Eggs and Fertilization
By Dr. Chris McDaniel, Associate Professor, Poultry Science Department, Mississippi State University -
The main goal of broiler breeder management is producing eggs. However, the only good broiler breeder egg is a fertilized egg. Fertility, the percentage of eggs laid that are fertilized, is very important in poultry production. If an egg is not fertilized, then, of course, it will not contain an embryo and will not hatch. Simply put, "Hatchability can never be better than fertility."
Hatchability is around eight percentage points lower than fertility because many chick embryos are usually lost during incubation. For example, even if 93 percent of the eggs laid are fertilized, then under normal incubation conditions only 85 percent of the eggs will hatch. This example illustrates how fertility must be very good to get above average hatchability and hatch bonus pay.
Breeders need to be kept under ideal conditions for maximum life of flock fertility. The chicken's reproductive system is very sensitive to the bird's environment, and under poor conditions the reproductive system will dwindle. For example, the environment can cause a rooster's testes to increase or decrease in size by several hundred fold. But, before we can understand which management factors influence fertility, we must first examine the fascinating process of fertilization in poultry.
Fertilization in any animal depends on production of eggs from the female and sperm from the male. A problem with either sperm or egg production can decrease fertility. The rooster's reproductive system is simple when compared to humans or other mammals. The rooster does not have a prostate gland or any of the accessory reproductive glands. Like all other animals, chicken sperm carry the genetic material from the rooster and are produced within the testes. The rooster has two very large testicles within the abdominal cavity on each side of the backbone. After sperm leave the testes, they enter the epididymis, where they gain the ability to swim. Next, the sperm enter the vas deferens, where they are stored until the rooster mates with a hen.
Sperm formation takes about 15 days. The rooster's semen contains around 5 billion sperm per cc, about 40 times as much as that of a human. Once a rooster is mature and if he is maintained properly, he will manufacture about 35,000 sperm every second of his life. However, just like the males of many animal species, the fertilizing potential of roosters varies, even within a flock. For example, some roosters are extremely fertile and create a maximum number of quality sperm; other roosters are subfertile and do not make enough good sperm. This variation in rooster quality is caused by management, environment, nutrition, and genetics.
The hen does not produce nearly as many eggs as the rooster produces sperm, but during her 40 weeks of production, the broiler breeder hen lays about 180 eggs. Egg formation requires about 25 hours. Since egg formation requires more than 24 hours, even the best hens cannot lay an egg every day in succession throughout their productive life. As is the case with roosters, some hens are more productive than others, and management has a major impact on variability among hens.
The hen's reproductive system can be divided into two major components: the ovary and the oviduct. The ovary produces the egg yolk. The oviduct adds the white, shell membranes, and shell to the yolk to complete egg formation.
The hen has only one ovary, which is on the left side of her abdomen. The ovary has several thousand ova (egg yolks) in different stages of development and looks like a bunch of grapes. Very immature yolks contain only genetic material from the hen, and as the yolks grow to around 1 mm in diameter, they become white. If the hen is managed properly, many of these developing egg yolks will mature in about 19 days into large, 35 mm, yellow yolks. As the egg yolk develops it will get water, sugars, fats, proteins, vitamins, and minerals from the hen's blood. These are all necessary for the embryo to develop. The egg yolk is surrounded by the perivitelline membrane. This keeps all of these nutrients in a ball-shaped package. One particularly visible region of the perivitelline membrane is the germinal disc. The germinal disc is a small white dot about half the size of a pencil eraser on the surface of the yellow egg yolk. Fertilization takes place here, and embryonic development begins.
When the egg yolk is mature, it leaves the ovary, and within 20 minutes it is captured by the infundibulum, the first part of the oviduct. Here fertilization takes place. Following mating, sperm enter the hen's oviduct and are stored within sperm storage glands. Only sperm that can swim will enter these sperm storage sites. These glands can store more than half a million sperm. Sperm can remain alive in these glands and fertilize eggs for up to 3 weeks.
A hen will have maximum fertility for only about 3 to 4 days after one mating. For this reason, the male-to-female ratio in a flock must be enough to ensure mating of every hen every 3 days or so. Sperm do not break through the eggshell. Instead they travel up the oviduct to the infundibulum to join with the egg yolk.
The sperm bind to the perivitelline membrane and make a hole as they enter the egg. Hundreds of sperm may enter the yolk. As a matter of fact, the more sperm that enter the yolk, the more likely the egg will be fertilized. Around 30 sperm must enter the egg near the germinal disc to insure a 95 percent chance of fertilization. While it is true that only one sperm is necessary to fertilize an egg, the probability of an egg's being fertilized by only one sperm's reaching and penetrating it is very low.
After about 15 minutes, the yolk leaves the infundibulum (fertilized or not) and receives the egg white, shell membranes, and shell over the next several hours from the magnum, isthmus, and uterus sections of the oviduct. When the hen lays a fertilized egg, the chick embryo has already developed for about 25 hours into approximately 20,000 embryonic cells and is a live, breathing organism. If this fertilized egg is handled properly before and during incubation, a healthy baby chick is the result.
