Numerous factors affect the functional quality of the egg shell mostly prior to the egg is laid. The thickness of the shell is determined by the amount of time it spends in the shell gland (uterus) and the rate of calcium deposition during shell formation. If the egg spends a short period in the shell gland, the thickness will be less. Also, the time of the day when the egg is laid determines the thickness of the shell. In general, the earlier in the day or light portion of the photoperiod, the thicker the shell will be.

1. Strain: Some strains of the birds may be able to deposit calcium for the egg shell at a faster rate than others, resulting in better deposition. It is observed that darker brown eggs have a higher shell quality than lighter brown eggs.
   
   
2. Diseases: Diseases like infectious bronchitis (IB), Newcastle disease (ND), avian influenza (AI) and egg drop syndrome (EDS) affect the shell quality. IB virus causes soft/rough shelled eggs, discolouration and wrinkling of the shell. EDS virus affects only the shell gland but with ND or IB, every portion of the reproductive tract can be affected.
   
   
3. Management: Poor housing, high ambient temperature, rough handling of the eggs will affect the eggshell quality. Since large eggs are more prone to cracks, the egg size must be managed through proper nutritional and lighting management. Management: Poor housing, high ambient temperature, rough handling of the eggs will affect the eggshell quality. Since large eggs are more prone to cracks, the egg size must be managed through proper nutritional and lighting management.
   
   Eggs from hens in the 3L:1D (3 days light : 1 day dark) regimen had a significantly greater shell breaking strength than eggs from hens in the 16L: 8D (16 hours light : 8 hours dark) regimen.
   
   
4. Moulting: The management practice of "forced" or "induced" moulting has shown to improve shell quality in all ageing flock. Following the moult, egg specific gravity, shell weight, shell thickness and percentage shell were either the same as they had been prior to the moult, or had improved, for all strains. Egg shell breaking strength improved in all strains as the result of the induced moulting.
   
   
5. Age of Bird: As the hen ages, the thickness of the shell usually declines. Older flocks lay larger eggs, which break easily. The hen is genetically capable of placing only a finite amount of calcium in the shell. Secondly, hen looses some of her ability to mobilize calcium from the bone, and is less able to produce the needed calcium carbonate. The absorption and mobilization of calcium decreases to less than 50% of normal after 40 weeks of age.
   
   
6. Drugs: For example, sulfa drugs affect the eggshell quality whereas tetracyclines have some beneficial effects.
   
   
7. Water Quality: Many studies showed that saline drinking water, including tap water containing sodium chloride supplied to mature laying hens at concentrations similar to those found in underground bore water, has an adverse effect on eggshell quality while having little effect on feed intake, egg production or egg weight. In contrast some reports indicate that there were no visible shell defects and specific gravity was also not adversely affected.
   
   
8. Stress: While a genetic predisposition for egg and eggshell quality exists, good genes can be upset by environmental stresses. The shell is formed by the activity of cells lying the oviduct and uterus. Under stress the secretions of these cells become acidic and the cells can be damaged or destroyed. In extreme cases, stress induced effects can result in eggshells that have excess deposits of calcium - a sort of powdery "bloom" on the surface and result in misshapen eggs. Relocation stress is known to have effects on the visual appearance of eggs produced; increasing the incidence of calcium coated and checked (misshapen) eggs.
   
   Major types of relocations, such as movement from one type of housing to a completely new housing environment, can produce severe visual defects of the egg.
   
   
9. Environmental Temperature: One of the factors contributing to poorer eggshell quality in hot weather is inadequate feed intake. Eggshell quality is somewhat compromised during summer months. During exposure to warm environmental temperature, the hen reacts by increasing its rate of breathing (panting) in order to cool itself. This causes the lowering of CO² in the blood and produces a condition termed "respiratory alkalosis". The pH of the blood becomes alkaline and the availability of calcium for the eggshell is reduced. This disturbance in acid-base balance causes an increase in soft-shelled eggs during summer.
   
   Temporary thinning of the egg shell may occur during periods of high ambient temperature (above 25ºC) since feed intake is reduced. The shells quickly regain normal thickness when temperatures are reduced and feed intake increases.
   
   Respiratory alkalosis also causes increased carbonate loss through the kidney resulting in competition between kidney and uterus for carbonate ion, consequently resulting in poor eggshell thickness. During heat stress calcium intake is reduced as a direct consequence of reduced feed intake and this stimulate bone resorption resulting in hyperphosphatemia. This inhibits the formation of calcium carbonate in the shell gland. Also heat stress reduces carbonic anhydrase (Zinc dependent enzyme) activity in the uterus. Under heat stress more blood is shunted to the peripheral tissues with concomitant reduction in flow of blood to the oviduct resulting in poor shell quality. Lastly the ability of layers to convert vitamin D³ to its active form is reduced during heat stress.
   
