Poultry production: meat chickens and turkeys

Meat chickens

A layer and a broiler hen, both at 6 weeks of age
(c) Coop cam

Meat chickens (broiler chickens) are brought to European countries from Scotland (Ross-breed). The grandparent birds are brought to the country as eggs, and hatched there. The grandparents will produce the parent generation, which are raised until 18 weeks of age in another poultry house. The 18 week old broiler chickens are then transferred to a breeding facility, where the live until 60 weeks of age producing the progeny. A fourth poultry farmer incubates and hatches the production generation birds. Finally a fifth farmer buys the day old hatchlings, which arrive in crates and are simply poured on the floor. The birds are reared for 32-39 days, at which time they are slaughtered. Before another flock is brought in, the hall is disinfected and left to dry for 2-3 weeks. Flock sizes in Norther Europe are usually 20 000 - 80 000 birds.

Broiler chicken females used for breeding grow very fast. But since fast growth would cause severe injuries and deformations, the females receive only 1/3 of the feed they need. The nearly starving birds are constantly hungry and frustrated. Only because malnutrition their growth slows down, and the birds are able to mate with the roosters and lay fertile eggs.

Broiler chickens meant for meat production (instead of breeding) live only 5-6 weeks before slaughter.  During this time the currently used chicken hybrids grow from 40 g to 2,1 - 2,5 kg. In comparison, a layer-breed chicken of 6 weeks of age would weigh less than 800 grams. The temperature of the poultry hall is decreased from 34 C to 20 C as the birds mature. Injured and dead birds are removed from the flock and destroyed. During the rearing time mortality of 3-4 % is expected and accepted. In a flock of 54 000, ~1890 birds will die of various reasons before slaughtering. In Europe, broiler chickens are not medicated in any way. If the hall is disinfected properly between flocks and the rearing conditions are good, the birds stay free from any diseases or parasites, eliminating any need for treatments or preventive medication.

The rearing is mostly automatized. Ventilation, temperature and humidity control, feeding, watering and light programming are all automated. While it saves work, even relatively short power outages can kill the entire flock to suffocation due to rapidly raising levels of ammonia, H₂S and CO₂ in the air. Because the light/dark program is carefully designed and obeyed, broiler halls have no windows or any inlet for daylight (organic farms are an exception). Meat chickens are still manually inspected twice a day. The farmer ensures all the automated systems work, and that the litter is dry and clean.

Meat chickens are fed crumbled feed from the floor during the first two days. The chicks would survive from nutrients in their yolk sacs, but solid feed helps the development of  the digestive tract and production of gastric juices. Later the chicks eat granulated feed from feeding cups. Feed and water are freely available, and the height of water nips and feed cups are altered as the birds grow. Feeding is usually phased with four different feed mixes suitable for birds of different ages. Each successive phase has less amino acids, calcium and phosphorus than the previous phase, but the energy content stays at 12,4 MJ/kg ME. This is because the bones of the birds develop fast and need Ca, P and proteins to grow strong. The last phase has no coccidiostates to ensure drug-free meat at slaughter.

Australian broiler shed. (c) Animals Australia

The EU directive for meat chicken welfare sets strict requirements for animal density. Densities are measured as kg / square meter at the end of the rearing period. Density of < 33 kg / m² has no additional requirements. If each meat chicken weighs 2,5 kg, a density of 33 would mean 13,2 birds / m². Animal density of 33-39 kg/m2 requires the farmer to have a writter description of the rearing facilities and equipment available at all times. A secondary power source is required, and the indoor air must fulfill cleanness, humidity and temperature requirements.  The highest allowed animal density is 39-42 kg/m2, roughly 17 birds / m2. In addition to the previous requirements, a farmed using the highest density must have impeccable results from animal welfare auditions and daily bookkeeping on bird deaths and removals with reasons attached. The mortality per flock is limited, and exceeding the limit forces the farmer to drop the animal density to 39 until the conditions improve.

Turkey production

Wild Turkeys (c) Turkey Management

The turkey is the largest bird used in animal production. It's meat has more protein and less fat than that of chickens, geese, sheep or cows. The current production turkeys are white hybrids, descendants of the still-living wild turkey (Meleagris gallopavo).  Turkeys are classified by color (bronze, white or black) and by size (heavy, medium heavy and light). Heavy males weigh 15-16 kg, medium males 8-10 kg. Hens weigh only half of the males' size.

Turkeys are raised in flocks like meat chickens. During the long rearing time of 3-4 months (compared to the 5-6 weeks of broiler chickens) the turkeys grow to weigh 6-12 kg. Hens and roosters are reared separately because of the size difference. Otherwise they are reared much like meat chickens. Turkeys are, however, much more demanding. They must be given time to develop strong bones at first, so their feed cannot be too strong. Young turkeys must have 8-10 cm of warm, dry and clean litter.  The first few days they are kept under a heat lamp in 38 C degrees and in bright lightning. The behaviour of turkey chicks is closely monitored. If they are huddled together, the temperature is too low, or they are stressed. If they are scattered away from the lamps, the temperature is too high.

After the first few days all birds know where to find water and food, so the light intensity can be lowered. Light program is changed gradually to allow for a dark time of 8-10 hours. Humidity is kept at 60-75 %, and by the end of the rearing period the temperature has been decreased to 14-17 C . 

