Biology of Domestic Animals YAS-10806 Past Paper PDF

Contents of reader of Biology of Domestic Animals YAS-10806 Cases: Case 1 Egg p. 1 Case 2A Sperm p. 36 Case 2B Meat...

Contents of reader of Biology of Domestic Animals YAS-10806 Cases: Case 1 Egg p. 1 Case 2A Sperm p. 36 Case 2B Meat p. 46 Case 3 Nutrition p. 88 Case 4 Milk p. 129 Case 5 Fish p. 154 Case 6 Horse p. 166 Case 7 Dog p. 198 Case 8 Behaviour p. 243 Case 9 Immunology p. 275 Case 10 DNA p. 296 Every case has selfstudy material and a manual or description of the corresponding practical. Some manuals are available on Brightspace. 0 Case 1: Egg Self-study material p. 2 Practical manual p. 24 Animal species: Chicken Study objectives: To be able to reproduce the composition of the egg To be able to describe the formation of the egg with hormone regulation and influences from outside To be able to name quality characteristics To be able to explain laying cycle, laying series and laying curves To be able to identify differences between eggs To be aware of sensory analysis 1 Self-study material 1. EGGS 1.1 The function of the egg Animals’ activities are aimed at survival of the individual and the species. Reproduction is essential for the latter. The reproduction process starts amongst others with the egg cell (ovum). The dimensions of animals’ egg cells vary widely from species to species; this variation is linked to the quantity of yolk. Mammalian egg cells have very little yolk. These are called oligolecithal egg cells. In the case of certain insects, fish, reptiles and birds we encounter large egg cells, with the yolk filling the largest part of the cell and in which the rather small quantity of cytoplasm has collected at one pole: telolecithal egg cells. The yolk mass is so large that we simply refer to the cell body of a chicken’s egg as 'the yolk', ignoring the small quantity of cytoplasm also in it. Most animal species are oviparous; the females lay eggs from which the young later emerges. This applies to most animal species that live in water, most insects and reptiles and all birds. Most of the remaining animal species are viviparous; they give birth to live young. Certain species are ovoviviparous; they produce eggs with a lot of yolk that develop further within the female’s reproductive organs. The embryos are not attached to the wall of the oviduct (or the uterus) and do not receive any further nutrients from the mother animal. Examples of ovoviviparous animal species are: certain insects, snakes, sharks and lizards. Mammals (and a few other species) are viviparous: they produce small eggs that develop in the uterus and continue to be fed by the mother animal. This means that the mammalian egg cell can be small, given that the developing foetus can receive all its nourishment from the body of the mother, to which it is attached by means of the placenta. A bird embryo by contrast develops entirely outside the body of the mother, so that the bird egg has to be large enough to contain all the nutrients needed for the growth of the embryo. Yolk-rich egg production is comparable to some extent with milk production; both form a source of nourishment for the young. However, an embryo in a yolk-rich egg is more dependant on its egg than a young mammal is on milk, because the young mammal has already received a lot of nutrients in the liver and other body tissue. The closed environment of the egg demands concentrated nutrients. 1.2 Egg production Whenever one thinks of eggs, one generally thinks of chicken eggs. This is certainly understandable, seeing that (global) egg production can be ascribed overwhelmingly to the chicken. Ducks and geese can also be used for egg production. Particular lines and crosses have been bred that have outstanding laying capacities (the best-known duck laying breed, Khaki Campbell, lays approx. 300 eggs per year, each weighing around 70 grams or more), but the product has much lower acceptance than the chicken egg. The shape and taste of duck and goose eggs are to some extent different from chicken eggs, but not to the extent that this can explain the much lower popularity. 2  Up to the end of the 1950s, chickens were allowed to wander around freely on the farmyard, or they were kept in a run or stable with floor housing. However, a new housing system was developed in the United States in the 1960s allowing a more intensive way of housing: the battery cage. This made it possible to mechanise and automate chicken farming for egg production. Conditions that could be controlled with respect to hygiene also played a major role here. More chickens could now be kept in the same space, looked after by fewer people than in the old way of keeping chickens. This cost-cutting meant a considerable improvement in the competitive position of the chicken egg over the duck egg, given that ducks could not be housed in a battery cage. At the start of the 1960s, duck egg farming declined sharply, partly on account of the declining supply of fresh fish, which is necessary for high egg production in ducks, export problems connected to Salmonella infections and environmental problems. However, the battery cage has the disadvantage that the animals have limited space and suffer welfare problems. The battery cage for chickens has been banned in the European Union since 2012. Chickens may now only be kept in suitable cages (with perch, nest, litter and more space) or in non-cage systems, such as free-range or aviary. In 2010, 10.1 billion chicken eggs were produced in the Netherlands, with a large proportion being exported as eggs or egg products (mainly to Germany). The domestic market consumed approx. 