Introduction

Egg is a good source of nutrients such as vitamins A, D, E, and K, B2, B6, B12, as well as minerals including calcium, phosphorus, selenium, zinc, iron, and magnesium [97]. Egg quality is a key criterion for egg producers all over the world and has significant economic ramifications. While poor egg quality can result in losses, improving quality can help increase the value of the finished product. The nutrition and digestive health of hens are directly related to this criterion. The rearing system of the hens significantly affects the concentration of nutrients including elements in the egg. Eggs from organic farming systems have richer nutrients like magnesium, calcium, and zinc compared with eggs laid from caged hens [97].

Mineral supplementation in diets of layer chickens contributes to boosting egg quality criteria. Minerals are important for laying hen diets because they contribute to the biochemical processes that support normal development and growth of body and eggs [82]. Based on the National Research Council [69], it can be concluded that the total Zn, Mn, Se, I, Fe, Mg, and Cu requirement for laying hens is around 29–45, 17–25, 0.06, 0.32–0.48, 38–60, 370–600, and 4–5 mg/kg. Also, the requirement of sodium and potassium is around 0.13–0.19% in the laying hen diets. Maintaining the optimal concentration of minerals in diets is necessary to obtain high egg quality for sale [59, 61]. Incorporating minerals in layers’ feed is essential to produce superior external traits of egg quality and to diminish the problem of limiting levels of minerals in hen commercial diets which based on the ingredients of soybean meal and corn with low concentrations of minerals [32]. Mineral contents in the whole egg and eggshell are quite variable, according to the dietary element form and dose, as well as other aspects such as physiological reactions, management practices, geographic area, and supplemented feed additives [11, 37]. The consideration of the mineral source, organic or inorganic, that requires for supplementation must be taken into account. Dietary organic mineral supplementation significantly improved internal egg quality besides the external one [86]. Minerals can also enhance certain physiological reactions including the immune reaction [70] and play crucial roles in the virulence of pathogens and the antimicrobial resistance of hosts [110]. For instance, guanosine 5′-monophosphate-chelated calcium and iron inhibited the growth of Salmonella gallinarum, boosted percentage of egg production, and decreased the proportion of both cracked eggs and broken eggs [70]. Therefore, from another side, supplemental minerals can be used to improve the egg quality criteria. This review was aimed to summarize the benefits of mineral supplementation for laying hen diets, factors interfering them and to fill in gaps in the knowledge of the influences of dietary minerals on laying hens’ egg quality (Fig. 1).

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Effects of minerals in the laying hen nutrition on egg quality

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Minerals Sources in Diets of Laying Hens

Minerals are the main nutraceuticals required for physiological functions and optimum health. They are necessary as a part of the activator of enzymes and hormones for the eggshell formation and skeleton. The advanced knowing of the significance of minerals in reproduction and the changing of mineral levels in feed ingredients resulted in their supplementation in high quantities to the laying hens feed in the commercial sector with a considerable safety margin often surpassing the nutritional requirements [6, 7]. The inorganic sources are often incorporated into diets at higher proportions due to uncertainties associated with absorption (Araújo et al. 2008). However, this form results in a lack of nutritional balance and a potential for toxicological concerns [64], as well as a greater excretion of mineral than does the organic from [81, 102].

The bioavailability of minerals refers to the utilized portion by the organism, depending on their form (organic or inorganic). Inorganic minerals encompass oxides, sulfates, phosphates, and carbonates, while organic ones include proteinates, amino acids, polysaccharides, yeast, and complexes/chelates analogous to methionine. Feed-grade sources of trace minerals can differ greatly in purity. The variation in minerals’ bioavailability of these sources is also observed, wherein sulfates generally exhibit higher bioavailability than oxides [64]. Moreover, the element in some sources may need some enzymes to be available to the bird. For example, the bioavailability of P from plant sources is relatively limited, even though certain feed ingredients such as wheat and barley contain phytase which enhances the availability of P [14].

