Abnormal Egg Problems: Problems and Causes
Part 1 - Egg and Eggshell Formation
In order to evaluate the causes and solutions of problems associated with shell quality, it is necessary to understand the processes of egg and eggshell formation. 1.1 The ovary and oviduct Figure 2 show the reproductive system of the domestic hen. Birds are unique among animals because only one ovary (the left) matures to the stage where it releases eggs or ova into the oviduct. The ovary of a mature hen contains thousands of immature oocytes that develop sequentially into follicles (known also as ova or yolks) ready for release into the oviduct. The oviduct is a tube-like structure that completes the formation of the egg. The oviduct is divided into different sections, each performing its own individual role in egg formation; these sections are the infundibulum, magnum, isthmus, shell gland and vagina. 1.2 Egg formation The process of egg formation is a well-researched area. The structure of the egg and the physiological role of the oviduct during egg formation are quite well understood. This brief summary will concentrate on the structure of the egg as it is formed rather than on the physiology of the reproductive tract. At ovulation, the yolk is released from the ovary into the body cavity. The infundibulum is the first section of the oviduct and its main role is to actively capture the yolk from the body cavity and direct it into the oviduct. The yolk remains in the infundibulum for 20 minutes. During this time, if sperm are present, fertilisation occurs and the egg will have the potential to form a new chicken. The infundibulum also has roles in the addition of the membrane that immediately surrounds the yolk (perivitelline membrane) and in the formation of the chalazae (the ‘anchor-like’ structures that hold the yolk in place). The magnum is the next and longest section of the oviduct. However, the yolk spends only four hours travelling through it. During this time, the addition of egg white protein occurs. There are many different proteins that make up the egg white. The layers of proteins provide mechanical and bacterial protection for the yolk as well as creating a template for the formation of the shell membranes and shell.
This template is created from a layer of sulphated mucus applied in the final portion of the magnum. The yolk with its layers of egg white now passes into the isthmus where it remains for one hour. The isthmus contains many secretory cells that produce the fibres that make up the inner and outer shell membranes. This is a rapid procedure and the egg moves quickly into the next section of the oviduct.
The egg then enters the shell gland, which is responsible for producing the final volume of egg white (albumen) and completing the eggshell. The shell gland is divided into two sections: the tubular shell gland and the shell gland pouch. The egg spends about 19 hours in the shell gland and the first five hours are spent in the tubular shell gland. The tubular shell gland has two responsibilities: (1) It causes an increase in the albumen volume by facilitating the movement of water (approx. 8 mL), rich in electrolytes, into the egg; and (2) It arranges the outermost fibres of the shell membrane into mammillary cores (which are chemically modified end portions of the shell membrane) through the transfer of calcium salts on to the shell membrane fibres. The mammillary cores are of importance because they act as points of contact for the crystals of calcium carbonate that form the majority of the eggshell.
Although the time spent in this section of the shell gland is a lot shorter than the time spent in the shell gland pouch, this organisation of the mammillary cores is a critical step in determining the eggshell quality of the completed egg. The egg then moves into the lower part of the shell gland, the shell gland pouch. Here the process termed calcification occurs, where calcium carbonate layers are added to form the eggshell. The process of calcification can be divided into two phases. Phase one occurs over the first four hours and involves a relatively slow rate of calcification. The calcium is transferred from the blood through the shell gland into the shell gland fluid. Once the calcium is in the shell gland fluid it supports the precipitation of calcium carbonate crystals (in the form of calcite) into the shell structure. ‘Plumping’ (hydration), a process that involves the uptake of salts, glucose and water into the albumen, occurs during phase one of the calcification. This movement of fluid causes the albumen volume to increase and it is thought that this swelling exposes the mammillary cores on the shell membrane. The now exposed mammillary cores initiate the next phase of calcification.
Phase two involves the bulk of eggshell formation as the layers of calcium carbonate crystals are laid down rapidly. During the last two hours of shell formation the bulk of the pigment is produced and deposited into the outer layers of the shell, including the cuticle, which is laid down to provide protection against penetration of the shell by microbes and the loss of water from the egg’s contents. The result of the calcification process is an eggshell that consists of approximately 95% calcium carbonate and 5% organic material. The completed egg is pushed into the external environment through the vagina and cloaca
Here is a list of egg shell problems and their causes:
The Problem : Stress can occur at any time or place where a stimulus causes mental, emotional or physical strain to the animal. General stress can influence laying hens, like most other animals. Stress can be caused by a number of factors that often owners overlook. It can come in the form of some thing as simple as a bird experiencing something new, to visitors such as possums and rodents in the night. In other words, unfamiliar objects or actions will cause the bird to experience stress as a fright response. The Cause The cause of stress can be anything from a change in colour of the handler’s clothing from day to day, a snake in the immediate area, or a loud thunder storm outside, possums or rodents in or around the coop. Stress can be can also be caused by poor management including food deprivation, extreme temperatures in the shed and overcrowding. Stress is a natural state that will be experienced in cage, barn and free range systems, and it becomes a problem when it occurs too frequently or for prolonged periods as this affects the productive performance of the hen. Stress can have lasting affects on birds oviducts.
