The present study explored the mechanism of Zn-methionine (Zn-Met) influencing eggshell quality of laying hens and investigated whether the mechanism was related to Ca deposition. Hyline grey layers (n 384, 38 weeks old) were divided into four groups: 0 mg Zn/kg, 40, 80 mg Zn/kg as Zn-Met, and 80 mg Zn/kg as zinc sulphate (ZnSO4). Eggshell quality, Zn contents, Zn-containing enzyme activities and expressions of shell matrix proteins in eggshell gland (ESG) were analysed. Zn-Met treatment at 80 mg/kg increased (P < 0·05) egg weight and eggshell strength throughout the experiments. The 80 mg/kg Zn-Met group (P < 0·05) had decreased mammillary knob width and larger relative atomic weight percentage of Ca, stronger signal intensity of Ca in linear distribution and the Ca was more evenly distributed in the transversal surface of eggshell. Zn contents (P < 0·001) in yolk and serum, Ca, albumin (Alb) levels in ESG as well as carbonic anhydrase (CA) activity in serum (P < 0·05) and mRNA levels of CA and Ca-binding protein-d28k (CaBP-D28k) (P < 0·001) in the 80 mg/kg Zn-Met group were the highest among all treatments. In conclusion, shell strength as one of eggshell qualities was mostly related to mammillary cone width in ultrastructure caused by the pattern of Ca deposition in eggshell formation. Also, the increase in Zn-Met-induced Ca deposition may be due to the increased Zn contents in serum and tissues, which were attributable to the increased CA concentrations in serum, Ca, Alb levels and up-regulated CA and CaBP-D28k mRNA levels in ESG.

Vitamin D plays many important functions in the animal, as it is necessary for proper calcium (Ca) absorption and use in different biological processes. The aim of several recent experiments with poultry was to evaluate the efficacy of different forms and levels of vitamin D3 in the diets for broiler chickens and laying hens. The results of these studies are presented and discussed in this review paper. Based on these results, the modern high-performing poultry requirement for supplemental vitamin D to maximise mineral digestibility, performance and immunity indices, bone health, and eggshell quality is about 3,000 IU/kg, i.e. much higher than NRC (1994) recommendations. The results of several recent poultry studies have shown that 25-hydroxycholecalciferol (25-OH-D3) is more efficient in commercial poultry nutrition than the basic form of vitamin D3 (cholecalciferol).

Manganese (Mn) is an element required in nutrition, functioning largely in the enzyme systems involved in lipid and carbohydrate metabolism. It plays an important role in growth, bone development, perosis prevention, optimal eggshell quality and performance maintenance of poultry. The current NRC (1994) guidelines recommend 20 and 60 mg/kg of Mn for laying hens and broilers, respectively. In practical poultry feeds, however, the dietary requirement of Mn is considered higher than these values. Traditionally, Mn is added to the diet in the sulphate form, which is typically used as the standard in nutritional studies. However, in recent times, other, more natural forms have been made available to the feed industry, e.g. organic chelates. From this current review of published data, it appears that inorganic Mn supplementation up to 200 mg/kg is ineffective on the performance of both laying hens and broilers but at lower levels it improves eggshell quality. The dietary Mn requirement of laying hens and broilers appears to be 90 mg/kg, and the availability of Mn-sulphate is higher than that of other inorganic Mn sources but lower than its organic sources. More studies should be undertaken to determine the optimum Mn requirement in its various forms for laying hens and broilers.

The shell colour of the eggs laid by pheasants is determined genetically and can vary from very light to dark brown. In general, four basic colours may be distinguished: dark brown, light brown, olive, and blue. A number of studies indicate that there is a relationship between the colour of pheasant eggshells and their quality. Pheasant eggs with different shell colours differ in terms of morphological (shape) and physical features (weight, egg specific gravity, yolk and albumen share and indices) as well as eggshell quality (thickness, strength, ultrastructure). Studies conducted by previous authors demonstrated a range of structural and physical properties of pheasant eggs. The microscopic structure of the blue-coloured shells demonstrates some abnormalities in the structure and thickness of the shell layers and the shells of these eggs are the thinnest. This may be the cause of poorer hatchability in blue-coloured shell eggs.

