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DL
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Reproduction in the Hen
« Thread started on: Sep 17th, 2006, 2:27pm » |
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http://ag.ansc.purdue.edu/nielsen/www245/lecnotes/avianrepro.html
POULTRY REPRODUCTION
Puberty and Breeding in the Avian Species
The hen reaches puberty and starts to produce eggs at 4-5 months of age. As with mammals, the reproductive system is not functioning completely normally at the onset and hens at puberty produce small egg sizes and high percentages of eggs with twin yolks. Since these eggs do not produce viable offspring, hens are not bred to produce young until they reach 5-6 months of age.
The cockerel is capable of insemination at 4-5 months of age, but, like the hen, is not used for breeding until 6 months of age to insure viable sperm.
The natural instinct is to lay a "clutch" of eggs, become "broody", stop laying eggs (ovulating), and set on eggs to hatch. This broodiness has, for the most part, been bred out of our commercial breeds of poultry, and hens will produce eggs continuously. Hens do not have an estrus cycle, and will lay an egg nearly every day.
In contrast with mammalian species in which both ovaries are functional and either ovulate simultaneously, or alternately, 99% of hens have only one functional ovary.
Reproductive Physiology of the Hen
The reproductive tract of the hen is also different from mammals, and different functions are performed in different segments of the tract. The major structures are as follows (see illustration below):
OVARY - containing immature and mature follicles. The mature follicles consist of the egg "yolk" and the unfertilized ovum.
INFUNDIBULUM - yolk with attached ovum is snatched up by the infundibulum. It is at this point in the reproductive tract that the ovum is fertilized if the hen has been mated with a cockerel. Spermatozoa from the cockerel are stored in "sperm nests" located within the infundibulum and are capable of fertilizing ova for up to 30 days after mating.
MAGNUM - while traveling through this part of the oviduct, the albumin or egg white is formed.
ISTHMUS - the tough outer membrane located just beneath the egg shell is formed in this part of the oviduct.
UTERUS - also referred to as the "shell gland", this is where the egg shell is formed. Most of the transit time from ovulation until the egg is laid is spent in the uterus.
VAGINA - the egg travels through the vagina into the cloaca, from which it is "laid."
CLOACA - this is the common external opening from which the contents of the urinary tract (urates), the intestinal tract (feces) and the reproductive tract (eggs) exit the hen.
This image was taken from the Purdue Avian Sciences web page. For additional pictures of the reproductive tract of the hen, check the University of Georgia, Department of Poultry Sciences, PS 202 lab review materials. Slides 68, 70, 71, 73, 77 and 84 are particularly good examples.
General Information:
-A hen is capable of producing an egg every 25 hours.
-Eggs are produced and laid regardless of whether the hen has been mated and the eggs are fertile or not.
-A hen is capable of laying approximately 270 eggs per year.
-The embryo in a cracked fertile egg will not develop.
Incubation and hatching of fertile egg:
humidity & temperature control are important factors in the hatchability of fertilized eggs.
Chicken -- eggs incubate 21 days; spend 1 day in hen; 22 days from fertilization to birth of chick
Turkey, duck -- eggs incubate 28 days for a total of 29 days from fertilization to birth.
In addition to differences in the reproductive physiology, the development of avian offspring differs significantly from mammals in a number of other ways:
The shell takes the place of the uterus or womb in providing a dark, warm, moist sterile environment in which the offspring develop.
The albumin or egg white serves as a shock absorber for the developing embryo, just as the amniotic fluid does within the mammalian amniotic membranes (birth sac).
The yolk provides nourishment to the developing chick just as the umbilical cord provides nourishment to the developing mammal. The yolk also provides maternal antibodies to the chick to protect against infectious agents in the environment to which the hen has developed an immunity (this is similar to colostrum in mammals).
The chick develops outside and independent of the hen, and does not need the hen for survival provided that the proper environment is provided by man for incubation of the egg. Mammals develop inside the uterus of the dam and are dependent upon the health and well-being of the dam throughout the entire gestation for their health and well-being.
