i>p</i> < 0.05), and a reduction in feed conversion rate (<i>p</i> < 0.05). The general model (GLM) analysis of data from blood samples collected before and after OEO addition showed a significant decrease in plasma levels of procalcitonin (PCT), calcium, albumin (<i>p</i> < 0.05), and aspartate aminotransferase (AST) (<i>p</i> < 0.01). In contrast, a significant increase in estradiol, total protein globulin (<i>p</i> < 0.01), and phosphorus levels (<i>p</i> < 0.05) was recorded. The changes in endocrine profiles were significantly related to a restoration of calcium-phosphorus balance and a decrease in hepatic activity of AST and gamma glutamyl transferase (GGT). These results reveal the investigative value of PCT, in conjunction with metabolic profiling and reproductive hormones, for evaluating the effectiveness of phytogenic additives. Further studies are suggested to determine whether essential oil components can improve health and production performances of laying hens by a potential concurrent modulation of their metabolism, inflammatory response, and reproductive axis function.