Zinc (Zn), an essential trace element for laying hens, plays a significant role in many
biological processes, such as growth, tissue growth and repair, skeletal devel- opment,
and immune competence (Salim et al., 2008; Abdallah et al., 2009; Richards et al.,
2010; Gheisari et al., 2011). Generally, inorganic Zn (Zn sulfate, Zn oxides, etc.)
is used in poultry production. However, various disadvantages of the inorganic form,
such as high excretion, low bioavailability, high hydrophobicity, and oxidation and
destruction of nutrients in the diet, were found in previous studies (Wedekind et al.,
1992;Cao et al., 2000). Currently, organic Zn products are used as an alternative source.
A series of experiments have been conducted to estimate the effect of organic Zn
sources relative to inorganic Zn based on performance, tissue Zn concen- tration,
and relative metallothionein (MT) expressions in chicken (Wedekind and Baker, 1990;
Mohanna and Nys, 1999; Jahanian and Rasouli, 2015). Some studies demonstrated that
organic Zn sources were more available than inorganic Zn sources (Wedekind et al., 1992;
Cao et al., 2000, 2002), whereas others showed no differences in bioavailability between
the 2 types of Zn sources (Mohanna and Nys, 1999; Huang et al., 2009, 2013;
Liu et al., 2013). Such discrepancies might be explained by the types and quality
characteristics of tested organic Zn sources or the criteria used in bioavailability
assays. Numerous studies on the relative bioavailability of Zn-methionine complex
for chicks have been reported (Mohanna and Nys, 1999; Cao et al., 2002; Jahanian
and Rasouli, 2015). How- ever, studies have rarely been conducted to estimate the
effects of methionine hydroxy analog chelate zinc (MHA-Zn) on aged laying hens.
Chemically, MHA-Zn is more stable than other organic Zn forms and has
higher bioavailability in broilers (Yan and Waldroup, 2006; Yi et al., 2007) and
laying hens from 39 to 52 wk of age (Sun et al., 2012).
Laying performance and egg quality are often used as parameters to estimate dietary Zn
requirements for laying hens (Batal et al., 2001; Berg et al., 2004). However,
laying performance and egg quality are usu- ally not sensitive when hens are fed
with a conventional corn-soybean meal diet containing higher Zn levels (Tabatabaie
et al., 2007). More recently, tissue Zn con- centrations have been considered to be
sensitive criteria for estimating the Zn requirements of chicks (Huang et al., 2007;
Sunder et al., 2008; Liao et al., 2013). Previous studies suggested that tibia Zn
concentration might be a more suitable and reliable parameter for the Zn requirement
estimation of laying hens (Qin et al., 2017). In addition, the Zn concentrations of liver
tissue and liver MT mRNA level were also regarded as potential indices to estimate the
Zn requirement of birds (Dewar et al., 1983; Bao et al., 2007).
Therefore, the objective of this study was to inves- tigate the effects of MHA-Zn
compared with inorganic Zn on Zn concentrations in the tibia and liver tissue and
liver MT mRNA level for aged laying hens (from 57 to 72 wk) to estimate the optimal
dietary Zn level of aged laying hens fed a corn-soybean meal diet from 57 to
72 wk of age.