3.0 billion eggs, translating into approximately 185 eggs per person per year. 1.3 Structure of the egg The egg consists, simplified to a certain extent, of four components: the yolk, the egg white (albumen), the shell membranes and the shell. In this article, ‘egg white (albumen)’ means the white of the egg; the chemical substance will be referred to as ‘protein'. A chicken egg is made up to almost a third of yolk and to 10% of shell. These proportions may vary widely for different bird species (see Table 1.1). The fact that the composition of the egg determines the development of the embryo may be deduced directly from the relationship between the composition and the degree of development in the young on emerging from the egg. Nidicolous young emerge less developed from the egg than do nidifugous, and so the eggs of nidicolous bird species such as the pigeon contain fewer reserve substances, and the yolk is smaller. 3 Table 1.1 Average egg composition of different bird species (Horbanczuk 2002, Kuli et al. 2004, Kozuszek et al. 2009) On closer examination, the structure of the egg turns out to be somewhat less simple. A schematic cross-section of the egg is shown in Figure 1.1. The yolk of the chicken egg has an average cross-section of around 3 cm. It consists of a series of alternating light and darker concentric layers and is surrounded by a thin yolk bag (or vitelline membrane), to which the chalazas (or chalazae) are attached. As the yolk has a lower specific gravity than the egg white, it lies slightly above the centre in a fresh egg. The germinal spot is a small light-coloured spot on the surface of the yolk. This germinal spot (within a couple of hours a germinal disc, or blastodisc, in a fertilised egg) is structured and incorporated in such a way that it always faces upwards, irrespective of the egg’s position. Eggs for consumption are usually unfertilised. 4 Figure 1.1 Cross-section of a chicken’s egg. The white of the egg has a nutritional function, but it also has a protective function; it is elastic, cushions impact and insulates. It provides not only mechanical protection, but also chemical: certain substances in the egg white have an anti- bacterial effect. The egg white consists of four layers: (1) an outermost thin layer (the ‘thin white’, 23% of the egg white), (2) a thick layer (the ‘thick white’, 57%), (3) an innermost layer of thin white (17%) and (4) a layer of thick white (3%) around the yolk. The last layer extends into the chalazas. Chalazas are spiralling strands of egg white that keep the yolk in the centre of the egg. There are two chalazas at the rounded end of the egg, and three at the pointed end. As they age, the chalazas lose their attachment to the thick white, with the result that the yolk, which is lighter than the egg white, will start to rise. The egg shell consists to 93-98% of calcium carbonate (CaCO3). The shell is porous (approx. 10,000 pores), so that gas exchange between embryo and environment is possible. But these pores also make it possible for bacteria to penetrate the shell. However, the shell is covered on the outside by thin film, the cuticula. This cuticula dries out quickly after laying and reduces the gas exchange through the pores and the bacterial penetration. During incubation, the permeability of the cuticula increases again under the influence of the high temperature, so that almost 15% of the water in the egg evaporates during the incubation process. 5 Figure 1.2 General egg and eggshell microstructure diagram. (Figure credit: Shaena Montanari) There are two thin shell membranes just inside the shell. These two membranes lie close together, except at the rounded end of the egg, where the air chamber is located between the two membranes. The egg shell is at its most porous at this point, so that air can be sucked in here most easily, when the egg contents shrink as they cool after laying. This air chamber increases in size over time, especially in a dry and warm environment. The shell normally has a structure such that it is strong enough to bear the contents and yet fragile enough for the chick to break through easily as it emerges from the egg. The total thickness of the shell is approximately 0.35 mm, but may vary from 0.25 to 0.45 mm. Figure 1.2 shows an egg shell in cross-section. 