Turkeys are fed mostly with complete feeds consisting of wheat, soybeans and peeled oats. Feeding is phased. For example, the recommended phasing for Nicholas-breed turkeys is:

• prestarter (0-2 weeks)
• starter (2-6)
• Grower 1 (6.9)
• Grower 2 (9-12)
• Finisher 1 (12-16)
• Finisher 2 (week 16 onwards)

Commercial breed turkeys. (c) Zimbio

Prestarted feed has the most proteins, espcially lysine and methionine. Energy content grows slightly towards the finisher feeds, but otherwise the contents stay rather similar. The phasing is designed by weeks of age, but weight of the birds is a more accurate measurement since each flock is different. However, it is important to stay with the prestarter and starter feeds long enough for the bird to develop strong bones before the period of fast growth. 4-phased feeding can also be used with the same principle of gradually lowering the amino acid content.

Since turkeys are the largest of domesticated birds (not including ostriches or emu), their rearing densities are much lower than that of broilers. At the rearing phase, 3-4 hens or 1 rooster / m2 is used. In farms producing turkey eggs the density is 2 hens or 1 rooster / m2.

Healthcare of poultry

Vaccinating hens against H5N1. (c) Reuters

To understand healthcare of poultry, one must first understand the conditions in a poultry house. Apart from organic production, there are no limits to how many animals can be kept in one hall. Meat chickens and layers are reared in flocks of  10 000 - 40 000 or higher. Turkey and goose flocks are usually  smaller. It is easy to comprehend that medication and health checks are flock-based: it is impossible to monitor and care for each individual animal, even though their pains and problems are unique. Therefore a disease outbreak is often noticed as the production or growth of the entire flock are decreased.

Viruses and parasites spread nearly immediately to every bird in the flock. If there are several poultry farms in a small area, the disease can spread from one farm to another in car tires, boots, dirty hands, clothing, by wind etc. "Backyard hens" and wildlife are also a risk to commercial poultry producers. Disease control, preventive medication and quarantines are vital in animal import/export. As the bird influenza -epidemics have shown, international and even global disease outbreaks are possible, and carry heavy consequences to poultry and, in extreme causes, to humans as well.

Each poultry keeper should take and send samples to an official laboratory for disease screening. Easy and quick methods of taking samples are

• Shoe cover sample: Put on dispensable shoe covers, and walk amongst the floor-reared birds. Swap the covers between different sections of the hall. Send the individually-packed dirty covers for screening.
• Feces sample: Mix an adequate amount (100-200g) of feces from all feces collector mats or different parts of the poultry house.
• Incubation sample: Mix 100-200g of broken egg-shells and floor litter from the incubator
• Dust and swab samples: Take and store individual samples from all around the poultry house: floors, corners, feed trays etc.

(c) The Poultry Guide

All diseases are cheaper to prevent than to cure. Most diseases lower production, and may lead to eggs or carcasses being rejected entirely, which decreases profits and increases costs as decreased feed utilization rate and medical costs.  The steps for disease prevention are simple:

• Allow only necessary guests to the poultry house. 
• Ensure all visitors wear disposable shoe covers and other protective clothing provided by the farm.
• Ensure no one enters the poultry house if they've been abroad during the last 48 hours.
• Follow the official and recommended vaccination and medication programmes.
• Monitor the animals and production levels closely.
• Repel indects, flies, rats and other vermin. Ensure wild birds cannot enter the poultry house or feed storage through air ducts or ventilation shafts.
• Use barriers. Make sure all clothing and equipment used in the poultry house is kept there, and all "dirty" clothes and tools are left in the barrier area. 
• Follow the all-in, all-out method with thorough cleaning and disinfection between flocks.

Common diseases

Salmonellosis
Several bacterial strains can cause salmonellosis. Salmonella Enteritidis and S. Typhimurium can infect poultry and humans alike. Both bacteria are infectious if swallowed, and are common in eggshells and feces. Infection through infected feed is rare but possible. S. galliarum infects poultry only, and may cause 100% morbidity in birds of any age. It survives months in a normal climate, but is susceptible to disinfectants. S. pullorum infects only poultry aged three weeks or older. 

Symptoms of all types of salmonellosis on poultry are ruffled feathers, closed eyes, diarrhea, loss of appetite and thirst and stunted growth. Post-mortem lesions can occur. Morbidity is low to medium. Salmonella can be cured with antibiotics, which are added to the feed or drinking water. The meat of infected birds may be accepted for normal food processing after heat processing.

Infectious bronchitis, IB
Poultry can and should be vaccinated against IB. IB is caused by a Coronavirus, which evolves rapidly but infects only chickens. The symptoms depend on the age of the chicken. Chicks get flu-like symptoms. Their oviducts are damaged, preventing them from laying eggs later on. Young chickens show only mild symptoms. Adults have respiratory problems, their productivity decreases and their eggs have faulty shells. Some Corona-strains cause damage to kidneys. IB infection lasts 2-8 weeks. Post-mortem lesions can occur. Morbidity is 0-25 %.

IB can be treated with sodium salicylate. Antibiotics are often needed to treat secondary infections by E. coli.

Skin hemorrhage (c) CFSPH

Newcastle disease, ND
ND is caused by some strains of paramyxovirus. In the EU, ND-infections must be confirmed in a reference laboratory in England. The viruses are extremely resistant: it can live months in feces, weeks in carcasses and years in a freezer.  ND infects all birds. If an ND-infection is confirmed, a stamp-out may be ordered and all poultry within 3 kilometers must immediately be destroyed.

The symptoms of ND vary, but common ones are decrease of production, moulting, hemorrhage of the comb, skin and eyelids, respiratory problems, paralysis and sudden deaths. There is no cure for Newcastle Disease.