According to Suttle et al. (2010), bioavailability has been controlled by four steps: (1) the availability of such mineral to the absorptive enterocytes of mucosa that is affected by its form and its agonistic or antagonistic relations in the feed or in the gut; (2) the possibility of absorbed mineral transference through the mucosa layer (absorbability) which is depending upon the mucosa’s ability to uptake accessible minerals; (3) the ability of circulating minerals to avoid excretion through the kidneys or gut, and it measures the rate of mineral retention of the transferred ones (retainability); (4) the capacity of the minerals that assimilated into functional forms of retained ones which is influenced by the mineral absorbed forms and the site of its retention.

Organically chelated minerals showed superior bioavailability than those of inorganic metal salts [66, 101],therefore, using chelated or organic complexes of minerals in layers’ diets has been reported. The bioavailability of trace minerals determines their efficacy. Therefore, it appears that the additives of minerals in organic are more effective in supporting criteria of egg quality and egg production in laying hens [24, 32, 92, 106]. Furthermore, the complete substitution of inorganic mineral sources with organic ones in laying hens’ diets allowed the decrease in excreted minerals and did not impact egg production or the eggshell quality [20, 21]. Trace metal-amino acid complexes have the potential to mimic the mechanism by which trace elements are absorbed, making them available to animals more than inorganic forms [95]. However, such complexes must be robust enough to withstand natural dietary antagonists while still providing the complexed element to the tissues in a form that can be utilized [95]. The role of some minerals in enhancing egg quality will be shown in this review along with the difference between the effects of organic forms of minerals versus inorganic forms in laying diets (Table 1).

Table 1 Effects of the mineral supplement source and its doses on egg quality in laying hens
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Effects of Minerals on Egg Quality

Zinc (Zn)

Zinc performs multiple functions in the formation of the skeletal system, the regulation of metabolic processes, the maintenance of antioxidant systems, and the enhancement of the immune response in poultry, as well as egg formation [6, 7]. Zinc is a constituent of several metalloenzymes, for example, carbonic anhydrase, which has crucial role in eggshell formation [116], where the crystal and texture morphologies of the eggshell are affected by carbonic anhydrase’s catalysis of carbon dioxide into bicarbonate ions [51]. Zinc is a vital trace mineral necessary for forming eggshells and can contribute to the process of calcium deposition as well as impact the structure and physical properties of the shell [51, 65]. Zn serves as a significant cofactor in carbonic anhydrase enzyme which is responsible for facilitating the hydration of circulating CO2 into HCO3 − and providing the precursor for eggshell carbonates [122]. Zinc is important during albumen deposition in the magnum, eggshell membrane formation in the isthmus, and shell formation in the uterus [12]. Augmented Zn supplementation may decline egg loss from breaks and cracks. Several research has demonstrated superior eggshell thickness and strength and less egg breakage in birds administrated with Zn in organic form or in a combination of inorganic and organic forms [40, 71, 92]. The addition of zinc methionine chelate to layer diets is recommended because it results in improved bone mechanical properties, without compromising the quality of eggshell at the end of the egg-laying cycle, indicating its positive impact in the overall maintenance of bone mineral reserves through the end of the egg-laying cycle [70].

Mixing ZnSO4 and Zn-amino acid (ZnAA) complex in broiler breeder feeds diminished the percentage of cracked eggs and enhanced the eggshell quality in comparable to the basal diet that contained ZnSO4 [40]. Swiatkiewicz and Korelski [96] illustrated that 50 and 100% substitution of Zn oxide with ZnAA complex increased the eggshell breaking strength in the aged hens (62:70 weeks of age) that received the basal diet enriched with 30 mg Zn/kg. Moreover, Manangi et al. [62] exhibited that using Zn–Cu–Mn chelated with hydroxy analog of methionine (20–5-20 and 40–10-40 ppm) as an organic Zn supplement augmented both eggshell strength and eggshell thickness in layers (44 to 80 weeks of age) comparing to those consumed the basal diet only or the basal diet enriched with sulfate salt of Zn–Cu–Mn (20–5-20, 40–10-40, and 80–10-80 ppm). Thus, Zn might ameliorate the adverse influence of age on the quality of eggshell.