1.4 The chemical composition of the egg Table 1.2 shows the average chemical composition of the chicken egg. It is noticeable that the egg white consists of water to a very large extent and contains a lower percentage of protein than the yolk. 6 Table 1.2 Average chemical composition of a chicken egg of 58 grams, in grams and in percentages (after Romanoff and Romanoff, 1949). Yolk Egg white Shell Total grams % grams % grams % grams % Water 9.1 49 28.9 88 0.1 2 38.1 66 Dry matter 9.6 51 4.0 12 6.3 98 19.9 34 Organic matter 9.4 50 3.8 12 0.2 3 13.6 23 Proteins 3.1 17 3.5 11 0.2 3 7.0 12 Fats 6.1 33 0.0 - - - 6.1 10 Carbohydrates 0.2 1 0.3 1 - - 0.5 1 Inorganic matter 0.2 1 0.2 1 6.1 95 6.3 11 Total 18.7 100 32.9 100 6.4 100 58 100 Energy (kJ/100 g) 1533 86 202 11 50 3 614 100 The most common protein in the egg white (75%) is ovalbumin. When used in preparing food, albumen lends a firm structure to for example cakes and custard, as it coagulates under the influence of heat. Other proteins occurring in egg white are ovomucoid (13%) and ovomucin (7%). These are both glycoproteins, or proteins bonded to a carbohydrate. They stabilise the egg white foam, and are thus important for making cakes, sweets etc. A fourth protein occurring in egg white is ovoglobulin. This substance contributes to being able to beat egg white into a foam, which is important for example for the production of meringues and other sweets. The yolk consists for the most part of lipids, which form the most important source of nutrition for the embryo. The lipids largely consist of triglycerides (62%), most of which contain only saturated fatty acid chains. There are also 33% phospholipids and 5% sterols (primarily cholesterol). The relatively high cholesterol content of eggs (0.4%), in combination with the possible link between cholesterol levels in the blood and the occurrence of cardiovascular disease, is a reason for certain people to limit egg consumption. However, in the majority of epidemiological studies into this link no indications were found (Nau et al., 2010). The protein fraction in the yolk contains 77% vitellin (a phosphoprotein) and 23% livetin (which contains a lot of sulphate). The lipids in the yolk occur primarily bonded to proteins and so form lipoproteins. Lipoproteins have an emulsifying effect (important in salad dressing) and also help the capacity to foam and coagulate that is important for cakes, poffertjes, etc. Many pigments are fat soluble and so occur in the yolk of the egg (for example carotene, xanthophyll and riboflavin). These pigments are essential for the colour of products such as mayonnaise and egg noodles. Riboflavin also occurs in egg white. 7 1.4 Quality of eggs There are large differences between eggs, and these can influence the quality of the egg. Asked for their standards on egg quality consumers turned out to be aware of size (with a larger egg representing higher quality) and freshness. However, an experiment revealed that people could not tell the difference between an egg one to two days old and one kept in cold storage for three to four weeks. The egg trade can assess the freshness of an egg by candling. Light is allowed to pass through it. A fresh egg is translucent, the yolk is dimly visible and does not move much, and the air chamber is small (Figure 1.3). Figure 1.3 A candled fresh egg. On the far right is the air chamber, the red spot is the yolk. The consumer can assess the freshness by shaking the egg gently; with a fresh egg nothing is heard, but with an older egg (Figure 1.4) the contents can be heard shaking or lapping as a result of the large air chamber. Another option is to place the egg in a bowl of salt water. A fresh egg remains on the bottom. The older an egg is, the more it stands up in the water with the rounded end upwards. In the end it floats as a result of the increasing size of the air chamber. This happens with eggs six weeks and older. They may then even be rotten. Figure 1.4 A less fresh egg with an enlarged air chamber on the right. 8  On cracking, a fresh egg will have a lot of thick white with a flat and firm yolk and thus will not spread a lot. The variation in the proportion between thick and thin egg white and in the viscosity of the thick white depends not only on the freshness of the egg, but is also partially genetic in nature. The temperature and age of the hen are also an influence. To be able to make an assessment on the thick white, its height is measured with a cracked egg on a flat surface (so-called Haugh unit) (Figure 1.5). The Haugh unit is of particular significance for the Asian market, where it is important for certain dishes that the raw egg remains lying on the rice for example. Figure 1.5 A fresh egg. The yolk (centre), the thick white (around the yolk) and the thin white (around the thick white) can be distinguished. The relationship between the height of the thick white (left figure) and the weight of the egg is the Haugh unit and determines the freshness of the egg. The shape of the egg is mainly determined genetically; each hen lays eggs one after another of practically the same shape. The variation between chickens is fairly large: round, cylindrical, pointed, pear-shaped etc. In general, small eggs are almost round, whereas large eggs are often conical, elliptical, oval or biconical. The shape index is used to assess the shape quantitatively. It is calculated by multiplying the proportion of the width to the length of the egg by 100. Eggs with an ideal shape index (72-76) fit readily into the usual packaging, with the result