Typical posture for a hen with MD.
(c) Poultry Club SA

Marek disease (MD)
Marek is caused by a herpesvirus. It is transmitted as aerosols via respiration, and may infect all animals in a flock in a short time. The virus proliferates in the roots of the feathers, and is extremely resistant. Many healthy birds carry and spread the virus. Marek infects mostly hens, but the symptoms and susceptibility to infection depend on many factors. Most infected are aged 4 weeks or older, most commonly 9-24 weeks of age.

Symptoms include staggering walk, paralysis, dangling wings, enlarged crop, bent neck, respiratory problems, eye deformations and weight loss. Sick birds have normal appetite. There is no cure for MD, and even cured birds will get infected again later.

(c) The Poultry Site

Coccidiosis
Coccidiosis is caused by one-celled parasites of the Eimeria-family. There are nine known contagious Eimeria-species. The infection happens when chickens eat the eggs of the parasite, often brough to the poultry house in contaminated equipment or cargo boxes. The eggs proliferate in the chicken gut, and 4-7 days after the infection there are Eimeria-eggs in the chicken's droppings. The eggs need 1-3 days outside the chicken's body to become contagious. Eimeria infects mostly chickens of 3-6 weeks.

Symptoms are increased mortality, stunted growth, decreased immunity, diarrhea, bloody feces, dehydration and decrease in egg-laying. Coccidiosis can be prevented by adding coccidiostates to the feed, and by keeping the litter clean and dry. The only treatment is keeping the litter clean and treating the secondary infections with antibiotics.

(c) Cornell University

Avian Encephalomyelitis (AE)
AE is a viral disease of the central nervous system, affecting chickens, pheasants, turkeys, and quail. Grandparent, parent and production line birds can be vaccinated against AE. Developing fetuses can get infected by the parent, but oral infections may also occur. Mortality is high.

Symptoms of AE include sitting, paralysis, tremors, imbalance, muscle weakness and dull apprearance. There are no visible lesions in live animals. There is no treatment for AE. 

(c) Science Alert

Gumboro or Infectious bursal disease virus (IBDV)
IBDV attacks the immune system of chickens 14-28 days old, causing severe effects in young birds. It is caused by Birnavirus, and infects chickens, turkeys and ducks. White leghorns are more susceptible to Gumboro than brown breeds. The disease is highly contagious, with mortality of 0-20 %. Gumboro increases susceptibility to all other viral and bacterial diseases, including Newcastle Disease.

Symptoms of IBDV are depression, lack of appetite, diarrhea, hiding and unsteady gate. There is no treatment, but vitamins and water may help. Antibiotic medication may be indicated if secondary bacterial infection occurs. All laying chickens and parents to meat chickens must be vaccinated against Gumboro.

CAV virus (c) ICTV

Blue Wing Disease (BWD) / Chicken Anemia Virus (CAV)
CAV and BWD are different names for the same viral disease caused by Gyrovirus. Mortality is 5-10 %. The virus is very resistant to disinfectants, but is destroyed in 5 minutes in 80 C.

Symptoms of BWD/CAV include poor growth, paleness and a sudden rise in mortality. There is no cure for the disease. Good hygiene and management, and control of other diseases as appropriate, may be beneficial. All parent chickens should be vaccinated 6 weeks before their eggs are collected for incubation.

Evaluation of foot health

Leg deformations, broken bones and infections in the soles of the feet are common welfare problems for fast growing birds all over the world. The birds are bred to grow faster than their bones develop, and to a larger size than their feet can carry. Feet health can be improved by

• Enough dry and clean litter
• Allowing roosting
• Optimizing the ratio of calcium and phosphorus in the feed
• Optimizing the amount of minerals in the feed
• Keeping low animal density
• Allowing locomotion
• Using slow growing breeds  

Foot and leg health can be measured by gait scoring, latency to lie -test or post-mortem from carcasses. In gait scoring, a sample size of 100 birds is selected. Each bird is put in a small enclosure, when the animal is encouraged to walk. The gait is scored from 0-5, and the average of all results is calculated. A score of 0 means a normal gait, 3 is a gait problem affecting the animal's locomotion, and 5 means the animal is not able to walk. Gait scoring is a part of Welfare Quality assessment. In a  Latency to lie (LTL) -test, 2-4 birds are set in a cage with sawdust litter. After 15 minutes, when the birds are relaxed, 3 cm of lukewarm water is poured to the bottom of the cage. The birds will stand up and stay standing as long as possible before falling down to the water. The maximum measured time is 15 minutes. The longer the birds stay standing, the better their score. 100 birds are tested altogether.

Post-mortem examinations are done in slaughterhouses by evaluating the quality of foot soles. This can be done on live animals as well, but may cause unnecessary stress. Same visual evaluations can be done on hocks. Chemical and physical measurements such as tensile strength can also be done post-mortem. The dry matter content and ash composition can be measured from bones. Phosphorus and calcium concentrations in bones can be measured after cremation in a spectrofotometer.

Visual evaluation on foot health. (c) Welfare Quality

Breeding and growth of chickens

Egg-laying hens and meat chickens are sometimes infertile. They are hybrids, produced from two selective breeds or lines in a breeding facility. Meat chicken and egg producers do not raise their own animals, but buy them in large batches from the sellers.

