Effect of Different Sources of Supplemental Zinc on Performance, Nutrient Digestibility, and Antioxidant Enzyme Activities in Lambs

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.