Although many methods have been developed in order to reduce the negative effects in laying hens reared at high cage density, researches on effects of dietary supplementation of oregano (Oreganum syriacum L., OEO) essential oil as a feed additive to the diets are extremely sparse. This study investigated the effects of OEO and caging density on performance parameters, egg quality criteria, some blood biochemical parameters, blood antioxidant capacity, and intestinal histopathology of laying hens. We used 276 white Lohmann laying hens aged 38–40 weeks in the experiment. The animals were divided to positive control (600 cm2 hen/cage, PC), negative control (360 cm2 hen/cage, NC), negative control+ 200 mg/kg OEO (NC+O2), negative control+ 400 mg/kg OEO (NC+O4), and negative control+ 600 mg/kg OEO (NC+O6). In this study, it was determined that egg weight and egg shell weight increased at higher caging density, but they did not change with dietary supplementation of OEO. In addition, the plasma calcium and phosphorus decreased with the higher caging density and dietary supplementation of OEO. Supplemented OEO to the diets of the laying hens reduced lipid peroxidation and improved antioxidant capacity and intestinal histopathology of laying hens. As a result, it was determined that OEO decreased the negative effects of high caging density. While this positive effect was found to be at a maximum dose at 400 mg/kg of OEO, it was determined that a dose of 600 mg/kg of OEO had a toxic effect, if even numerically.

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