Li et al. [50] stated that adding Zn-methionine (Zn-Met) in the basal diet at level of 100 ppm augmented egg’s Haugh unit (HU) and albumen height in comparable to the control that fed the basal diet supplemented with 80 ppm Zn as sulfate salt. Abd El-Hack et al. [1] illustrated that supplementing diets with 50, 75, or 100 ppm Zn-Met had considerable affirmative influence on the HU compared to the control with no Zn-Met supplements. These enhancements may be owing to the significance of Zn function in the egg formation. Besides, Amen and Al-Daraji [9] pointed out that the epithelium quality is affected by the deficiency of Zn owing to its role in the synthesis of protein.

As previously mentioned, zinc has an indirect impact on the secretion of the epithelium layer by altering its structure or directly influencing the secretion of the eggshell membranes. Also, it plays a vital role in the magnum and isthmus during egg albumin and eggshell membrane formation, respectively. Tabatabaie et al. [98] have demonstrated that HU and egg albumen percentage were elevated by dietary administration of 25 or 50 ppm organic Zn, in comparison to the control with no Zn administration. Nevertheless, Idowu et al. [41] declared no statistical changes in criteria of egg quality, except the values of HU, when layers consumed diets enriched with Zn-proteinate, Zn-carbonate, Zn-oxide, and Zn-sulfate at a level of 140 ppm. Also, zinc could maintain the quality of the stored eggs due to its ability to activate enzymatic antioxidant system in the egg. Organic zinc, at a concentration of 60 to 80 ppm, exerted a more pronounced influence in this regard compared to 80 ppm inorganic zinc [50]. According to Zhao et al. [144], zinc has an anti-oxidation function that makes birds more resistant to certain oxidative stresses. Furthermore, Zn can be used as a dietary supplement to control the negative effects brought on by various agents such as aflatoxicosis in addition to serving as a nutrient for birds [67]. Generally, eggshell traits of layers could be improved by the replacement of inorganic Zn with organic one, especially in older birds, but the mechanism was still unclear.

Selenium (Se)

Selenium revealed strong biological and nutritional influences in inducing physiology and production of poultry [6, 7] that were primarily mediated by the activity of selenoproteins [55, 93]. The dietary requirement of selenium for laying hens is comparatively low, at approximately 0.3 mg/kg [79]. Excess Se intake is toxic [101], and its inorganic form has limited biological availability, consequently its inclusion by high doses limits its utilization in poultry nutrition [120], as well as its emission to the environment is higher than the organic form [117]. Organisms take inorganic selenium and turn it into organic form [94]. Selenium-enriched yeast culture, bacterial Se, SeMet, OH-SeMet, Se-cysteine, and semethyl-Se cysteine are examples of selenium organic sources. Organic form of selenium can be transferred to eggs [18] with a higher rate than inorganic Se through its addition to diets of egg-laying hens. In addition to the enhanced efficacy in transferring to eggs, a better internal egg traits (e.g., HU) was documented as a result of the stimulation of selenoprotein, methionine sulfoxide reductase B enzyme, which is demanded for preventing protein oxidation and maintaining the albumen water-holding capacity [42, 44]. Moreover, it is hypothesized that the presence of chromium is essential for maintaining the physical properties of albumin [44]. Dietary chromium supplementation (400 ppb to the basal diet contained 1285 ppb) linearly increased the HU and shell thickness of layer chickens kept at low temperatures compared to the control diet [84]. Lien et al. [52] displayed no changes in the shell thickness in response to supplemental Cr picolinate at dose of 200, 400, or 800 ppb under thermally neutral conditions. Similarly, in 24- to 33-week-age brown-type layers, supplemental Cr picolinate in diets at 400 or 600 ppb had negligible effect on shell thickness and strength, yolk color, and HU compared to non-supplemented Cr group. Torki et al. [100] observed greater weight and eggshell thickness of heat-stressed hens that received 400 ppb of Cr picolinate compared to the Cr-non-supplemented control birds. Subsequently, Torki et al. [99] pointed out that adding Cr to diets of laying hens exposed to heat stress increased the HU and shell weight of eggs produced from laying hens after exposure to heat stress. Nevertheless, supplemental Cr exhibited insignificant impact on internal egg quality. These studies demonstrate that chromium supplementation is important to improve the shell quality of eggs produced at non-neutral temperatures.