that breakage can be limited as far as possible. The curved shape of the egg ensures that normally shaped eggs are able to withstand extremely high static pressure. However, point loads resulting from impact are withstood much less. The shell is the most variable among the egg components. The formation process of calcium carbonate is very sensitive, in particular with high production birds. The following factors may lead to a decline in shell thickness: high ambient temperature, calcium deficiency in the feed, stress, disease, medications and large eggs during the last part of the laying. There are sometimes abnormalities in the structure of the egg shell, for example a very rough surface or a lot of pores. It is often the same chickens that show an abnormality. A flawless, strong shell is very important, not only in connection with product loss as a result of breakage (for example during transport), but also in connection with penetration by microbes that cause spoiling and increased moisture loss as a result of hairline cracks or too-great porosity. 9  Moreover, the shell colour of an egg is a noticeable characteristic. In chickens, the colour is determined by pigment in the shell, whereas in other birds, the cuticula can also contribute to the colour. Depending on breed, chickens lay white or brown eggs, but there is also a breed (the Araucana hen from South America) that lays blue eggs. The amount of variation in the colours white and brown is noteworthy; there are large differences between breeds and between chickens of the same breed. Usually the first brown eggs in a laying series are darker than the subsequent eggs. Free-range eggs are almost always brown in the Netherlands, because the Dutch consumer prefers this colour. The association between egg colour and housing system is however heavily dependent on the country: in Germany for example a lot of white free-range eggs are sold. The most important colour substance of the yolk is xanthophyll, which originates in the feed. The colour intensity of the yolk is mainly determined by the xanthophyll concentration in the feed. The colour of the yolk is assessed by comparing it with a colour standard; a chart showing the various colour shades. The colour of the yolk can be completely controlled by the feed composition. Sometimes traces of blood occur in an egg; these are blood clots that remain attached to the yolk on ovulation. The occurrence of these traces is determined by a number of factors: feed, age of the hen and hereditary predisposition; in addition brown eggs reveal more blood traces than white eggs. Figure 1.6 An egg with a blood trace. 10 2. EGG PRODUCTION 2.1 Formation of the egg Although there are many animal species that propagate using eggs, in the following, only birds will be discussed, because these are the only animals that are 'kept' for their eggs (and then again, almost exclusively the chicken). In the chicken, fertilisation of the egg cell is not necessary for laying an egg (a cow provides milk only after the birth of a calf). A modern laying hen can continue to lay eggs without mating or even in the absence of a rooster (a pigeon for example needs the presence of another pigeon - male or female - as a stimulus for ovulation). Humans have used this biological phenomenon for the production of unfertilised eggs for consumption. The entire production process for an egg depends on hormone synchronisation and balances (see Figure 2.1). The hormonal cycle of the hen is under the influence, just as with mammals, of the gonadotropic hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Figure 2.1 Diagram of the most important effects of hormones and their mutual relationships in the chicken (Nesheim et al, 1979). The FSH promotes yolk formation and the growth of the follicles in the ovarium; once a follicle is ripe, the LH presumably ensures ovulation (ovulation takes place 8-10 hours after the release of LH; see Figure 2.2). 11 Figure 2.2 Comparison of the times of LH-release and ovulation in three domesticated bird species under a constant daily 14-hour light regime. Open block with 'ovulation' indicates the time of ovulation of the first egg in a laying series. Hatched block with 'LH-release' indicates the period in which the LH-release for the first ovulation is initiated; open block indicates the period in which the release is completed (Cole and Ronning, 1974). The egg cell then enters the funnel, or infundibulum, of the egg guide, or oviduct. The oviduct is stimulated at the appropriate moment by the hormones of the ovarium (presumably an interplay of female and male hormones; see Figure 2.3) to receive the egg cell. The egg cell is then surrounded during its passage through the oviduct by egg white, which is formed by the glands in the wall of the oviduct. At the end of the oviduct, in the isthmus, the egg is surrounded by the two shell membranes, after which the calcium shell is deposited around the egg in the shell gland, or uterus. Figure 2.3 shows a diagram of the reproductive organs of the hen. 12 Figure 2.3 The reproductive organs of the hen. As soon as faults occur in the system, abnormal eggs often occur, such as an egg within an egg, wind eggs (no shell or very soft shell), yolkless eggs and double yolks. This occurs primarily at the start of the lay with young birds, with overstressed hens or as a result of disease or feeding errors. Figure 2.4 shows an average time schedule for the formation of an egg in the chicken. 