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DL
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Re: Reproduction in the Hen
« Reply #1 on: May 12th, 2007, 05:19am » |
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http://edis.ifas.ufl.edu/PS020
EGG QUALITY
http://www.ams.usda.gov/poultry/pdfs/EggGrading%20manual.pdf
(scroll down to the section >STRUCTOURE AND COMPOSITION OF THE EGG and to the link above for some excellent illustrations)
(excerpt)
"The parts of an egg are the yolk, the white, the shell
membranes, and the shell (figs. 26 and 27).Yolk (Ovum). The yolk consists of the latebra, germinal
disc, concentric rings of yolk material, and vitelline mem-
brane (a colorless membrane that surrounds and contains the yolk). The yolk constitutes approximately 31 percent of the total weight of the egg.
White (Albumen). The white consists of four distinct
layers which together constitute about 58 percent of the
weight of the egg.The chalaziferous layer immediately surrounds the yolk and is continuous with the chalazae (pronounced kah-lay-za). This is a very thin, but firm, layer of white and makes up 3 percent of the total white.The inner thin layer surrounds the chalaziferous layer and comprises about 17 percent of the white.The firm or thick layer of white provides an envelope, or
jacket, that holds the inner thin white and the yolk. It
adheres to the shell membrane at each end of the egg.
Approximately 57 percent of the white is firm white.The outer thin layer lies just inside the shell membranes,
except where the thick white is attached to the shell, and accounts for about 23 percent of the total white.
Shell Membranes: The shell membranes are tough and
fibrous and are composed chiefly of a protein similar to
that in hair and feathers. The inner membrane is
thinner than the outer, and together they are only about 24 ten-thousandths of an inch (0.00609 mm) thick.
Shell: The shell is composed of three layers and
constitutes approximately 11 percent of the egg.The mammillary or inner layer covers the outer shell
membrane. Next is the spongy layer, then the cuticle.
Pores connect the surface and the mammilla.The egg, as laid, normally has no air cell. The air cell forms as the egg cools, usually in the large end of the egg, and develops between the shell membranes. The air cell
is formed as a result of the different rates of contraction
between the shell and its contents............
FormationThe reproductive system of the hen is divided into two main parts: Ovary and oviduct (fig. 28). Most female animals possess both a right and a left functioning ovary but in the hen, the right ovary and oviduct normally remain dormant and the left ovary and oviduct develop the egg.
Ovary: This is a cluster of developing yolks, each separate from the others, attached to the middle part of the back about midway between the neck and the tail. This organ is fully formed, although very small, when the chick is hatched. It contains approximately 3,600 to 4,000 minute ova (future yolks) each within its own sac or follicle.
Oviduct: This is a long, tubelike organ lying along the
backbone and attached to it loosely between the ovary
and the tail. It is approximately 25 to 27 inches (63.5 to
68.6 cm) long, and may be divided into five areas which
perform certain functions in completing the formation of
the egg. It is here that the white, shell membranes, and shell are secreted.
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DL
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Re: Reproduction in the Hen
« Reply #2 on: May 12th, 2007, 05:42am » |
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(contd.)
FORMATION OF THE YOLK
...........Ovulation is the release of a mature yolk (ovum) from the ovary. Each developing yolk is enclosed in a yolk sac in which many blood vessels are profusely distributed. These blood vessels carry yolk-building substances to the developing yolk. One particular area of the yolk sac is free of blood vessels (stigma or suture line) and it is normally here that ovulation or release of the mature yolk takes place. However, the rupture of the yolk sac and release of the yolk sometimes occur at sites other than the stigma, with the result that one or more blood vessels are ruptured and blood spots occur on the yolk, or the white becomes bloody.Occasionally, reddish brown, brown, tan, or white spots,
commonly known as “meat spots,” may be found in the
egg. It has been demonstrated that meat spots may be
either blood spots which have changed in color due to
chemical action, or tissue sloughed off from the reproductive organs of the hen. Ovulation occurs again about 30 minutes after an egg is laid, during the laying clutch of a good producing hen.
FORMATION OF THE WHITE
.............The white contains ovomucin, secreted by the magnum as fibers or strands, which makes the white thick. These strands draw together as the developing egg continues its spiral movement through the oviduct. From these strands the chalaziferous layer and the chalazae are formed (1st layer). The continued twisting and drawing together of these strands tend to squeeze out thin white to form the inner thin white (2nd layer). The remaining thick white is a homogeneous gel (3rd layer).The quality of the white is largely dependent on the amount of ovomucin secreted by this part of the
oviduct. (note>see the TROUBLESHOOTING section for symptoms of disease through internal abnormalities in the egg) The portion of the gel immediately adjacent to the yolk undergoes partial liquefaction before the egg leaves the magnum.
