During egg formation in the reproductive tract, minerals are deposited into the egg and eggshell. Firstly, the egg needs to be of high quality, and robust enough to withstand the environment for the duration of incubation. The eggshell is a vital component of the egg, not only because it contains the embryo and keeps pathogens out, but also because it provides the nutrients required to build a strong skeleton and healthy skin during embryonic development. Later, these same nutrients are also critical for supporting the extraordinary growth rate of the commercial broiler.
Secondly, the embryo contains not only the unborn chick, but also includes many other tissues and membranes. The embryo requires various minerals to be available for uptake and utilisation during incubation for development and growth. All nutrient deposits that need to be made are done by the time the egg is laid and the embryo is solely supplied of all its trace minerals and all the other nutrients needed to survive by the egg contents. If mineral levels in eggs are sufficient, this contributes to the optimum functioning of the enzymes critical for the health, growth, and survivability of the embryo up to hatching. Any deficiency in trace minerals in the egg may lead to growth and/or health issues of the embryo, dysfunction of the membranes, or retarded development of the bird. Immunoglobulins, transferrin, and glycoproteins are what bind trace minerals to the yolk, and ovalbumin and ovo-transferrin are the proteins that bind trace minerals in the albumin fraction.
The most critical minerals for embryonic development: Zinc, manganese, and copper
Zinc is part of carbonic anhydrase, an enzyme involved in supplying carbonate ions during eggshell formation. In this way, zinc plays a role in the deposition of calcium in the eggshell. Zinc is also an integral part of superoxide dismutase, an antioxidant enzyme that maintains the integrity of the internal membranes of the egg and protects the embryo from oxidative stress. Egg trace mineral content increases when organic sources, rather than inorganic sources, are fed to breeder hens (Abd El-Hack et al., 2017). This indicates that, if there is more zinc available to the hen, it may deposit more readily into the egg.
Manganese forms part of glucosyl-transferase, an enzyme involved in eggshell formation, and may thus promote improved eggshell quality. It plays a role in the formation of mucopolysaccharides, which are components of proteoglycans in eggshells. It may further contribute to egg quality by regulating and orientating the construction of the mammillary layer of the shell. Additionally, and similarly to zinc, manganese is required for superoxide dismutase and its antioxidant activities inside the membranes and the embryo.
Copper forms part of the lysyl-oxidase enzyme and assists manganese in its role of forming calcite crystals, polymerising collagen, and incorporating it into eggshells during eggshell formation. Furthermore, copper is vital in building and maintaining the internal membrane lying between the albumin and the eggshell. Like zinc and manganese, copper is also associated with superoxide dismutase and thus plays a role in the protection of the embryo from oxidative stress by maintaining the integrity of internal membranes soft tissues.
The use of zinc, manganese, and copper supplementation, especially when used in more bioavailable forms, has been shown to increase eggshell quality and membrane integrity. This increases hardness and the improved eggshell breaking strength leads to a decreased prevalence of cracked eggs and a longer egg shelf life. As mentioned previously, zinc, manganese, and copper form part of cellular antioxidants, specifically superoxide dismutase, which scavenges free radicals and thereby protects cell membranes from oxidative damage. The presence of these enzymes in the egg maintains the integrity of the albumin, yolk, and other soft membranes during incubation.
Additionally important trace minerals: Iron, selenium, and chromium
Eggshell colour has been linked to the hatchability of broiler chicks, with higher hatchability being observed when eggshells are darker in colour. According to a study by Ebbing et al (2019), iron supplementation in breeding hen diets darkens the colour of eggshells. This is linked to iron being a component of protoporphyrin-9, a precursor of the brown pigment in eggshells. The effect was even more pronounced for organic sources of iron as compared to inorganic sources. This research indicates that increased iron availability at the time of eggshell formation may lead to increased hatchability of chicks. Iron is also required by the developing embryo to build the blood system of the chick and to facilitate the flow of oxygen into and around the egg.
Selenium is a component of glutathione peroxidase, an important antioxidant in cells and egg membranes. By increasing the efficacy of free-radical scavenging, glutathione peroxidase has been linked to increased hatchability of fertile eggs. It has been shown that supplemental selenium in the diet will increase the selenium status of the egg yolk and other internal egg membranes, including the albumin, which may lead to improved integrity of the internal egg membranes during incubation. The Haugh unit is a measurement of albumin quality and is obtained by measuring the albumin height (thickness of the membrane) of broken table eggs. If the internal membranes’ integrity are maintained, the embryo may survive better and thus hatchability may be improved. Selenium deposits into the egg are more preferentially directed to the yolk when dietary selenium is below the hen’s requirement and more evenly distributed between yolk and albumin when selenium levels are increased.
Chromium may play a role in alleviating high cortisol levels in the developing embryo, which will reduce the negative effects of heat stress or dehydration during embryonic development. If chromium is deposited into the egg at a higher rate, it will be available and thus be absorbed by the embryo.
An egg laid by a breeding hen must be of premium quality. The eggshell must be well constructed and robust to withstand reasonable handling and processing, while also acting as a barrier to pathogens. All the membranes and soft tissues inside the egg must be able to withstand dehydration, environmental stress, and oxidative damage in order for it to maintain integrity and protect the embryo during embryonic development. Finally, the embryo must be well supplied with nutrients by its immediate environment – the membranes and eggshell – to build systems and tissues. A breeding hen deposits trace minerals and other nutrients into the eggshell and membranes, but if the hen’s mineral status is optimised, the transfer may be more pronounced and so the whole process and the outcome are optimised. This is the topic of the last instalment in this series: Broiler health and performance.