13 Figure 2.4 The production process of an egg in the chicken (amended after Bell and Freeman, 1971, and Ensminger, 1980). The size of the egg is largely determined by the size of the egg cell during its passage through the oviduct. Small egg cells, such as occur at the start of the lay in young chickens, receive less egg white allocated in absolute and in relative terms, so that in the end the egg will be considerably lighter than the egg of an adult hen. The hormones of the ovarium also cause the so-called secondary sexual characteristics. Oestrogen ensures the female plumage, female sexual behaviour and the absence of spurs (see figure 2.5). Androgen, or testosterone, is responsible for the large, bright red comb and lobes in a normal hen when laying (chickens are the only birds with a comb). 14 Figure 2.5 Rooster and hen of the brown laying breed. A chicken shows no oestrus during its oestrous cycle. Under natural circumstances, the chicken, like other birds, does have a clear reproductive season, which is primarily influenced by the length of the day. In contrast to mammals, no corpus luteum is formed after ovulation; the corpus luteum in mammals after all produces a large quantity of progesterone, which prepares the uterus for the fertilised egg cell and which is necessary for the development of the foetus (it also prevents the ovulation of new egg cells during gestation); so a chicken is never gestating. The hen stores the rooster's sperm in its sperm store and in this way the eggs are fertilised for several days. 2.2 Laying the egg Once an egg is fully formed, it is usually rotated 180 (around its width axis), so that it lies with its rounded end pointing towards the vagina. The egg is then pushed through the uterus to the vagina largely through muscle contractions. The vagina bulges out past the cloaca, so that the egg is laid without coming into contact with the cloaca. By contrast to mammals, the uterus has no cervix. The laying of an egg (also referred to as oviposition) lasts only a few minutes. Another hormone deriving from the hypophysis, oxytocin, presumably plays a major role in the muscle contractions and thus in oviposition. It is remarkable how much this oviposition process resembles the birth process in mammals; the same hormones and the same mechanisms (such as contractions of the uterine muscles) play a role in parturition. Both the ovulation and the laying of the egg are influenced by external factors, light in particular. Wild birds in our climate zone lay in spring. Once the length of the day increases, a signal is sent from the eyes to the hypothalamus, stimulating hormone 15 secretions (see Figure 2.1). The latter then leads to egg cell ripening and subsequently to the laying of eggs. This seasonal pattern ceases to exist as a result of the current housing method with laying hens. Most laying hens are kept with a regulated daily light/dark rhythm (16 hours of light and 8 hours of darkness); and they lay eggs all year round. The light's wavelength also plays a role in certain bird species. Ultraviolet light does not have a stimulatory effect; certain bird species do react to red and not to green light. Usually the hen lays its egg during the day. On average, 30 minutes after an egg has been laid, the ovulation of a new egg cell occurs; at that point the chicken 'cackles' (see also Figure 2.2 and 2.4.). In principle a hen can ovulate every day, because no corpus luteum is formed. The second egg is laid later in the day than the first egg and the third later again etc. When an egg is ultimately laid late in the light phase, the subsequent ovulation does not occur after 30 minutes. The next egg cell then ovulates 16 to 18 hours later, meaning that no egg is laid for a day. This unproductive day forms the end of a laying series; a laying series is defined as the number of eggs that a hen lays on subsequent days (see Table 2.1). Poor layers have laying series of 1 or 2 eggs, whereas good layers have series of 4 up to 200 eggs (see for example Figure 2.6). Good layers presumably have higher hypophysis hormone concentrations (FSH and LH). 16 Table 2.1 Average number of eggs per laying series (series size) and maximum number of eggs per year if the eggs are removed from the nests (size of the laying cycle) of different bird species, in captivity (Campbell and Lasley, 1969). Bird species Average series Size of the laying cycle size (number of eggs) Chickens: Laying chickens 10 - 14 300 - 500 Meat chicken (broiler) mothers 10 - 14 190 - 200 Ornamental chickens 10 - 14 60 Ducks: Laying ducks 14 - 20 250 - 310 Meat ducks 14 - 20 120 Turkeys 15 - 20 220 Geese 12 - 15 100 Ostriches 12 - 15 100 Pheasants 10 - 12 104 Quail 12 - 20 130 Pigeons 2 50 Canaries 4- 6 60 Figure 2.6 Diagram of different laying series with chickens (Cole and Ronning, 1974). 