This publication is a joint effort of the Mississippi State University Extension Service and the Mississippi Agricultural and Forestry Experiment Station.
Source: Mississippi State University Extension Service - August 2004
Chickens’ complicated sex life
A recent research on poultry has revealed the truth which farmers have long suspected. Roosters can control the amount of sperm that is inseminated during inseminations. This behavior is believed to be the result of roosters’ ambition to mate with as many hens as possible and thereby spread their genes. If you look at the mating ritual from the roosters’ point of view it is actually an economical problem. A rooster who doesn’t have any competition from other cocks in the poultry farm can control their sperm allocation and thereby inseminate every hen in the farm. If there are other cocks in the flock the roosters must allocate their sperm supplies in an economical way in order to attain maximum result: inseminate as many hens as possible.
PhD student Hanne Løvlie grew up in a big city and just as regular urban citizen never came in contact with hens or other farm animals. At the age of 23 with six months left of her university studies in Zoology, she decided to undertake a degree project in ethology. By chance it turned out to be a study about hens and roosters mating behavior. Today, about five years later, she works as a doctoral student at Stockholm University and has analyzed the mating behavior of hens over a number of years. The study is a cooperative project between Stockholm University, The University of Sheffield, The University of Leeds and The Swedish University of Agricultural Sciences. In order to achieve the best results, scientists from the different universities gather at one location and perform the studies together.
A doctorate takes about 4 to 5 years to complete, which can seem a long time, but when it comes to science 5 years is almost nothing.
Hanne Løvlie has limited her study to tame hens and jungle hens, the wild ancestor to today’s domestic hens.
A question of sperm resources
At Tovetorps Zoological Research Station, south of Stockholm, hens were exposed for several experiments to simulate the actual mating behavior of chickens. There were two kinds of mating. One in which the rooster was allowed to inseminate sperm and one in which a “rump protection” was placed over the buttocks of the hen to prevent insemination. This was to imitate the so called fake mating, which roosters are known to perform. Afterwards, the subjected hens were studied in order to determine what effect sperm insemination had on the promiscuity of hens. The study showed that irrespective of whether the insemination was carried out or not, the hens reacted in the same way. Either way, fowls were not interested in mating with new roosters. This is contrary to fowls that were not exposed to mating at all. Decreased promiscuity has been seen in other species but the effect was in those cases caused by large amounts of sperm inseminated or sophisticated products in the semen fluid. The study is a breakthrough because it shows that roosters can easily decrease the promiscuity of hens through exploiting hens’ incapability of deciding irrespective whether they received sperm or not during mating.
Roosters can thereby ensure guaranteed control over several hens in the flock and in this way spread their genes more effectively. As a result of this, cocks are more interested in mating with hens which they have not mated before.
May the best comb win!
But there is more to it than just finding new hens to mate with. Since roosters, like every other animal, are interested in having the best and strongest offspring they want to invest their sperm in hens which are more likely to lay larger eggs in greater numbers. But how do roosters know which hens to choose? Hens that are sick, old or not even fertile are of course not of interest for the successful rooster. A hen which has a big, red comb is much more likely to produce many large eggs which will turn into healthy chickens. You can say that the comb of the hen represents its status and ability to lay good eggs.
Hens play their part
However, the sex life of hens is just as sophisticated as the roosters’. Since the roosters are bigger and stronger, the hens are not able to reject a mating demand. This does however not mean that hens do not have their favorite mating partners. Dominant roosters are those which have the largest combs and have the highest status in the chicken flock, which makes them also more attractive to the hens. Dominant roosters have often better genes than subdominant, something that hens are aware of and wish therefore to lay the dominant roosters eggs. But when the dominant roosters are absent, the subdominant roosters take the opportunity to mate with the unwilling hens. To prevent the hen from laying the unwanted eggs, they spurt out the inseminated sperm and therefore make the mating useless. This behavior is not only used towards subdominant roosters but also against related cocks. If a hen is inseminated both by a brother and a rooster which is not related to her, she uses less sperm from the related cock’s sperm in order to avoid inbreeding. The research can however not tell how the hen is able to recognize which roosters are related to her. Hanne Løvlie is now carrying out a research to find out whether this behavior is a physiological mechanism occurring inside the female, or if the hen actually can recognize and in turn choose to eject the ejaculate from brothers.
Roosters are able to control the amount of sperm ejaculated during mating. This means that they are able to decrease hens’ promiscuity without having to waste valuable semen. Cocks can decide which hen to inseminate through studying the comb of the hen. A large and red comb is an indication that the hen is in good physical shape and will produce strong and healthy offspring. But hens are also able to tell if the interested rooster is of good breed, if this is not the case the hen will squirt out the rooster’s sperm. This is done in order to increase the chances of laying the eggs of a more desired rooster. As opposed to human beings and other primates, hens do not get orgasms and therefore they’re not able to enjoy the sexual act. This means that the behavior of fake mating which roosters perform is only in order to get granted access to more hens and not for personal sexual pleasure.
The studies which Hanne Løvlie and her crew have performed have led to increased knowledge in the sexual life of poultry and other animals with similar social conditions. The studies at Tovetorps Zoological Research Station are still in progress and will most certainly proceed for some more years.