Depending on the nature and severity of the stimulus that produces the stress, the hens may exhibit stress as only a loud chatter for a few minutes or a long-term reduction in the overall eggshell quality. A short period of stress causes defects such as bodychecked and misshapen shells as a result of the egg being squeezed in the shell gland by contraction of the uterine muscles during the initial fright. Calcium coated eggs are also common if the bird is stressed. The Solution Usually stress can be addressed directly by changes in the management strategy. After consultation with a poultry veterinarian or other qualified person, a management strategy to minimise the cause of the stress and therefore the amount of stress the bird experiences can be implemented.
Additional Information: Research has shown a direct link between the type of shell defect and the time during egg formation at which the bird is stressed, as well as the amount of time the egg is retained. It has been shown that, if a bird is stressed during the early stages of egg formation, a misshapen egg will result. However, if the bird is stressed late in egg formation, a calcium-coated egg will appear. Evidence of this type suggests that handling or stressing the birds during early calcification is likely to cause eggs that are body-checked and misshapen. Note that good stockmanship is very important in reducing the amount of stress experienced by the hen. Incorrect treatment may cause a stress response in the hen, which will be reflected in her eggshell quality
HEAT STRESS: Laying hens can also experience stress through temperature fluctuations. High temperatures cause hens to consume less food and produce fewer eggs, and result in an overall reduction in eggshell quality that is displayed in a variety of eggshell defects. Typically, eggs are smaller and lighter in colour.
The age at which birds are changed from a prelayer to a layer diet is of great importance. The balance of calcium to phosphorus is different between these two diets, and changing too early may result in the development of kidney damage. The age at which diets should be changed is dependent on strain and also the age at which the birds are brought into lay.
Experimental work has shown that the ability of the bird to absorb the required amount of calcium is not a limiting factor in the production of eggs with good shell quality. Calcium limitations in terms of eggshell quality result from not enough calcium in the diet, other compounds such as phosphorus restricting the amount of calcium available for absorption, or a metabolic limitation that occurs in the hen’s body after the calcium has been absorbed.
The Problem: Calcium is required for many metabolic functions in the hen, with a major demand being the production of eggshells. Inadequate levels of calcium lead to problems with eggshell quality.
The Cause: The amount of calcium present is influenced by environmental conditions, the physiological state of the bird, age and the diet. The cause of inadequate calcium is usually low levels in the feed. In general, as a short-term solution to a low level of calcium the bird will maintain its positive calcium balance by regulating the amount of calcium it uses to produce each eggshell (by both the number of eggs produced and how much shell is on each egg). Generally, a reduction in calcium causes a decrease in the amount of calcium deposited on the eggshell, and therefore a reduction in eggshell quality results. Inadequate calcium in the diet may also have long-term negative effects on bone strength. The particle size of the calcium provided in the diet is also important.
Eggshell Quality Problems Any significant decrease in the availability of calcium will have an adverse effect on the eggshell. This will be evident in the increase in shell defects as a whole – in particular, the size of the egg, shell thickness, the number of thin/shell-less eggs, and the number of cracked shells. The Solution Ensure that the diet is adequate in calcium, and that the calcium is provided at the appropriate particle size. It is recommended that particle size should exceed 2 mm (2-5 mm) for 50% to 70% of the calcium supplied and that this is particularly important towards the end of lay and in hot climates. Additional Information: During the intense calcification process when most of the eggshell is formed, up to two grams of calcium can be deposited, which is a significant proportion of the calcium present in the hen’s body. Therefore, it is quite obvious why, when the supply of calcium is limited, the eggshell suffers the first and greatest loss of the calcium. Vitamin D has a very important role in the bird’s ability to control calcium movement. Vitamin D stimulates calcium reabsorption from the bones and increases the amount of calcium absorbed from the gut. It also helps control the amount of calcium lost in the urine and increases the production of calcium binding proteins, which are important in moving the calcium to where it is required.
Drinking water quality Drinking water is an essential requirement. Compounds that are found in water are generally rapidly absorbed into the animal’s body. For this reason, many of the supplements that are given to animals, and particularly in disease control, are administered through the drinking water. However, the opposite is also true, in that undesirable compounds that cause harm to the bird easily enter the bird’s body through this route.