In a breeding facility chickens are either inseminated or allowed to mate with a rooster. The sperm  cannot enter the ovaries immediately, because there are always developing eggs blocking the way. The sperm is deposited in the vagina, which has small membrane sacs for storing the sperm. When the oviduct is free after the oviposition (laying of the egg), the sperm can continue towards the ovaries. The funnel-like end of the oviduct, the infundibulum, has membrane sacs similar to the vagina. Here the sperm stay fertile for a long time: 8-10 days in chickens, less than a week in geese and 3 weeks in turkeys. Most fertilizations happen 2-3 days after the insemination. In poultry houses, best results ar achieved by inseminating non-pregrant hens once a week.

(c) Margaret Gunning

Roosters are sexually mature at the age of 16-20 weeks, but aren't selected for breeding until the age of 24-32 weeks when their sperm quality has increased. Most roosters cannot produce high-quality sperm for long. Roosters' testicles are located in the body cavity right behind the lungs. They have no penises, so the ductus deferens leads to the cloaca. Roosters and chickens mate by rubbing their cloacas together, during which the sperm enters the vagina. Breeding henhouses have one rooster for 10 hens, but using artificial insemination, the sperm from one rooster can be used for 48 hens weekly.

Two hormones affect the development of spermatozoa: FSH (follicle stimulating hormone) and LH (luteinizing hormone). FSH increases the development of spermatozoa, and LH increases the secretion of androgens from the testes.

Most production-line hens no longer sit on their eggs. This vital trait has been lost during the animal breeding, so the fertilized eggs must be incubated in an automated incubator. The incubation temperature is 37,6 - 38,6 C. Ventilation must allow constant supply of oxygen and removal of CO₂. The developing fetus "breathes" since day 1. Air humidity must be 60-70 %, and the eggs must be turned to avoid the fetus from touching the egg shells. Before incubation most eggs are treated with gas. The gas kills 96 % of the bacteria covering the egg shell (dirty eggs can have 50 000 - 200 000 bacteria). If needed, the eggs can also be medicated by dipping them into a liquid medicine.

The fetal cells start dividing within three hours after the internal temperature of the egg has risen above 22 C. The development of a chick proceeds as follows:

• Day 1: a yolk sac is formed, providing nutrients for the embryo. The Embryo weighs 0,0002 g.
• Day 2: The fetal membranes are developed. The heart starts to beat and ears are formed.
• Day 3: Nostrils, feet and wings can be seen.
• Day 4: Tongue starts to form. The fetus weighs 0,05 g.
• Day 5: Genitalia starts to form, the gender of the fetus is developed
• Day 6: The beak starts to develop
• Day 8: The feathers start to develop. The fetus weighs 1,15 g.
• Day 10: The beak hardens
• Day 14: The fetus turns around into a hatching position, head towards the blunt end where it can breathe from the air bubble. The weigh is 9,74 g.
• Day 19: The yolk sac starts to retreat to the chick's abdominal cavity during day 19, and is completely assimilated by day 20. 
• Day 21: The chick uses a sharp "tooth" on its beak to break the egg shell, and the bird hatches. The chick weighs over 30 grams.

The hatching times vary in different bird species. For chickens the development of the fetus lasts 21 days, 30-32 for geese, 28 for turkeys, 18 for doves and 42 for ostriches.  If the incubation temperature varies from the optimum (depends on species), the incubation time will also decrease (in colder temperatures) or increase a few days.

Growth of chicks and young chickens

(c) Unknown

Newly hatched chicks need water and warmth immediately. Their yolk sac has enough nutrition for 2-3 days, after which the chick will need to eat to survive. During the first days the chicks need a temperature of 31-32 C, and it can be decreased by 3 degrees weekly until the temperature is 16 C. For hygienic and safety reasons the chicks are raised separately from hens. The animal density is max 12 chicks / m² until they reach 12 weeks of age. Young chicks must be kept in a draftless area with good ventilation and 5-10 cm of litter.

The sex of the chick can be determined during the first three days after hatching. Male chicks are deemed useless and destroyed, usually by crushing . Chicken sexing is done manually, and can be very painful for the bird.

Egg and meat chicken producers byu their animals in batches (flocks). Therefore the breeders must make sure that each bird in a flock grows evenly: 90 % of the birds should weigh +- 10 % of the eaverage weight of the flock. Chicks are weighed 2 or 3 times between 5 and 15 weeks of age to assure even growth. In practice a selected sample will be taken and weighed, since each flock may have tens of thousands of birds. Young birds are also vaccinated. The light/dark program and light intensity are very important for young animals. If they will be raised in a poultry house with roosts, the chicks should be allowed to roost already at a young age.

The chick feeds depend on the breed of the bird. Chicken breeders have exact feeding programmes for the rearing poultry houses to use. Starter feed is usually fed for the first 3 weeks, or until the bird has reached a certain weight. The started feed includes all needed nutrients in pressed crumbs, preventing the chicks from selecting their feed. Grower feed has less energy, and is fed during weeks 1-8. Developer feed has as much energy as the grower feed, but less amino acids. Pre-laying feed accustoms the ~17 week old chickens for the feed they will be eating when they start laying eggs.

Egg formation and quality

Unlike any other bird, chickens lay eggs even if they are not pregnant. Chickens will lay approximately one 50-60 g egg each day. Layers are never inseminated, not artificially or naturally. Their eggs will thus not develop into chicks even if they were incubated. Hens lay eggs for 12-15 months, after which their production decreases and they are often culled.

Eggs are developed in the oviduct and uterus. 2-3 hours prior to ovulation, the hormone levels of progesterone and luteinizing hormone increase for a short time. The first ovulation of a chicken happens in the morning right after sunrise. Each subsequent ovulation occurs ~15 minutes later. After the delay has increased to several hours, the hen will not lay an egg for a day or two. Then she ovulates at sunrise again, and a new egg-laying period begins.