17 2.3 The laying cycle The total egg production (E) of a hen in number of eggs is determined by the length of the laying cycle in days (T) and the intensity of lay in this period in percent (I): E = T  I. The starting time of the lay is a measure of the sexual maturity of the hen. Light again appears to be the most important influencing factor; an increasing day length advances sexual maturity, whereas a decreasing day length causes the opposite. But other factors also have a role, because not all hens lay their first egg at the same age when bred under the same circumstances. This implies that there are also genetic differences for instance. When a hen is sexually mature early, this usually indicates high reproductive activity. There are also many factors that can influence the end of the laying cycle. Broodiness is one of them; if the eggs are not removed from the nest, the hen may become broody. The pressure of the eggs against the breast of the hen informs her whether sufficient eggs have been laid ('sufficient' means as many eggs as she can cover with her body and incubate effectively). This leads to her ceasing to lay eggs. The hormone prolactin appears to play a role here. Broodiness is largely genetically determined and also dependent on the temperature, the light regime and the opportunity to build a nest. Commercial laying chickens usually no longer become broody as a result of different housing methods (a lot of light, battery cages etc.) and selection. A second factor that may go together with the end of a laying cycle is moulting. Moulting is the falling out of old and growth of new feathers; this usually goes together with ceasing to lay eggs. The moulting period is a type of physiological resting phase or pause in the laying. The thyroid hormone presumably has an important role here. By means of housing and genetic measures, humankind has caused hens to go into moulting later and later; complete suspension of the moult is still not possible, but laying chickens can currently be kept up for more than 100 weeks of age without moulting. On the other hand, the moult is often initiated artificially. This can be done by means of a sudden decrease in the amount of light and by a sudden reduction in the provision of essential feed components or water. The aim of this artificially caused moult ('forced moulting') is that the hens are able to recover physiologically in order to commence a second good laying cycle after the moult. The hens are then kept for two relatively brief laying cycles, instead of one relatively long laying cycle. This method is mainly used outside Europe. Forced moulting is banned in the European Union. The end of the lay can also be reached as a result of a shortage of essential nutrients for egg production through a reduction of reserves in the body. A reduction in the quantity of calcium in the feed in particular, and thus also in the body, could lead to a halt in laying. The intensity of lay (I) is determined by the length of the laying series, the length of the pauses between the laying series and the regularity of the laying series; in brief the number of productive days per laying cycle. The shorter the interval between oviposition and ovulation, the more eggs are laid per laying series, and the more productive days there are. This is largely genetically determined. With reference to the intensity of lay, the laying cycle of a hen can be subdivided into three sections (see Figure 2.7). 18 Figure 2.7 Egg production curve (Identification of promising mutants with egg production traits revealed by GWAS, Jingwei Yuan et al, 2015). The first period forms the start of the reproductive activity, from the first ovulation up to the start of the regular lay and usually lasts 2 to 3 weeks. Laying in this period is rather irregular. Irregularities that often occur include: (1) production of more than one egg per day, one or both of which are often abnormal, (2) production of wind eggs, (3) production of double yolks, (4) irregular production with long intervals between the eggs, and (5) production of yolks not captured by the infundibulum, also referred to as 'internal laying' (see Figure 2.7). The second period is characterised by a relatively regular lay and lasts the longest, so that this period has the most influence on total production. The third period is relatively short and characterises the end of the lay. This period looks like the first period with respect to the production of abnormal eggs. 19 Figure 2.8 Certain frequently occurring 'errors' at the start of the lay. SS = “softshell” or wind eggs (Bell and Freeman, 1971). Egg production is most intensive in the first laying cycle; in the subsequent cycles production declines almost linearly (see Table 2.2). In addition, the hen starts laying later and stops increasingly soon. Laying hens are usually kept for just one laying cycle. Table 2.2 Egg production over the years (Why have my hens stopped laying eggs, submitted by Neil Armitage, 2018) Age (years) Hybrid chicken Heritage breed Number of eggs per year Number of eggs per year 1 340 240 2 310 220 3 200 180 4 30 130 5 0 90 6 0 40 7 0 20 Total 880 920 Working life is negatively correlated with speed of reproduction; hens with relatively high production life live shorter than hens with relatively low egg production. Independently of the hereditary predisposition of a hen for a good laying performance, it should also have a good hereditary predisposition for vitality and constitution to be able to withstand the physiological strains resulting from high egg production. These strains on the body could affect health and resistance to disease, and shorten working life. Figure 2.9 shows the external factors that can influence egg production. 