CALCIUM A Key Factor for Eggshell Quality By Iain Campbell, UK Poultry Technical Manager, Techna UK


Eggshell quality is one of the major concerns for egg producers. Cracked, pale and thin shelled eggs aren’t suitable for whole egg sale so are downgraded, which reduces the price paid to the farmer. Downgraded eggs account for 6-12% of all produced eggs of the flock. At 50 weeks of age, the hen’s eggshell quality drastically decreases with up to 20% downgrades at the end of the laying period. Egg shell integrity is therefore a crucial condition for ensuring the profitability of egg producers within the length of current cycles, but a very large issue with extending the length of the laying cycle. Calcium is an essential mineral in poultry nutrition, with levels


between 3.5% to 4% of a layer’s diet. It plays a key role in vital bodily functions such as the participation in acid-base balance and enzymatic system and is the main component of bone structure. It is also the main component of the eggshell. It is estimated that each egg contains 2.2g of calcium, the majority of which is found mainly in the eggshell. In fact, 95% of the eggshell is calcium, which is deposited during the later stages of egg formation in the composition of calcium carbonate deposited on the egg membrane by the shell glands. Since demand for calcium is very high during lay, avian species


can resort to special and unique ways to mobilise the needed quantity of calcium by drawing upon their bone reserves and increasing intestinal absorption. The maximum rate of calcium deposition occurs between 12 and 18 hours after ovulation, when the egg is in the uterus. The calcium transfer from the blood to the eggshell at that time is very high (rates of 100-200 mg/h). Due to productive, economic and animal welfare reasons, it is


essential to understand the structure and function of the bone and the calcium metabolism in laying hens. This knowledge will help us to apply the correct management of feeding strategies and nutrition in order to achieve appropriate mineral reserves in the skeletal system, an eggshell of good quality, and the prevention of problems or imbalances derived from an alteration of these factors. It is unlikely that there is another animal that can consume,


absorb, transport and metabolise more calcium per unit of weight than birds. All nutritional factors affecting egg composition, whether internal or external egg quality, are linked to nutrient intake. Therefore, it is important to manage the birds’ feed composition as well as feed distribution management. The size of the particles for dietary calcium is known to show


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different efficiencies on the utilisation of the calcium in the hen. Feed compounders primarily use two supplemental sources of dietary calcium; oyster shell or limestone. Oyster shell and limestone both provide calcium in the form of calcium carbonate, each contains about 38% calcium and both are usually presented as a ‘grit’ particle rather than a powder. The particle size of calcium sources may influence its availability


to the laying hens, as eggshell is usually formed during the night, when hens do not eat feed, it can be beneficial to use larger particles which have a slower passage through the gastrointestinal tract. Slower passage through the intestinal tract allows more calcium to be available for eggshell formation, with consequently lower mobilisation of bone calcium by the laying hens. Large particles (over 1.0mm) are speculated to remain in the upper digestive tract (crop and gizzard) for a longer period than smaller particles (smaller than 1.0mm), resulting in calcium being available to the hen for a longer period, especially during the 8 to 9 hours of the dark period when feed is not consumed.


Active and Passive absorption The skeletal system of birds is lightweight to facilitate flight, thanks to the fusion of several bones into one. There is a large presence of pneumatic bones, which contain air instead of bone marrow to further support a lightweight skeleton. The bones with bone marrow are the long bones distal to the humerus and pelvis. One such bone found in female birds is the medullary bone. It is developed during sexual maturity under the effect of estrogen and contains easily mobilised storage of calcium. The medullary bone tissue is physiologically active and plays an important role in the homeostasis of serum calcium levels to compensate for the temporary lack of intestinal calcium using the bone for the eggshell formation. During lay, calcium intestinal absorption in the duodenum and upper jejunum by passive ionic diffusion is not sufficient to satisfy the high demand. Hence, these calcium supplies are usually complemented by a specific active diffusion allowed by a calcium binding protein called calbindin. Once calcium provided in the diet reaches the blood vessel, it can


be used directly for eggshell through the shell gland. If this mineral is not available in sufficient supplies as the shell is forming, the hen will use her own bone reserve.


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