Effect of Nano-minerals on Egg Quality

Lately, the utilization of nano-minerals has garnered considerable interest owing to their elevated bioavailability; hence, their integration in poultry diets is capable of augmenting performance and health [2, 27, 38, 118] (Table 2). Incorporating nano-minerals in diets positively affects egg quality of laying hens [47]. The incorporation of selenium nanoparticles into the layer feeds exhibited a remarkable impact on the egg laying productivity, egg quality, and the activity of oxidative enzymes [68]. Sirirat et al. [89] stated that using chromium picolinate nanoparticles has the potential to enhance the quality of eggs. The latest authors revealed that layer-fed diets containing 500 and 3000 ppb nanometric chromium picolinate produced eggs with low yolk weight percentage. Laying hens fed on diets supplemented with zinc oxide nanoparticles achieved the highest significant improvement in egg shell quality, egg index, yolk and white quality, and HU compared to the control group [4]. The enhancement observed can potentially be attributed to the exceptional characteristics of nano-minerals, e.g., their remarkable biological availability, higher surface activity, improved mobility, increased solubility, and enhanced cellular uptake [63, 115].

Table 2 Examples of interaction of dietary constituents with minerals and subsequent effects on egg quality
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Interaction of Dietary Constituents with Minerals and Subsequent Effects on Egg Quality

The inorganic trace mineral absorption is limited by the interactions with other feed nutrients including the other trace minerals [111]. The mineral form and quantity determine how much mineral is deposited in the egg contents [13]. For example, chickens exposed for a long time to Cr showed drops in serum levels of Ca, Fe, Mg, Zn, and Cu [122]. Zinc is considered the first limiting trace mineral among others, i.e., Cu, Fe, Mn, and Zn, so supplementing Zn-deficient diets with organic form of these minerals alone or in combination led to nonsignificant change in bird performance and were primarily excreted [15]. Zn-methionine exhibits superior bioavailability compared to inorganic one owing to its stability and withstands interference against disruption caused by other ligands in the gastrointestinal system [51]. The availability of Cu could be decreased by consuming more Zn and Fe, and a lack of Cu would cause abnormalities in shell formation [33].

The most significant source of organic P, inositol phosphate 6 (InsP6), is regarded as an anti-nutritional factor because it forms complexes with minerals and inhibits the absorption of cations in poultry [108]. The high-calcium-content diets of layers, which is necessary for forming eggshells, may inhibit phytate-degrading enzymes, reducing the hen’s access to phosphorus [39] and additionally resulting in the inappropriateness of phytase superdosing in mixed diets of layers (Skřivan et al. 2018).

On the other hand, the synergy between minerals and other dietary supplements, e.g., phenolic compounds and essential oils that present in plant extracts, can maximize the effects of minerals. The chelation among these compounds augments the absorbed minerals and their utilization; this shows that the two together can be more effective than either used alone [91].

Dietary incorporation of inorganic mineral with 100 ppm of rosemary oil enhanced yolk color and the relative weight of egg albumen of laying hens [32]. Additionally, the advantages of using essential oils on digestibility [8] led to more absorption of nutrients, including minerals, from the feed, and enhancing the quality of eggs. A further strategy to lessen the negative effects of P, Cu, and Zn on the environment is to increase mineral digestibility using exogenous enzymes. For instance, exogenous phytase added to the feed improves the piglets’ ability to digest zinc [28]. Additionally, this would require less mineral supplementation ([28]; EFSA 2016). Therefore, research into the adverse effects and interferences of trace minerals, particularly in mixtures, on digestion, absorption, and utilization, should be done in order to spot nutritional solutions to maximize egg yield and quality while reducing the environmental pollution.

