V. Gerzilov - Department of Animal Science, Faculty of Agronomy, Agricultural University, ۱۲ Mendeleev str., ۴۰۰۰ Plovdiv, Bulgaria.
V. Boncheva - Department of Animal Science, Faculty of Agronomy, Agricultural University, ۱۲ Mendeleev str., ۴۰۰۰ Plovdiv, Bulgaria.
A. Alexandrova - Laboratory of Free Radical Processes, Institute of Neurobiology, Bulgarian Academy of Sciences, ۲۳ Acad. G. Bonchev str, ۱۱۱۳ Sofia, Bulgaria.
E. Tzvetanova - Laboratory of Free Radical Processes, Institute of Neurobiology, Bulgarian Academy of Sciences, ۲۳ Acad. G. Bonchev str, ۱۱۱۳ Sofia, Bulgaria.
A. Georgieva - Laboratory of Free Radical Processes, Institute of Neurobiology, Bulgarian Academy of Sciences, ۲۳ Acad. G. Bonchev str, ۱۱۱۳ Sofia, Bulgaria.
G. Nenkova - Laboratory of Free Radical Processes, Institute of Neurobiology, Bulgarian Academy of Sciences, ۲۳ Acad. G. Bonchev str, ۱۱۱۳ Sofia, Bulgaria.
N. Bozakova - Department of General Animal Breeding, Faculty of Veterinary Medicine, Trakia University, Student's Campus,۶۰۰۰ Stara Zagora, Bulgaria.

The aim of this study was to evaluate the effects of the immunostimulant Immunobeta® on egg production and oxidative stress parameters, influenced by the environmental conditions in free range laying hens. A total of ۸۱ laying hens and ۹ cocks (Tetra Super Harco) were divided into three groups: group ۱ (control) without supplementation; group ۲ with ۰.۲%, and group ۳ with ۰.۴% Immunobeta® as a dietary supplement. The laying period (۱۹-۵۵ weeks of age) was divided into three sub-periods depending on the ambient temperature – cold (from November to March ۲۰۱۵, from ۱۹ to ۳۷ weeks of age), thermoneutral (April and May ۲۰۱۶, from ۳۸ to ۴۶ weeks of age) and hot period (June and July ۲۰۱۶, from ۴۷ to ۵۵ weeks of age). Immunobeta® supplementation dose dependently improved the average egg production and mean egg weight, and in ۰.۴% dose treatment significantly reduced the feed conversion ratio for the entire laying period. Immunobeta® supplementation influenced the blood oxidative stress parameters, decreasing significantly the lipid peroxidation level during the cold period, increasing the glutathione level in hens supplemented with ۰.۴% dose in all periods, regulating the catalase activity during the hot period, and increasing glutathione peroxidase activity during the thermoneutral and hot periods. In conclusion, addition of Immunobeta® to the diet reduced the oxidative stress induced by thermal stress and enhanced the performance of free range laying hens.

Egg production, Feed conversion ratio, Free-range, Oxidative stress.