The formation of an egg. (c) Unknown

Ovulation is the release of an ovum. Nearly 500 000 immature ova exist in the ovaries at the time of hatching. 300-500 of them ever mature. Maturation of each ovum takes months, during which the ovum grows from 8 to 37 mm and 0,08 g to 15-18 g. Each day one ova fully matures into a follicle. The follicle develops into a yolk, which consists of darker and lighter layers, a latebra and a germinal disc. Yolk is mostly water, protein (16 %) and fats (33 %), and it is surrounded by a yolk sac. Originally the contents of the yolk were produced in the liver of the hen, and transported into the ovaries by blood. The formation of egg yolk is called vitellogenesis.

During ovulation the yolk is released and dropped into the funnel-shaped beginning of the oviduct. The funnel is called infundibulum. Infundibulum is the site of fertilization, if the chicken has been artificially or naturally inseminated. The yolk remains approximately 18 minutes in the infundibulum, before it moves forward in the oviduct.

The next phase of the oviduct is the magnum, a site where the egg-white is developed around the yolk. Egg-white consists of two thick and two thin layers. Unlike yolk, egg-white is mostly water and proteins. The thick layers form two chalazae, a pair of twisted "strings" which keep the yolk in the center of the egg. This protects the yolk from touching the egg shells and thus from being contaminated by bacteria and microbes. Both thick and thin layers of egg-white protect the yolk, which, if fertilized, would develop into a chick.

After magnum the yolk and egg-white move on to isthmus. In the isthmus two protective, thin layers are created on top of the egg-white. The inner shell membrane is attached to the egg-white, and the outer will be attached to the egg shell. An air bubble forms between the two membranes. As the egg is laid and stored for consumption, it will dry, and the contents shrink. The air bubble can expand, so the shrinking membranes do not break the egg shell.

So far the egg formation has lasted approximately 5 hours. The last phase is forming the shell. The shell is formed in the uterus. It is uncertain whether the egg shell is pigmented in the uterus or already in the isthmus, but generally brown chicken breeds lay brown eggs. Formation of the hard shell takes nearly 20 hours. When the egg is finished, it is moved from the uterus to the vagina. Oviposition, or laying the egg, occurs in 10 minutes. Only an hour after the oviposition a new ovum matures and the next yolk is released into the infundibulum.

Structure of the egg

(c) Encyclopaedia Britannica

The yolk consists of dark and light layers. 50 % of yolk is water, and one third of the dry matter is fat. The latebra is the "primal yolk", the part around which the rest of the layers are formed in the oviduct. Should the egg have been fertilized, the fetus would start developing from the germinal disc. Rest of the yolk would provide all the needed nutrients for the developing chick. Hence it is rich in vitamins (A, D, E and K),  cholesterol, B-vitamins, iodine, selenium, zinc, proteins and fats. One egg contains 20 % of the recommended daily amount of phosphorus for humans and 31 % of selenium. The yolk is surrounded by yolk membrane. As the egg is stored (in the shop or fridge), the yolk membrane gradually breaks. When the egg is broken, the yolk in old eggs spreads on the egg-white, while fresh yolks retain their shape.

The yellow color is caused by carotenoids received solely from chicken feed. The pigments in the feed can be either natural (paprika, pumpkin, shellfishes, etc) or synthetic. The color of the yolk is simply an esthetic quality factor, and has no impact on the nutritional value of the egg.

Contents of yolk and egg-white.

The egg-white or albumen consists 66 % of the mass of the entire egg. It is mostly water, but has some proteins, minerals and B-vitamins as well. One albumen has 41-51 kcal / 100 g, while a yolk has 348-378 kcal / 100 g. Considering a developing fetus, the albumen is only meant to serve as a protection. It has some antimicrobial features, and it protects the yolk from shaking and bouncing. The membranes around the albumen and inside the shell are 70 µm thick in total. Like yolk, egg-white dries as the egg is stored. In freshly laid eggs the albumen stays as a thick, round layer when the egg is broken. In older eggs it spreads into a thin layer.

The outermost layer of an egg is the hard shell, which is covered by a waxy layer called a cuticle. Cuticle protects the air ducts, which are formed between hard calcite shards. The cuticle is only 10 µm thick and very sensitive. If an egg is washed, polished or otherwise handled, the cuticle will rub off and the egg will spoil. All eggs are collected and sold to consumers as they are, without any washing. Otherwise the shelf-life of eggs could be reduced to less than a day. If the cuticle is intact, eggs can be stored in cool temperatures for months without any spoilage. The egg shell allows oxygen inside the egg, and carbon dioxide and water out from the egg.

Quality of eggs

The quality of eggs is controlled in many ways. The components of quality can be divided into three sections:

1. Inner quality: blood or meat spots, size of the air bubble, quality of the yolk, firmness of the albumen (Haugh-value)
2. Outer quality: egg size, cleanliness, qualities of the shell, form of the egg
3. Other quality factors: microbiological quality, taste, odor, nutritional content

Inner quality factors can be measured by candling, checking the egg against a bright light. This is done in an egg packing facility. Spots of blood, any develoment of a fetus and the position of the yolk can be observed in candling. The size of the air bubble is also examined. If the egg as any faults, it will be rejected. Good quality eggs are stamped, classified by size and packed. Grade A eggs are then sold to stores and to consumers. Grade B eggs, which are the wrong size or have some minor cosmetic faults, are sold to the food industry as raw material (as intact eggs or egg mass). Any egg which is unedible and/or dirty is entirely rejected by the packer and not used in any kind of processing.

