Role of Dietary Zinc Supplementation on Performance of Fattening Lamb: A Meta-Analysis

Introduction: Zinc (Zn) is well known as an essential trace element in animal nutrition to ensure optimal body functions and animal health. Previous studies have demonstrated that Zn supplementation has improved the rate of animal growth in practical feeding regimes. Traditionally, the farm animal diets have been supplemented with Zn in the inorganic forms as either zinc oxide or zinc sulfate. The use of organic Zn sources in the form of chelates and supplements for ruminant diets has increased in recent years. Many studies have been conducted to interrogate the relative bioavailability of organic Zn sources in comparison to inorganic forms in ruminant nutrition. Meta-analysis is the statistical combination of results from two or more independent studies for the purpose of integrating findings or results for the under study subject. Meta-analysis can be helpful in determining whether multiple tests of an intervention yield effects on an outcome construct of interest that are similar in direction and magnitude. In the current meta-analysis the organic and inorganic zinc supplementation effects on performance of fattening lambs have been compared. Materials and Methods: A literature search was initially conducted using PubMed, Medline, Science Direct, and Google Scholar data bases and investigated references in the papers. It was also based on the following key words: zinc, organic zinc, inorganic zinc, zinc sulfate, zinc oxide, zinc methionine, zinc proteinate (s), zinc polysaccharide, growth performance, and fattening lamb. The resulting 14 articles were examined for inclusion or exclusion in this analysis. Then seven studies were included in this meta-analysis; and prepared 14 comparisons for average daily gain, 12 comparisons for average dry matter intake and feed efficiency, 11 comparisons for serum/plasma zinc concentration and super oxide dismutase activity, and 13 for alkaline phosphatase activity. Meta-analyses were carried out using the Comprehensive Meta-Analysis package, version 2. The size effects of across studies were calculated with fixed and random effect models. The presence of true heterogeneity among studies was identified with Cochran’s Q –tests and quantification of the degree of heterogeneity was done with the I2 index. Possible publication bias was evaluated with funnel plot and statistical tests. Results and Discussion:
The results of this meta-analysis showed that the addition of Zn in the diet, has a positive effects on average daily body weight gain (+0.5 ± 0.17), increase in serum or plasma zinc content (+1.13 ± 0.225) and alkaline phosphatase enzyme (+1.4 ± 0.205) and superoxide dismutase activity (0.8 ± 0.226). However, Zn supplementation in either form (inorganic or organic) had no significant effects on some variable such as daily feed intake (-0.05 ± 0.161) and feed conversion ratio (0.06 ± 0.241). This quantitative meta-analysis of data from several experiments indicated that dietary Zn supplementation significantly increased average daily body weight gain, serum or plasma Zn concentration, alkaline phosphatase and superoxide dismutase activity in fattening lambs. Superoxide dismutase is a redox metalloenzyme involving in cell defense mechanisms against oxidative stress as well as animal health. This enzyme can reduce oxidative stress therefore improves immune functions.
 Conclusion:
The results of this meta-analysis confirmed that organic sources of zinc supplements especially the zinc-proteinates are more effective than the inorganic forms for improving lamb growth performances. However, more precise experiments are required for reaching to the practical implementations.