The Interaction of Intestinal Microbiota with Minerals and Subsequent Effects on Egg Quality

A complex community of numerous microorganisms makes up the gastrointestinal tract (GIT) microbiota of laying hens. This microbial community creates the intestinal microbial barrier, which is crucial for nutrient digestion and absorption, gut health, and the immune system [5, 45]. There are differences in microbial composition that can be seen related to feed and its nutrients (Leeming et al. 2021). A two-way relationship exists between trace minerals and the digestive tract microorganisms. Through their ability to directly affect the absorption of minerals within the digestive tract and the production of important enzymes like phytase, which assist in releasing minerals from the feed, microorganisms of gastrointestinal tract exert a profound influence on the metabolic processes of trace minerals [36]. Additionally, an acidic medium and strong gastrointestinal functions are necessary dissolving calcium and phosphorus to make them more absorbable and utilizable [112]. On the other way, the gut microbial content and role might be changed by trace minerals, and they could also compartmentalize metabolic inflammation [16].

Mineral Supplementation May Change Gut Microbiota Composition, Resulting in Improved Egg Quality

According to Dong et al. [24], the harmful cecal microbiota (Barnesiellaceae and Clostridia) significantly decreased in layers that consumed inorganic (full dose) or organic (half dose) Cu, Mn, Zn, and Fe when compared to control. Furthermore, they demonstrated an association between the gastrointestinal microbiota and the genes’ expression associated with the uterus (such as vocalyxin-32 (OCX-32), osteopontin (OPN), and ALAS). As a result, distinct microbiota was negatively correlated with various eggshell characteristics, including absolute and relative weight, quality, and breaking strength. Hence, the integrity gastrointestinal tract and higher absorbability of trace minerals may be related to the decrease in the load of dangerous Barnesiellaceae and Clostridiales bacteria. But the thickness and color of the eggshell, however, were not connected to any cecal bacteria [24]. According to Abdelqader et al. [3], supplementing diets of laying hens with Bacillus subtilis exhibited higher relative weight of the eggshell and higher calcium retention, along with a decline in the load of intestinal Clostridium. Adding organic yeast Se to laying hen feed (0.3 mg/kg) increased the diversity of intestine microbes in pre- or post-challenge with S. Enteritidis, maintained gastrointestinal well-being by raising the load of anti-inflammatory microbes (Barnesiella and Bacteroidales), and enhanced the egg quality [43]. According to the authors, selenium supplementation exhibited considerable influence on how the content of the gut’s microbes responded to antioxidants or immune stimulation. In order to enhance the quality of eggshells and hen health, intestinal bacterial regulation can therefore be crucial. However, Roth et al. [83] demonstrated that the distribution of the microbiota and predicted functions in two breeds of high production laying hen were unaffected by a 20% reduction in dietary Ca and P.

Conclusion

Using organic minerals in the nutrition of laying hens has a wide range of advantages that apply to different product categories. To achieve the beneficial potential of supplementing feeds of laying hens with mineral, it is required to pay attention to the different supplementation doses, the different sources, their bioavailability, and transport as well as understanding the mechanisms to obtain much greater bioavailability. The possibility of using reduced mineral doses from organic sources could improve egg quality and reduce Cu, Zn, and P emissions into the environment. However, the output of mineral supplementation in laying hen diets is affected by the strain, breed, age, or duration of supplementation, and interactions between trace elements and feed additives. Only the optimal mineral supplementation would support high productive and reproductive performance and egg quality (internal and external traits). To understand the mechanisms of augmentation or antagonistic effects and achieve the greatest beneficial effects for laying hens, it is possible to focus on the interaction of probiotics or the microbiome in general with mineral supplementation. In addition, the efficacy within the different types of organic minerals (complexes/chelates) and their mechanisms in laying hens should be highlighted in further studies. Some trace elements have antioxidants or immune-stimulating activities, such as zinc, selenium, copper, and iron have direct and indirect positive effects on laying birds. They are not only used as nutrients to support egg performance but they also play a significant protective role against the negative effects of stressful situations, such as illnesses that affect egg production and egg quality, or they can strengthen various body systems against these agents. Data concerning the organic minerals’ impact on egg quality traits are quite variable and molecular mechanisms of organic mineral actions need further research. In addition, substantially increased cost, in comparison to inorganic minerals, is an important factor to be considered in commercial poultry production. Therefore, it is crucial to assess the mineral requirements for laying hens to be advantageous on a variety of aspects of the bird’s body under healthy or disease conditions by choosing the ideal dose and form without having negative effects on the bird, its products, and subsequently the consumer, or the environment.