Zinc (Zn) is an essential
element in the growth of all animals and plays structural and catalytic roles in many
enzymes and functional
proteins. Two completely randomized trials were conducted to evaluate the effects of different sources of
zinc on performance, nutrient digestibility, blood
mineral profile, and
antioxidant enzyme activities in male growing lambs on a barley-based diet. The first trial was conducted for 70 days and consisted of 30 lambs (30.8 ± 2.8 kg mean
body weight, 4-5 months of age) which were randomly allocated to five treatments consisting of a basal diet (19.72 mg Zn/kg DM), or the basal diet supplemented with 30 mg Zn/kg DM, added as either
zinc-sulfate (ZnSulf; inorganic),
zinc-methionine (ZnMet),
zinc-proteinate (ZnProt) or
zinc-glycinate (ZnGly). For the second trial, to measure the effects of dietary Zn on nutrient digestibility, four lambs from each group of the first experiment were randomly allocated to individual digestibility cages for 12 days (first 7 days as an adaptation period followed by 5 days of sample collection). Among the groups, dietary Zn supplementation above basal level significantly improved average daily gain, average daily feed intake, feed/gain ratio, and
superoxide dismutase activity of red blood cells (P < 0.05).
Glutathione peroxidase activity of lambs supplemented with organic Zn was significantly (P < 0.05) higher than inorganic and control groups. At the end of the trial, the concentration of plasma Zn, tri-iodothyronine (T3),
thyroxine (T4), and the activity of
alkaline phosphatase was increased (P < 0.05) in all groups receiving Zn as compared with controls (P < 0.05). In addition,
thyroxine level in animals supplemented with Zn-
methionine and Zn-proteinate was greater than in animals receiving Zn-
glycine and Zn-
sulfate. The results of the second trial revealed that the supplementation with Zn-
methionine and Zn-proteinate increased the digestibility of crude
protein (CP) and
acid detergent fiber (ADF) compared to groups supplemented with Zn
sulfate and control (P < 0.05). All organic sources of Zn improved organic matter (OM) digestibility compared to inorganic and control (P < 0.05). Results indicated that, regardless of source, supplementation of Zn in growing lambs improved growth performance, blood
antioxidants, and
thyroid hormone levels. Furthermore, Zn-
methionine and Zn-proteinate supplementation appeared to improve the digestibility of CP, OM, and ADF more effectively than Zn-
sulfate.