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Introduction
Zinc (Zn) is an important nutrient required in poultry nutrition for various biological functions (Olgunand Yildiz, 2016). It has many activities in animal tissues as an antioxidant through the synthesis of metallothioneine (Naz et al., 2016) and as a component of DNA and RNA polymerase enzymes. In this respect, Weser et al. (1969) found increased RNA synthesis in rats injected with higher levels of Zn without any deficiency after a partial hepotectomy. The same authors reported that Zn injection into deficient rats elevated the synthesis of DNA. Zinc potentiates the activity of some hormones including glucagon, insulin, sex and growth hormones. It forms part of more than 300 enzyme systems that are involved in energy, carbohydrates, nucleic acids and protein metabolism. In addition, it plays a substantial role in the immune system, and in the transport and the use of vitamin A (Chand et al., 2014).
Recommended requirements for Zn by poultry (40 mg/kg of diet) are mainly based on the performance criteria for broiler chickens (NRC, 1994), and can be supplemented from inorganic or organic sources. The main difference between these source compounds is that most inorganic compounds do not have carbon; however organic compounds always contain carbon (Jahanian and Yaghoubi, 2010) and have carbon-hydrogen bonds (C-H). Inorganic minerals, such as sulphates and oxides, are usually supplemented in poultry rations above the recommended NRC requirement to improve feed utilisation and maximize production performances (Leeson and Caston, 2008). There are two inorganic feed-grade Zn sources in a commercial sense, which are utilised by the poultry feed industries (Leeson and Summers, 1997): Zn oxide (ZnO: 72% Zn) and Zn sulphate monohydrate (ZnSO4-H2O: 36% Zn). Most (80-90%) of the supplemental Zn used in poultry diets comes from ZnO source, which is less bioavailable than Zn sulphate for poultry (Sandoval et al., 1997). However, the sulphate (acid salt) is more water soluble, allowing reactive metallic ions to promote free-radical formation. This can support in chemical reactions responsible for the breakdown of vitamins and eventually the destruction of fats and essential oils, down rating the nutritive value of the diets. Oxides are less reactive, but are again less bioavailable (Batal et al., 2001). The organic sources, for example...