20 Figure 2.9 Schematic representation of different environmental factors that can influence egg production (Bell and Freeman, 1971). The hen makes only a limited contribution to its environment; humans 'make' the environment. Light is presumably the most important of the physical environmental factors with respect to production. The influence of the feed is considerable, particularly with high-producing birds. A shortage of a particular nutrient will cause a drop in egg production as a result of a shortage of the material needed to form an egg. 2.4 Differences between eggs Not only the quantity of eggs produced can be influenced by all kinds of internal and external factors, but also the size and quality of the eggs. Eggs can differ considerably in size and weight; the heaviest chicken egg recorded so far weighed 320 g and the lightest 1.3 g. Sources of variation include: - hereditary predisposition; certain hens lay larger eggs on average than others. - age of the hen. The size on an egg increases over the laying cycle. This increase is relatively large in the first part of the lay; thereafter it is much more gradual (see Figure 2.10). The hen lays smaller eggs again after moulting. - location in the laying series; the first egg in the series is usually the heaviest. After that the eggs get lighter and lighter. This is caused mainly by an egg cell that gets smaller and smaller. - body size; larger hens usually lay larger eggs. - intensity of lay; hens with very high egg production in general lay smaller eggs than hens with a low intensity of lay. - temperature; high temperatures cause a decline in egg weight. - feed; certain nutrients may influence egg size. Higher protein content in the feed for example leads to a higher egg weight. 21 Figure 2.10 Egg production and weight as a function of age. (Raising chickens for egg production, Dr Jacquie Jacob, 2015) It is noteworthy that a hen lays eggs that do not vary much in composition day to day. However, the first eggs that a hen lays consist of relatively small egg cells, around which the oviduct deposits less egg white than around large egg cells in absolute and relative terms. The result is, compared with an egg from an older hen, a smaller egg with proportionately more yolk, less egg white and just as much shell. See figure 2.11. 22 Figure 2.11 Egg mass components shell, yolk and albumen as a function of total egg mass. (Dao H. Ho, Wendy L. Reed, Warren W. Burggren, Journal of experimental Biology, 2011) 23 Practical manual: Egg (Watch the instruction clip on Brightspace before you visit the practical!) Animal species: Chicken Location: Forum Building P0720 24 Introduction In this practical, we will look into the quality of the egg (A) and into the quality of the taste of eggs (B) The egg: Figure 1. Cross section of an egg. 1. Scale - outermost protection of the egg, consisting of calcium carbonate (lime) - is white or brown, depending on the species - shell colour does not influence quality, cooking characteristics or nutritional value 2. Yolk - the yellow part of the egg - colour varies according to the feed that the chicken receives, but says nothing about the nutritional value - most important source of vitamins, minerals and fat 3. Blastodisc 4. Yolk membrane - a clear membrane that keeps the yolk mass in position 5. Chalazas - twisted, cord-like strands of egg white - they keep the yolk in the centre of the egg - strong chalazas indicate high egg quality 6. Air chamber 25 - a bag of air at the rounded end of the egg - caused by shrinking of the egg contents as a result of cooling after laying - increases in size as the egg ages 7. Shell membranes - two membranes, the innermost and outermost shell membranes, surround the egg white - these form a protective barrier against penetrating bacteria - the air chamber forms between these two membranes 8. Thin egg white - closest to the shell - surrounds the thick egg white of a quality egg 9. Thick egg white - outstanding source of riboflavin and protein - stands up higher and spreads less than the thin in quality eggs - it becomes thinner and more watery and less distinguishable from the thin egg white with eggs of lower quality Quality characteristics of the egg Egg quality is an increasingly important characteristic from an economic aspect. When speaking of egg quality, we distinguish two types: external and internal quality. 1. External egg quality Externally, the following characteristics are significant to a greater or lesser degree: 1.) size and weight; in the Netherlands there are 7 weight classes (the classes 1 to 7 are respectively: >70, 65-69, 60-64, 55-59, 50-54, 45-49,