Organic or Inorganic Zinc for Laying Hens? A Systematic Review and Meta-analysis of the Effects of Zinc Sources on Laying Performance, Egg Quality, and Zinc Excretion

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Abstract
The higher availability of zinc (Zn) from organic than inorganic sources is already established, but more assertive and cost-friendly protocols on the total replacement of inorganic with organic Zn sources for laying hens still need to be developed. Because some discrepancy in the effects of this replacement in laying hen diets is noticeable in the literature, the objective of this meta-analysis was to properly quantify the effect size of total replacing inorganic Zn with organic Zn in the diet of laying hens on their laying performance, egg quality, and Zn excretion. A total of 2340 results were retrieved from Pubmed, Scielo, Scopus, WOS, and Science Direct databases. Of these, 18 primary studies met all the eligibility criteria and were included in this meta-analysis. Overall, the replacement of inorganic Zn with organic Zn, regardless of other factors, improved (p < 0.01) egg production by 1.46%, eggshell thickness by 0.01 mm, and eggshell resistance by 0.11 kgf/cm2. Positive results of the same nutritional strategy on egg weight and Zn excretion were only observed at specific conditions, especially when organic Zn was supplemented alone in the feed, not combined with other organic minerals. Therefore, there is evidence in the literature that the total replacement of inorganic Zn with organic Zn improves egg production, eggshell thickness, and eggshell resistance. Factors such as hen age and genetics, organic Zn source, concentration of Zn in the feed, and the strategy of its supplementation have to be more carefully considered in protocols designed to address egg weight and Zn excretion by the hen.

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