As discussed in the preceding articles in this series, the hen deposits nutrients, including minerals, into the egg and eggshell during egg formation. If the hen consumes highly bioavailable minerals, the minerals are deposited at a higher rate and the egg contains more minerals that are then available to the embryo. The developing embryo will take up all required nutrients from the yolk and albumin during incubation to build tissues, proteins, hormones, and immune cells. Uptake of these minerals by the embryo will also build up the mineral and energy stores that will be available to the chick after hatching. This may lead to increased bone strength and thickness, helping chicks to break through the shell at hatch. Stronger bones will also support a heavier frame during rapid growth later, thus decreasing the incidence of lameness. In commercial operations, apart from some nutrients obtained from an absorbed yolk sac, resources are scarce immediately post-hatch. Any increase in nutrient stores to make the chick more robust by the time it hatches, is beneficial to broiler health and performance. Furthermore, the newly hatched chick must immediately be able to defend itself against external threats. Along with minerals, increased antibody transfer to the progeny will promote the immune function of broilers and later allow them to compete for feed and water. All the mechanisms described here may either impede or promote embryonic development during incubation and will determine if and how the chick hatches and how well that chick is able to survive.
Critical trace minerals for broiler health and performance: Zinc, manganese, and copper
Some studies (Noetzold et al., 2022; Pereira et al., 2020) have fed broiler breeders diets that are supplemented with zinc, manganese, and copper from highly available sources and measured the progeny’s performance. Various observations were made, including increased cellular and humoral immune responses, and increased macrophage tumoricidal activity. These are all linked to an increased robustness and decreased mortality rate. Zinc supplementation in breeder diets has also been correlated with reduced inflammatory gene expression and antioxidant enzyme activity in their progeny. Typically, during a challenge, an inflammatory response is activated, which requires minerals and other nutrients. A weak immunity will require a prolonged inflammatory response to defend against a threat and may deplete mineral stores in the process. On the other hand, a strong immunity will launch an acute inflammatory response that subsides more quickly (thus, a more effective response, using fewer resources). A chick with sufficient mineral stores has a stronger immunity and thus will not deplete minerals stores. Having the chick maintain its mineral status for a relatively longer period gives it the ability to better deal with these stressors. This is especially critical during the first days post-hatch, which require adaptation, acclimatisation, and competition.
Iron, selenium, and chromium
The iron status of the breeding hen may directly correlate with the amount of iron deposited in the fertilised egg. This may influence the rate by which it is absorbed by the embryo, and finally increase the utilisation by the broiler to improve its performance. In a study by Ebbing et al. (2019), iron chelated to an amino acid was supplemented in the diets of broiler breeder hens. When the progeny of these hens was studied, the researchers observed an increase in their average daily gain and higher final bodyweights. In the same study, haematological parameters such as haemoglobin and haematocrit in blood were also increased for the same progeny.
Due to selenium being a part of the cellular antioxidant glutathione peroxidase, the selenium status of a chick may directly correlate with its ability to withstand oxidative stress. A chick incubated in an egg that had relatively higher selenium levels, deposited by a hen that was fed highly bioavailable selenium, may have a higher amount of this mineral stored in its body. Therefore, this chick may be better off physiologically, because it is more able to deal with environmental or metabolic stress at a young age.
Chromium plays a crucial role in glucose metabolism and insulin signalling between cells. A chromium deficiency may lead to a decrease in glucose uptake and utilisation, which is critical during times of stress. If insulin functions optimally, cellular damage as a consequence of stress may be limited. The cortisol level in the blood is a marker of increased stress. Chromium has been shown to reduce blood cortisol levels, which shows the beneficial effect chromium supplementation may have on the stress levels and performance of the commercial broiler.
Conclusion
The hatchling chick faces several challenges that start at the point of hatch and continue throughout the next phases of development, and so, the better a chick is able to survive the initial hours and days post-hatch, the better its chance of survival. However, for the chick to survive immediately after hatching, it needs to come with a good storage of nutrients – especially trace minerals. For the chick to build up mineral stores, it requires a wealth of minerals available to absorb during incubation. Research has shown that breeding hens supplemented with highly bioavailable trace minerals more readily transfer these minerals to the embryo by incorporating them into the egg during egg formation. It is, therefore, possible to improve the mineral status and thus survivability of chicks, as well as broiler health and performance, by considering the mineral source in the hen’s diet.