Two trials were conducted in a 2 × 2 + 1 factorial arrangement based on a completely randomized design to evaluate the effects of different sources of selenium (Se) on performance, blood metabolites, and nutrient digestibility in male lambs on a barley-based diet. The first trial lasted for 70 days and consisted of 30 lambs (35.6 ± 2.6 kg mean body weight, about 4-5 months of age) which were randomly allotted to five treatments including: (1) basal diet (containing 0.06 mg Se/kg DM; control) without supplementary Se, (2) basal diet + 0.20 mg/kg Se as sodium selenite (SeS 0.20), (3) basal diet + 0.40 mg/kg Se as sodium selenite (SeS 0.40), (4) basal diet + 0.20 mg/kg Se as selenium yeast (SeY 0.20), and (5) basal diet + 0.40 mg/kg Se as selenium yeast (SeY 0.40). For the second trial, four lambs from each group of experiment 1 were randomly allocated to individual metabolic cages for 14 days to measure the effects of dietary Se on nutrient digestibility. The results revealed that there were no significant differences for average daily gain, average daily feed intake, feed/gain ratio, hematological parameters (packed cell volume, red blood cell, white blood cell, and hemoglobin values), serum total protein, albumin, globulin, aspartate amino transferase, alkaline phosphatase, and creatine phosphokinase due to supplementation of different amounts and sources of Se in lambs. Dietary Se supplementation significantly improved (P < 0.001) glutathione peroxidase activity in blood. Furthermore, at the end of the trial, serum tri-iodothyronine (T3) amount also increased (P < 0.05), while serum thyroxine (T4) amount decreased (P < 0.05). Digestibility of dry matter, organic matter, crude protein, neutral detergent fiber, and acid detergent fiber increased (P< 0.05) by Se yeast supplementation. It may be concluded that supplementation of Se in lambs had no significant effect on performance and blood hematology, but increased blood glutathione peroxidase activity and serum T3 amount and decreased serum T4 amount as compared to non-supplemented control lambs. Furthermore, Se yeast improved nutrient digestibility in lambs.
