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:
Stress
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.
Additional Information:
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.
Inadequate calcium
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.