Practical approaches for optimum egg quality

Protein level in the diet is an important factor affecting egg production and egg weight (Table 1). Fats also have similar effects on laying performance, probably due to their effects on the metabolism of oestrogen which controls the formation of albumen in the egg yolk and hence contributes to increased egg size and weight.

Dietary fat source affects the degree to which egg size and weight increase. In a study with laying hens, egg size and weight were greater when vegetable oil was included in the diet at 4% rather than with fish oil fed at the same level. This difference was attributed to the greater ability of the vegetable oil to stimulate oestrogen metabolism.

Reduced vitamin content of the diet, particularly the water-soluble vitamins, results in low levels of these vitamins in eggs. In one study, the level of riboflavin in the eggs was reduced by 1.9 μg/g of egg weight when an inadequate amount of this vitamin was provided in the diet.

A deficiency of some minerals in the diet of laying hens, particularly iodine, iron and copper, also results in low levels of these minerals in the egg. It was estimated that the level of iodine in eggs produced by hens fed an iodine-free diet was 450 times less than that in eggs produced from iodine-enriched diets.

The calcium content of the feed is probably the most limiting factor in this regard, being a major constituent of the shell. Increasing the dietary calcium level from 1% to 3% led to an increased shell thickness from 328 to 388 picometres, with a subsequent reduction in the percentage of cracked eggs from 26.2% to only 10.2%.

In most practical diets for laying hens, a calcium level of 3% represents the upper limit of this element to be incorporated in the diet, above which it can negatively affect other production parameters, such as feed intake and egg yield. In some cases, however, positive responses were obtained with calcium levels of up to 5.7%. This apparent discrepancy can be attributed to such factors as breed of hen, temperature, disease outbreaks, protein and energy sources in the diet, and the bioavailability of calcium which, in turn, depends largely on its source.

Heat stress

Heat stress reduces feed intake and limits the availability of blood calcium for eggshell formation. It may also reduce the activity of carbonic anhydrase, an enzyme that results in the formation of bicarbonate which contributes carbonate to the eggshell.

Heat-stressed birds usually pant with progressive respiratory alkalosis. In this situation, the carbon dioxide content of the blood first decreases and is then followed by an extra-renal elimination of bicarbonate ions which restores the blood pH near to the normal value. This should limit the ionic exchanges in the shell gland which is consistent with the immediate decrease in eggshell thickness.

Another factor affecting shell quality under heat stress is the reduced blood flow (down by 30-40%) through the ovarian follicles and shell glands due to peripheral vasodilation.

Heat stress also reduces egg weight. This reduction in egg weight varies from 0.17-0.98 g/°C of temperature increase, with an average value close to 0.4 g/°C. The dry-matter concentration in the yolk and albumen is generally not modified when the temperature is lower than 35°C but may be reduced beyond that temperature.

A decrease in yolk lipid content was also noticed after very early heat stress in pullets exposed to 36°C between three and five weeks of age. The Haugh units (a measure of egg protein quality based on the height of the egg white or albumen) rapidly reduce after laying if the egg is not immediately withdrawn from a poultry house under high temperature.

To alleviate the effects of heat stress on egg quality, the following points should be considered:

• Poultry house walls or roofs should be well-insulated
• A proper ventilation system should be provided
• High stocking density should always be avoided
• Feeding practices under heat stress should focus on ensuring that birds are receiving adequate levels of essential nutrients
• Cooling the water may be beneficial

Poultry diseases

The main disease of laying hens that has been reported to affect albumen quality is the infectious bronchitis virus which may cause a decrease in quality and more variable albumen quality. There is evidence that infectious bronchitis impairs the synthesis of albumen proteins in the magnum of the oviduct and is associated with histological changes in the epithelium of the magnum.

Salmonellosis is another disease caused by the S. enterica bacteria and leads to various health and production problems, especially in force-moulted hens. The disease also affects egg quality traits and hatchability, as well as the survival of hatching chicks, particularly during the first few days of life. Therefore, management and biosecurity measures should be taken to reduce the introduction of S. enterica from feed, water, wild birds, rodents, insects or people.

Coccidiosis is also mentioned as a cause of decreased egg production and quality in laying pullets. In one study, Eimeria maxima infection resulted in cessation of lay one week later, production being interrupted for about 7-10 days. Albumen quality was not altered but shell thickness declined until the birds went out of lay.

Egg storage

The main factor influencing internal egg quality is the duration of storage. Eggs lose about 1% of their weight each week but these losses can be restricted by oiling the shell which decreases the permeability of the shell by closing the pores. Storage temperature is also an important factor affecting quality. High temperature causes rapid changes in the Haugh Units and air cell size.

The critical limit for grade A eggs (i.e., the probability of exceeding 6 mm air cell size), is predicted for 10% of the eggs at 17 days when stored at 20°C or at 25 days when stored at 6°C. Therefore, cooling eggs from the beginning of storage is recommended whenever eggs are not sold and consumed within a short time.