Blood spot. (c) The poultry site

All eggs may have blood spots of tiny brown "meat" spots. Neither are dangerous or signs of a fertilized egg. Blood spots may occur if there has been minor haemorrhage during the ovulation. Haemorrhage can be caused by an unsuitable light program, lack of vitamins, unsuitable feed, fright during ovulation or a disease. Brown spots are tiny pieces of the hen's tissue, or most commonly a broken down blood spot. Older hens and some breeds lay more eggs with brown spots.

Physical methods for determining the quality of an egg include measuring the thickness of the albumen in an broken egg and measuring the egg's corrected weight by sinking the egg into salt solutions of different concentrations. The shape of the egg is described as the ratio of its length (from the blunt end to the sharp end) and height.

Sandpaper egg (c) The poultry site

The quality of the egg shell is measured as the weight and thickness of the shell, and the deformation of the shell. "Sandpaper eggs" have a very hard and uneven shell, because the egg has been too long in the uterus. It can also be caused by lack of water, disease or an unsuitable light program. Mottled or transparent eggs are caused by too high a humidity in the poultry house. The egg shell dries very slowly, causing the color errors. IB disease can also cause both sandpaper and mottled eggs. "Leather eggs" are eggs without the hard shell. They are caused by lack of calcium and some diseases. Leather eggs are more common for young layers than adults.


Read more online from Optimum Egg Quality: A practical approach in the Poultry Site.

Poultry nutrition and feeds

Poultry, like all animals, need energy for production and maintenance. There are two different ways of estimating the energy requirement of poultry: AME_N (Apparent metabolizable energy, nitrogen-corrected) and TME_N (True metabolizable energy, nitrogen-corrected). The first is used in Europe, the latter in the USA. Both rely on metabolizable energy - the amount of energy gained from feed and not excreted in feces. The difference is that TME_N takes into consideration the endogenous excretion of nitrogen, and thus gives more accurate energy estimates.

Poultry regulate their intake of feed based on the energy content of the feed. The higher the energy count, the less they eat. It is important to ensure that chickens eat enough to get all the required nutrients from their feed. Gaining body fat is not a problem in today's poultry breeds, which are bred for extremely high growth and efficiency. Egg-laying chickens require ~500 kJ / day for maintenance, or 770-840 kJ ME/day. Each kilogram of eggs produces requires 11,2 kJ ME. Young egg-laying hens need additional 10 kJ a day for their own growth.

Methods of determining energy content

Chickens in balance cages (c) ILVO

There are three methods for determining the energy content of poultry feed. The traditional method CAM is used to determine apparent metabolizable energy. Birds of all ages can be used, but usually this is done using mature roosters. The birds are held alone in collection cages (balance cages), which enable the scientists to measure exactly the amount of food eaten and feces/uric acid excreted. First the birds are 1-6 days in the cage just getting used to the test feed (1 day is enough for chickens). The collection period lasts for 3-4 days, during which feces and uric acid are collected and analyzed several times a day. This method is painless for the animals and allows determining the digestibility of the feed at the same time, but is very labour-intensive and take a lot of time.

The Farrell method is a more rapid determination of ME.  In the beginning and end of the collection period, the animals are denied food to increase the reliability of the test results. Farrell method uses only experienced test animals, which are able to eat high amounts of test feed in one hour. 24 or 36 hours after the animal has eaten ,the feces (including the uric acid) are collected and analyzed. The Farrell method is a lot faster than CAM, and requires the animals to spend only a short time in the balance cage. However, fasting is unnatural for poultry, which may raise welfare concerns.

The third method is the Sibbald method. Unlike Farrell and CAM, Sibbald is used to determine the true metabolizable energy. The animals are kept on a fast for 24 hours before the trial to empty their digestive tract. The test feed is then force-fed through a tube directly into the crop of the birds. 25-50 grams of feed is inserted at one time. Feces are collected 24 or 48 hours after the force-feeding. To determine endogenous excretion of nitrogen, the trial is repeated several times, and each time one animal is not fed at all. The results for that animal show the level of endogenous excretion. While the Sibbald method is fast, reliable and enables studying all kinds of feeds, it raises severe welfare concerns due to fasting and force-feeding.

Amino acids 

Like mammals, poultry need  to get necessary amino acids from their feed. Young chickens need amino acids more than older ones, since their growing feathers, meat and bones require a lot of protein. The most needed amino acid is lysine. Young chickens need lysine 9,5 g/kg. Because poultry regulate their intake of feed based on the energy content, it is important to make sure they eat enough to get the needed proteins. To estimate this a ratio between energy content and amino acids is calculated:

amino acid content / energy content = AA/energy ratio

For a 6 week chick the ratio should be 155-160 g / 10 MJ, 165-180 g / 10 MJ for a meat chickens and 140-150 g/10 MJ for egg-laying hens. The feed for chicks younger than 6 weeks should have 200 grams of protein / kg dry matter.

Poultry can only use amino acids which are absorbed from the small intestine. Indigestible and endogenous amino acids are microbially transformed into ammonia in the caecum. The ammonia, which is useless for the animal, is excreted in uric acid. An egg-laying hen aged 35 weeks may eat 2500 mg of nitrogen, of which over 1000 mg is excreted in feces, and a bit less than 1000 mg used in eggs. This leaves only a few hundred mg of nitrogen for the animal itself. Eggs contain 12,5 % protein. When the hen lays one 50-70 g egg a day, she needs 10-17,5 g of protein daily. If the protein content of the feed is not high enough, the hen will stop laying eggs. The protein need depends on the breed of the animal, phase of producction, type of housing, health, energy content of feed and the feeding method.