The objective of this review is to update and discuss the current findings from studies with laying hens on dietary factors that can beneficially affect eggshell quality, with special emphasis on microelements and feed additives. The crucial importance of dietary calcium, phosphorus and vitamin D3 levels and sources for eggshell quality has been well documented in scientific literature. Many recent studies regarding the effect of nutrition on eggshell parameters have focussed on dietary micromineral levels and sources. There has been also growing interest in the influence of feed additives on the improvement of intestinal health and mineral availability. The results of the experiments presented here demonstrate that efficacy of layer diet supplementation with microelements and feed additives is not consistent, however findings of several trials indicate, that eggshell quality may be positively affected in certain conditions by optimal dietary level and form of manganese, as well as by the addition of pre- and probiotics, organic acids, and herb extracts.

Mineral availability is one of the most important concerns in poultry nutrition. In modern, high-producing, meat and laying poultry, mineral metabolism disorders often lead to decreased eggshell quality and osteoporosis in laying hens and bone weakness in broiler chickens. This has negative effects not only on the economic profitability of production but also on the welfare of the birds. The article discusses the results of studies on the effect of chosen feed additives, namely organic acids and prebiotic fructans, on mineral utilisation in poultry. The results of some of the experiments presented in the article indicate that the use of these additives may have a positive impact on mineral metabolism in poultry.

Any approach which aims to improve the quality and safety of eggs, is of limited value unless it can relate to the incidence of breakages in the field, and can be applied either directly to breeding programmes or used as online tools for quality assurance. Online systems require the method of assessment to be non-destructive. This is probably not so important for breeding companies who tend to gauge the usefulness of an eggshell quality measurement by its ease of measurement, its heritability and, its genetic correlation with other phenotypic traits. Historically, qualitative measures such as shell breaking or puncture strength, non-destructive deformation and specific gravity have been widely adopted as tools in selection programmes. The dynamic stiffness is a novel eggshell quality measurement, which could be utilised by poultry breeders as it has a moderate heritability and high genetic and phenotypic correlation with other shell quality traits including eggshell breaking strength. This non-destructive measurement could also be applied on-line to monitor shell quality. An alternative approach is to use genetic markers as tools to aid conventional breeding programmes where an important eggshell quality trait is practically too difficult or expensive to fully exploit. Marker-assisted selection for the genes coding for organic matrix proteins may provide a way of incorporating other beneficial egg quality traits into selection programmes in the future.

Many studies have shown that saline drinking water supplied to mature laying hens at concentrations similar to those found in underground bore water has an adverse effect on eggshell quality and on the incidence of eggshell defects. This response can be observed in the absence of any adverse effects on production parameters such as food intake, egg production and egg weight. Strain differences appear to exist, and even within a strain there is considerable variation in the responses of individual hens to saline drinking water. The sensitivity of the hen also increases with age with a greater occurrence of broken and shell-less eggs. The incidence of shell-less eggs also increases with higher concentrations of NaC1 in the water. The use of saline drinking water with breeder flocks significantly reduces the production of day-old chicks through reduced numbers of settable eggs and lower hatchability. The primary metabolic lesion associated with the poor eggshell quality which results from the intake of saline drinking water appears to be related to the supply of bicarbonate rather than calcium to the lumen of the shell gland for eggshell formation. A reduced activity of carbonic anhydrase in the shell gland mucosa is of particular importance. This limits the supply of bicarbonate and the dependent calcium to the lumen of the shell gland. Treatments which have been found to be effective are preventive rather than remedial in nature. The two most promising treatments, apart from desalination of drinking water, appear to be the use of ascorbic acid supplements in the diet or drinking water appear to be the use of ascorbic acid supplements in the diet or drinking water and the use of zinc–methionine supplements in the diet. The mechanism by which ascorbic acid exerts its effect is unknown but the response is dose-dependent. The zinc component of zinc–methionine appears to be the important factor with this treatment, perhaps through its effect on carbonic anhydrase, a zinc-requiring enzyme.