The isthmus is a constricted area of the oviduct about 4 inches (10.16 cm) long, through which the developing egg passes in about 1-1/4 hours. Here some water and mineral salts are added and the two shell membranes are deposited.
The uterus: a heavy-walled part of the oviduct, approxi-
mately 4 inches (10.16 cm) in length, is where the devel-
oping egg spends about 21 hours. Here, the outer thin
white (4th layer) and minerals pass through the shell
membranes by osmotic pressure, and the shell and shell
pigment are added. The water and solids of the white
ultimately become equally distributed in the various lay-
ers of the white.
Formation of the Shell Membranes
The shell membranes are added as the partly formed egg enters the isthmus. The membranes are a closely knit lace-like nitrogenous compound similar to that present in the chicken’s toe nails.
Formation of the Shell
Calcium carbonate comprises about 94 percent of the dry shell. A hen may use as much as 47 percent of her skeletal calcium for egg shell formation. Two layers of the shell are formed in the uterus.The spongy layer consists of small calcite crystals that are not arranged in any order, except in the outer portion of the layer where crystals are set at right angles to the shell surface. Pigment, if any, is laid down in the spongy layer of the shell and is derived from the blood.
Pores are formed through the spongy layer connecting
some of the space between the knoblike mammilla with
the surface. Moving finally into the vagina, a 2-inch (5.1 cm) area, the fully formed egg enters the cloaca and the vent, and is laid. When the egg is laid, the pores are filled by the matrix material and covered by the cuticle. Cuticle, which is sometimes erroneously referred to as “bloom,” is of a chemical composition similar to the shell membrane.The time from ovulation to laying is usually slightly more than 24 hours. About 1/2 hour after a hen has laid an egg, she releases another yolk (ovulation), and it will, likewise, travel the length of the oviduct.
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DL
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Re: Reproduction in the Hen
« Reply #3 on: May 12th, 2007, 05:53am » |
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(contd)
Abnormalities
Double-yolked eggs result when two yolks are released
about the same time, or when one yolk is lost into the
body cavity for a day and is picked up by the funnel
when the next day’s yolk is released.
Yolkless eggs are usually formed around a bit of tissue
that is sloughed off the ovary or oviduct. This tissue stimulates the secreting glands of the oviduct and a yolkless egg results.
An egg within an egg is due to the reversal of direction of the egg by the wall of the oviduct. One day’s egg is added to the next day’s egg and the shell is formed around both.
Blood spots are caused by a rupture of one or more
small blood vessels in the yolk follicle at the time of
ovulation. Since the albumen of freshly laid eggs is usu-
ally cloudy, the detection of small blood spots during
candling is difficult.
Meat spots have been demonstrated to be either blood
spots which have changed in color due to chemical
action, or tissue sloughed off from the reproductive
organs of the hen, although most meat spots are from
sources other than blood spots.
Soft-shelled eggs generally occur when an egg is prematurely laid, and insufficient time in the uterus prevents the deposit of the shell.
Thin-shelled eggs may be caused by dietary deficiencies,
heredity, or disease.
Glassy- and chalky-shelled eggs are caused by malfunc-
tions of the uterus of the laying bird. Glassy eggs are less porous and will not hatch, but may retain their quality.
Off-colored yolks are due to substances in feed that cause off-color.
Off-flavored eggs may be due to certain feed flavors or
improper storage practices.
Eggs that are unusual in shape may have ridges, rough
areas, or thin spots (figs. 34 and 35). Abnormal shells may result from improper nutrition disease, or the physical condition of the hen. Sometimes
a shell is cracked while the egg is still in the body of the
hen. These eggs, which are commonly referred to as
“body checks,” are repaired by an additional deposit of
shell over the cracked area, generally resulting in a
ridged area.Shells with thin areas and some other types of defects are usually weaker than normal shells, and the danger of breakage en route to the consumer lowers the utility value of the egg........."
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