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KING’ORI cuticle (Bell, 2007).Within the different layers are crystalline interfaces, hundreds of hidden fault lines, and thousands of pores (tiny tunnels which travel from outer to inner surfaces of the shell). The small amount of organic matter mostly consists of matrix proteins (mixture of proteins and polysaccharides rich in sulphated molecules) and shell pigment. The matrix proteins are critically important in determining the egg shell structure and serves as foundation for the deposition of calcium carbonate during the mineralization process (Burley et al., 1989; Lavelin et al., 2000; Gupta, 2008). Without the matrix proteins, the crystal structure would be too brittle to keep its form. The chicken eggshell is 95-97% calcium carbonate crystals, 0.3% phosphorous and magnesium and traces of sodium, potassium, zinc, manganese, iron and copper and organic matter (Arias et al., 2001; Nys et al., 2004; Hunton, 2005; Neospark, 2012)). The structure and composition of the avian eggshell serves to protect the egg against damage and microbial contamination, prevention of desiccation, regulation of gas and water exchange for the growing embryo, and provides calcium for embryogenesis. The eggshell should be as strong as possible to maximize the number of eggs reaching the consumer. No single factor is usually responsible for egg shell quality. Factors related to eggshell quality include nutrition (the nutrient balance and intake levels of the ration), health (the flock’s general health and previous health history), flock management and its age, environmental conditions and breeding (the strain and breed of the flock) along with other factors (Bell, 2007; Neospark, 2012). About 5-7% of eggs produced do not reach the consumer; 2-3% of the damage is due to problems during laying and 3-4% during the process after laying. Types of Shell Deffects They include shell thickness, gross cracks, hairline cracks, star cracks, misshapen egg, pimples, sand paper, pinhole, leatherly and glossy eggs. The complexity of structure of the shell gives rise to differences in breakage in eggs. The crack severity, shape and length are results of variations in the structure with fractures occurring at the points of least resistance (Bell, 2007). Shell deffects account for 0.5-5-6% of total production. Nutrition plays a key role in maintaining egg quality. The hens should be offered a balanced ration. Vitamin D, calcium, phosphorous, manganese, copper and zinc play a major role in maintaining the integrity and shell quality. Excess or reduced concentration of phosphorous, chlorine, or mycotoxin contamination affects the availability of calcium and vitamin D. 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).Numerous factors affect the functional quality of the egg shell mostly prior to egg laying. 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 (Neospark, 2012). 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. 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. Shell quality affect consumer appeal, packaging, egg breakage, storage/shelf life, and hatchability (Rogue et al., 1994; King’ori, 2011). An egg shell that is smooth is desirable because rough shelled eggs break more easily. Large sized eggs usually break more easily than small ones because the hen is genetically capable of placing only a finite amount of calcium in the shell (Neospark, 2012). Reduction in shell quality lowers egg shelf life, hatchability and increases breakage. Shell thickness and porosity regulate the exchange of carbon dioxide and oxygen between the developing embryo and the air during incubation (Rogue, 1994). Shell thickness has significant effect on moisture loss during storage and incubation (Bennett, 1992). Thin shelled eggs loose more moisture than do thick –shelled eggs, causing 352 J. Anim. Prod. Adv., 2012, 2(8):350-357
EGG QUALITY DEFFECTS: TYPES, CAUSES AND OCCURRENCE … difficulty during hatching (Rogue et al, 1994) and deterioration on quality for table eggs. Viral infections (Infectious bronchitis (IB), Newcastle disease (ND) and Egg drop syndrome (EDS)) can lead to loss of shell colour, as well as egg deformities and a reduction in egg numbers (Thear, 2005). IB virus causes soft/rough shelled eggs, discolouration and wrinkling of the shell. Egg drop syndrome (EDS) is a viral infection that results in a reduced number of eggs, as well as an increased number of pale-shelled eggs. EDS virus affects only the shell gland but with ND or IB, every portion of the reproductive tract can be affected. The roughness of the egg shell increases with age and when serious may result in some downgrading. Molting corrects the problem to a degree, but when flocks are kept in lay beyond 30–40 weeks, many eggs become very rough and consumers may find this displeasing. The hen loses 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. The practice of "forced" or "induced" moulting has been shown to improve shell quality in all ageing flock. Following the moult, egg specific gravity, shell weight, shell thickness and percentage shell are either the same as they were prior to the moult, or had improved, for all strains. Induced moulting improves egg shell breaking strength in all strains. Use of certain chemotherapeutic agents can affect the intensity of pigmentation. Sulfa drugs affect the eggshell quality. The presence of the coccidiostat Nicarbazin in the feed can produce paler shells (Thear, 2005). 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. The hens on the 3L: 1D regimen have longer dark hours when calcium absorption is highest (Neospark, 2012). Eggshell quality is somewhat compromised during summer months. One of the factors contributing to poorer eggshell quality in hot weather is inadequate feed intake. 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. 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. The ability of layers to convert vitamin D3 to its active form is reduced during heat stress. 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 carbon dioxide 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. 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. The shell is formed by the activity of cells lining 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. The presence of parasitic red mites that emerge at night to feed on the perching hens can have a debilitating effect on the birds, leading to anaemia, poor shell thickness and loss of egg colour. The presence of parasitic worms can also debilitate the hens to the extent that their nutritional intake is affected that leads to the production of eggs with deffects. A sudden disturbance to the normal routine may result in a hen retaining the egg 353 J. Anim. Prod. Adv., 2012, 2(8):350-357
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