(c) Ajinomoto-Eurolysine

The ideal protein for poultry is a bit different for meat chickens and egg-laying hens. Lysine is the most important amino acid for both. Layers need more amino acids than meat chickens. The exceptions are histidine, leucine, phenylalanine and tyrosine, which are more important to meat chickens than layers.  Layers need amino acids in relation to their weight and egg production capability. For example, a layer needs 9,99 mg lysine a day / g of eggs produced, and further 73 mg / weight in kg. A 2 kg chicken laying one 60 g egg would thus need 9,99*60 + 73*2 = 745,2 mg lysine / day.

Meat chickens should be fed in phases to optimize their utilization of lysine. The need for lysine decreases as the meat chicken grows, so each feeding phase can contain less lysine. Optimizing feeding for meat chickens and layers decreases feed costs and waste of nitrogen.Varying the phase feeding lengths for meat chickens can increase the weight at slaughter over 150 grams, and decrease the amount of abdominal fat 0,5 %.

Studies show that optimizing poultry feeds based on amino acids is more reliable than optimizing based on metabolizable crude protein. Optimizing amino acids can produce 18-24 % less nitrogen in feces and better utilization of amino acids in the feed.

(c) Ultra Bio-Logics

Minerals

Egg-laying hens have strict requirements for minerals in their daily feed. Lack of minerals in the beginning of the laying season may affect productivity for the rest of the season.  Layers need especially

• Calcium: needed for bones and egg shells. Calcium is important for chickens of all ages.
• Phosphorus: calcium and phosphorus must be in the right balance for either to work. Phosphorus is also important for vitamins and cell metabolism.
• Magnesium: activates enzymes in energy metabolism, and builds up nerves and bones. Magnesium is said to have a calming effect on chickens.
• Sodium: Balances the nervous system and pH
• Chloride: Balances the fluid balance of the body, is needed for gastric juices
• Potassium: A balancing factor
• Sulphur: Important for building sulphur-based amino acids, and for healthy feathers

(c) Beaverhausen's Blog

Chickens also need the following macrominerals: iron, copper, iodine, manganese, zinc, selenium. The most important mineral, however, is calcium. Each day a hen loses 2-2,5 grams of Ca as she lays an egg. The recommended daily amount is 35 g Ca/kg feed, or 4 g / hen / day.  A high amount of Ca in the daily diet improves the strength of the egg shell, but amounts of over 2,5 g gradually decrease the absorbancy of Ca.  Chickens do not eat during the night (dark period), so Ca is absorbed from bones. Therefore slowly digestible Ca source is recommended. Also the coarser the Ca powder in the feed is, the stronger the chicken bones are. Coarseness of Ca has no impact on egg shell strength.

When the egg shell forms, calcium ions are combined with HCO₃^-. HCO₃^- is originally created from water (H₂O) and carbon dioxide, CO₂. When the Ca⁺⁺ and HCO₃^- are mixed, a nitrogen is lost and the resulting CaCO₃ creates a sturdy, white egg shell.

Phosphorus and phytic acid are important in maintaining good layer productivity. In one study a feed with 3 g P/kg and no added phytic acid results in a laying rate of 92,4 %,  egg weight of 56,8 g and dry matter content of the tibia of 4,19 g. The same feed with 460 units of added phytase resulted in a laying rate of 95 %,  egg weight of 58,4 g and dry matter content of the tibia of 4,87 g. Another study has proven that adding phytase increases the utilization of inorganic phosphorus, but do not improve the apparent ileal digestivity of P.  Added phytase also increases the mineralization and tensile strenght of bones.

(c) unknown

 Feeds

Chickens are usually fed either with a complete feed or concentrated feed + grains. Complete feeds include everything the birds need: minerals, proteins, energy, fats and calcium. It is an easy but expensive alternative. Using concentrated feed and grains requires more planning, but can be cheaper if home-grown grains can be used. All ingredients must be analyzed to ensure their correct balance in the feed, and mixed well. The ratio of ingredients must be changed throughout the growth/egg-laying period as the requirements of the animals change.

In Northern Europe, the most important ingredients in poultry feeds are barley and oats. They contain noticeably less energy than soybeans and corn, which are commonly used in the USA and elsewhere. Vegetable oils can be added to the grains to increase the energy content, because poulty cannot eat very much at one time, and will lose weight if the feed is too low in calories. 70-80 % of the energy in grains is metabolizable, because the energy in grains is mostly starch.  Wheat has 65 % of starch, barley 55 % and oats 40 %. Poultry can metabolize starch almost entirely.

(c) Fowl Visions

Wheat has 12,5 MJ/kg ME and 12-13 % of proteins. Wheat can be used for adult meat chickens and layers, but is too strong (the energy content is too high) for young birds. Wheat has only little lysine. Barley is the most used grain in poultry feeds. It has 11,5 MJ/kg ME and 12 % protein, which makes it less strong than wheat, and thus better suitable for poultry of all ages. However, the betaglucans in barley reduce the digestibility of all nutrients, and may cause sticky feces. Adding betaglucanase-enzyme to the feed eliminates this problem. Oats have only 10-11 MJ/kg ME due to a high content of crude fibres. Oats also have twice the amount of fats than other grains, and more lysine than barley. Oats are mostly used for chicks and growing chickens, but high-quality oats can also be used to replace barley for layers and meat chickens. For layers, oats and barley are interchangeable. By-products of grain processing, such as brans and barley flour can also be used. By-products of alcohol production have too little energy to be of value to poultry.

