Several environmental and nutritional management approaches have been used to mitigate heat stress and improve performance of farm animals in semi-arid and arid regions.  The present study was designed with the intention to alleviate the negative effects of heat stress and to promote the performance of growing lambs reared under hot environmental conditions. The study was conducted on 18 male Naimey lambs with average body weight of 22.78±0.49 kg, and 4-5 months old. The animals were randomly divided into 3 equal groups (A, B and C), and fed diets containing different concentrations of seaweed (Ulva lactuca) for 90 days. Group A served as control and was offered diet containing 0.0% seaweed. Groups B and C served as treated groups and were offered diets containing 3.0 and 5.0% seaweed, respectively. Dietary inclusion of seaweed to the diet of growing lambs exposed to heat stress (max T_a 43.9^oC, max RH 81.1%, max THI 84.6) neither influenced (P>0.05) the thermo-physiological parameters (rectal and skin temperatures), nor affected (P>0.05) the performance parameters (feed intake, body weight gain, feed conversion efficiency). Furthermore, dietary seaweed supplementation did not alter (P>0.05) blood constituents or blood antioxidant capacity. However, dietary seaweed supplementation significantly (P<0.05) reduced respiratory rate, and increased serum potassium concentration. Based on the data of the present study, seaweed (Ulva lactuca) supplementation to the diets of growing lambs reared under heat stress conditions did not show any indication of promoting their production performance or heat tolerance.

Key words: Growth performance, Heat stress, Lambs, Seaweed