   
10. Nutrition: There is a complex relationship between calcium, phosphorus, vitamin D³ and the hormonal system of the layer in calcium metabolism during lay. Calcium and phosphorus balance is critical for proper egg production and eggshell quality. Layer ration should be formulated with correct amount of calcium and phosphorus (usually 3.5 - 4.0% calcium, 0.35-0.40% phosphorus)

1. Calcium: Both excess and deficiency of calcium will negatively affect the shell quality. An egg contains almost 2 grams calcium; hence an average of 4 grams of calcium intake per day is required by a layer to maintain good shell quality since only 50 - 60% of dietary calcium is actually used in shell formation.
   
   Calcium requirement of a laying hen is 4 - 6 times that of a non-laying hen. The egg enters the shell-gland region of the oviduct - the uterus - 19 hours prior to oviposition, and the shell does not store calcium ions to attach on protein matrix.
   
   During the last 15 hours of shell formation, calcium movement across the shell gland reaches a rate of 100-150 mg/hr. This process draws calcium from two sources: diet and bone. Normal blood calcium level is about 20 - 30 mg/dl with a normal layer ration of 3.56% calcium or higher, while layers on a 2% calcium diet, 30- 40% of the calcium is derived from bone. It is therefore important to have pullets, prior to lay, on a high level of calcium to store it on body.
   
   Intestinal absorption of calcium in the diet is about 40% when the shell gland is inactive, but reaches 72% when active. This time closely coincides with late afternoon or the dark hours for the layer. Having higher calcium levels in the gut during this time is important to ensure calcium is being taken from the diet and not bone. Large particle sizes of calcium sources allow calcium to be metered throughout this time.
   
   In growers, most importantly, high calcium levels during the growth period will interfere with the proper development of the parathyroid gland by increasing gut pH, which will decrease absorption. The damage to the parathyroid would be permanent and would affect the bird's laying cycle afterwards.
   
   
2. Phosphorus: The phosphorus content of the eggshell is small i.e. 20mg, compared with 120mg in the egg contents. There is also uneven distribution of the phosphorus in the inner and outer layers of the shell. Phosphate ions have an inhibitory effect on the CaCO³ and bring the shell formation to an end.
   
   High levels of phosphorus in the blood will inhibit the mobilization of calcium from bone. The absorption of calcium and phosphorus are interrelated and can be influenced by:
   
   Source and form of calcium and phosphorus: Calcium source and particle size plays a role in calcium level in the gut when needed. Phosphorus must be in a form that is available and usable by the layer.
   
   Intestinal pH: Phosphorus absorption is optimal at pH 5.5-6.0. When the pH is higher than 6.5, absorption of phosphorus markedly decreases. Excess free fatty acids in the diet can cause the pH to decrease and therefore, interfere with calcium and phosphorus absorption.
   
   Calcium and phosphorus ratio: High calcium or phosphorus levels in the intestine reduce the absorption of both. High calcium increase the pH in the gut and phosphorus absorption is decreased along with zinc and manganese absorption. High plasma phosphorus decreases calcium absorption from the gut and calcium mobilization from the bone. Phosphorus is an integral part of the acid-base balance in the body. The proper ratio of calcium to phosphorus (Ca: P ratio) for growing birds is 1.5-2.0 Ca: 1.0P.
   
   Vitamin D³ : Vitamin D³ metabolite is essential in absorption of the Calcium.
   
   
3. Vitamin D³ is vital for absorption and mobilization of calcium during shell synthesis. The importance of adequate vitamin D³ intake by the hen is obvious and it is essential for proper calcium and phosphorus utilization. However, excess vitamin D³ and its metabolites have not shown to benefit eggshell quality when normal hens are already consuming adequate vitamin D³ .Vitamin D³ is the major control element in stimulating calcium absorption from the intestine. This effect is facilitated by the synthesis of calcium-binding protein (CBP). Vitamin D³ intake must be adequate. The function of vitamin D³ is related to its metabolite 1,25 dihydroxy D³ that is formed in the bird's liver and kidneys. Any problem that affects the integrity of these organs or the parathyroid gland will have an adverse effect on the action of vitamin D³ and thereby calcium absorption and metabolism.
   
   
4. Diet formulation: Shell breaking strength was greater for the sorghum diet than wheat or barley based diet and less for maize-soya diet. High levels of calcium and phytate in the diet of laying hen reduce the availability of trace minerals, especially manganese and zinc. Addition of non starch polysaccharides breaking and phytase enzymes to the feed tends to improve eggshell quality.
   
   No deleterious effects on egg and eggshell quality were observed when levels of chloride and magnesium were upto three times higher than recommended levels. Excess dietary chlorine, however, decreases blood bicarbonate concentration, which plays a pivotal role in eggshell calcification. Low dietary cationic-anionic balance, presence of non starch polysaccharides, mycotoxins and contaminants results in poor shell quality.

Part 1: Maintaining Egg Shell Quality
Part 2: Factors Influencing Shell Quality
Part 3: How to Improve Shell Quality

March 2008