Soybean meal has 50 % of protein and much lysine, but it has to be supplemented with methionine. Soy can be used for meat chickens. Rapeseed meals (canola) are the "left overs" from the production of rapeseed oil. Depending on the oil production method, rapeseed meals contain 2-10 % fat and roughly 8 MJ/kg ME. The protein content is similar to soybean meals, but is less digestable. If the rapeseed used contains no erucic acid, sinapic acid or glucosinolates, rapeseed meals can be used to replace soybean meals entirely. However, in one study canola meals were found to decrease egg-laying percentage and egg weight.

Peas and broad beans have some antinutrients, such as oligosaccharides (pea), visine and convisine (broad bean). Visine and convisine cause aplastic anemia, which is uncurable and lethal to chickens. The amount of visine and convisine in broad beans cannot be reduced in feed processing. Peas have 20-22 % of protein and broad beans 26-28 %, and both have a lot of lysine but little methionine. Due to the antinutrients, the feed of layers can have up to 30 % peas and 5 % broad beans.

Fish meal is used for chicks in small amounts. Adult layers and meat chickens develop a fishy taste and smell to eggs and meat if fed fish meals.

The digestive tract of a chicken

Birds have no teeth. (c) Earth Song farm

Poultry (chickens, geese, turkeys etc) have a distinctive digestion system, which has some clear differences to the digestion of ruminants or other monogastric animals. Their digestive system is comparatively short, so each food particle remains only approximately 6 hours in the digestive system. For ruminants, the time is almost 48 hours, and 24 hours for pigs. In such a short time the chicken must be able to absorb as much nutrients as possible from its feed. It's also interesting to note that the digestive system of a chicken develops very fast: a chick can digest fibers as well as an adult chicken.

Poultry have no teeth or soft palate. They peck food with their beaks, and especially chickens have no problem eating hard or stringy material such as dead mice, earthworms or broken eggs. All food particles are swallowed whole without chewing. The saliva has no active amylases so no digestion happens in the mouth or esophagus. Some poultry keepers routinely cut the beaks of the chicks, which causes extreme pain when pecking, and can cause problems eating. This practice is highly unethical, even though it is effective in preventing chickens from pecking each others in frustration.



Anatomy of a chicken (c) Poultryhub



After swallowing, the food slides down the esophagus into the crop. Chicken esophagus is approximately 35 cm long, covering 17 % of the length of the entire digestive tract. It secretes mucus which lubricates the esophagus. The crop is not a stomach, it is simply a small sac used to store and moisten feed (some wild birds feed their young by regurgitating food stored in their crop). The crop secretes enzymes which digest starch and proteins, and has some microbial activity as well.

After the crop the moist, partially digested food mass enters the proventriculus (gladular stomach). Like crop, the proventriculus is found only in birds. The proventriculus secretes HCI, pepsin and mucus, which start the actual enzymatic digestion. Food travels trough the proventriculus very fast: it is covered in gastric juices, but doesn't stay in the proventriculus to be digested. For actual digestion and mechanical breaking food enters the actual stomach, called the gizzard. The gizzard is surrounded by strong muscles, which contract and break food particles. The muscles are set in two pairs, thick and thin pairs of muscle. Muscle contractions also mix the enxymes well with the food mass. The internal wall of the gizzard has koilin fibres, which stick out from the wall like tiny teeth, further aiding in breaking food particles.

Liver and pancreas excrete important digestive enzymes. The relatively large liver of a chicken has two segments. Bile is excreted directly from the left segment of the liver to duodenum (beginning of the small intestine). The right segment has a duct to the gall bladder, which again has a duct to duodenum. Pancreas are located near duodenum, and excretes lipolytic, protelytic and amylolytic enyzmes. It is interesting to note that chickens produce no lactase, because their nutrition does not include milk derivatives. Chickens also lack the enzymes needed to digest cellulose, hemicellulose and beta glucans. Commercially available enzymes can be added to chicken feed to enhance digestibility.



Digestive system of a chicken (c) University of Kentucky

After the gizzard the food enters the small intestine, which is short compared to mammals. Most nutrients are digested and absorbed in the small intestine. The intestine has two kinds of glands: intestinal glands secrete lipolytic, protelytic and amylolytic enyzmes, and glands of the mucuous membrane secrete maltase, isomaltase, peptidase, saccharase and palatinase. The small intestine of chickens is divided to two, duodenum and ileum, compared to the three-part intestine of mammals. Ileum alone is 120 cm long, and comprises nearly 60 % of the length of the entire digestive system.

Chickens have two caeca (singular: caecum), which assist in digesting fibers and non-starch polysaccharides. Caeca are approximately 8 cm long. Caeca are full of microbes, and they are located in the junction of the small and large intestine. Sometimes in feed digestibility trials the caeca are surgically removed, so the intestinal digestibility can be measured without interference from the caeca. In other digestibility trials a fistulae is surgically inserted to the end of the small intestine. This way samples of the feed can be collected before it is digested in the caeca. Both methods enable to animal to be kept alive after the trial, and possibly used in another feed trial later. The most common trial method is to slaughter the chickens for collecting of feed samples from the digestive tract.

The large intestine is very short, and ends in a cloaca. Together they are about 6 cm long (5 % of the length of the digestive tract). The oviduct and uric acid are also secreted to the cloaca, so both feces, uric acid and eggs come out from the cloaca, which ends in the anus.

Retention times in each part of the digestive tract. (c) unknown