Background: Heat stress is one of the most critical problems confronting the poultry industry. Stinging nettle (SN) is a medicinal plant with potent antioxidant properties. Objective:The goal of this study was to evaluate the effects of dietary SN at two different levels (2 and 4%) on the serum levels of cortisol and some selected parameters of broilers exposed to chronic heat stress.Methods: A total of 240 broiler chickens were randomly assigned to six dietary groups as follows: (1) control: fed the basal diet; (2) HS: heat-stressed broiler fed the basal diet; (3) HS-SN2: heat-stressed broiler fed 2% dietary SN; (4) HS-SN4: heat-stressed broilers fed 4% SN; (5) SN2: no heat-stressed broilers fed the basal diet supplemented with 2% SN; (6) SN4: no heat-stressed broilers fed the basal diet supplemented with 4% SN. Diet supplementation with SN was performed from days 14 to 35 and chronic heat stress was induced from days 22 to 29. The serum parameters were evaluated on days 14, 21, 29 and 35.Results: HS had higher serum levels of cortisol, total cholesterol (TC), aspartate aminotransferase, alanine aminotransferase and creatine kinase (CK) compared to the other treatments. HS-SN4 had significantly lower cortisol, TC, alanine aminotransferase and CK compared to HS and HS-SN2. Conclusions:The inclusion of 4% SN powder in the broilers' diet alleviated the negative effects of heat stress by decreasing cortisol, TC and tissue damage indices. It seems that dietary SN could be used as a feed additive in the poultry diet for improving the health status and defence mechanisms of the birds under stressful conditions.

Heat stress is known as one of the most prevalent environmental stressors in poultry production, which is associated with oxidative stress. Stinging nettle is a medicinal herb with strong antioxidant properties. The present study was conducted to evaluate the effects of dietary stinging nettle at two different levels (2% and 4%) on growth performance and oxidative stress indices of broilers exposed to chronic heat stress. On day 14, a total of 240 broiler chickens were randomly assigned to 6 treatment groups as follows: 1) NC: negative control; 2) HS: heat-stressed broiler; 3) HS-SN2: heat-stressed broiler fed by 2% dietary stinging nettle; 4) HS-SN4: heat-stressed broilers fed by 4% stinging nettle; 5) SN2: no heat-stressed broilers fed by 2% dietary stinging nettle; 6) SN4: no heat-stressed broilers fed by 4% dietary stinging nettle. Diet supplementation with stinging nettle was performed from days 14 to 35 and a chronic heat stress was induced from days 22 to 29. The growth performance and oxidative indices were evaluated on days 14, 21, 29, and 35. Rectal temperature and panting frequency were assessed on days 22, 25, and 29. As a result, weight gain and food intake decreased in the HS compared to the NC, and these parameters increased in the HS-SN2 and HS-SN4 compared to the HS (P<0.05). The HS, HS-SN2, and HS-SN4 groups had a significantly higher rectal temperature and panting frequency. The HS had the higher circulating MDA and HSP70, and lower T-AOC, SOD, and GSH-Px compared to the treatments. The HS-SN4 had a significantly lower MDA and HSP70, and higher T-AOC, GSH-Px, and SOD compared to the HS and HS-SN2. In conclusion, the addition of 4% stinging nettle powder to the broilers' diet improved the oxidative status in heat-stressed birds. Furthermore, this herb could be utilized as a feed additive in poultry diet to improve bird's health and defense mechanisms under stressful conditions.