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Summary Biology of Domestic Animals YAS10806
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CASE 1 – EGGSFunction of eggs
- Reproduction
- Dimensions of eggs are related to the quantity of yolk birds lay large egg cells containing mostly
yolk and a small part of cytoplasm.
- Embryo develops in the egg outside of mother’s body, so egg must contain all nutrients for its
growth.
Egg production
- Chicken eggs are most popular, but also duck and geese eggs are produced. Ancestor of chickens
Red junglefowl
- In 2010, 10 billion eggs produced in the NL; only 3 billion were consumed in the NL 185
eggs/person/year, the rest was exported.
- Farming systems in NL:
o Organic organic feed; can go outside. Code on organic eggs is 0.
o Free range can move freely. Code on eggs is 1.
o Barn free to move. Code on eggs is 2; called “scharrell eggs”.
o Cage battery cages were banned in 2012 in the EU; only enriched cages can be used
(bigger, with nest etc). Code on eggs is 3.
- Types of eggs:
o Organic eggs
o Scharrell eggs (barn)
o Omega-3 eggs more linseed in feed, which is then converted into omega-3 fatty acids
after digestion.
o Maize eggs feed is 60% maize, so they contain 35% more linoleic acid and less fat.
Structure of the egg (study fig page 5 and 6)
- Chicken egg approx. 58g, 32% yolk, 56% albumen, 12% shell.
- Yolk section of 3cm approx.; surrounded by a vitelline membrane to which the chalazas are
attached; on the yolk surface there is the germinal disk, a small spot from which the embryo will
develop.
- Albumen nutritional function, mechanical and chemical protection because contains substances
with antibacterial effect. It consists of a thick layer around the yolk, and an outer thin layer.
- Chalazas strands of egg white that keep the yolk in the centre of the egg (3 alla punta e 2 al
bottom).
- Shell made for 93-98% of calcium carbonate (CaCO3); it is porous, so gas can be exchanged
between the embryo and the environment (but bacteria can also come in!); it is covered with the
cuticula, that reduces these exchanges, but its permeability increases during incubation so 15% of
the water in the egg evaporates.
- Shell membranes two inside the shell; they are close together except at the rounded end of the
egg, where they form the air chamber (here the shell is most porous, it becomes bigger with time).
,Chemical composition
- The albumen is the most watery part ; the yolk contains mostly fats and also the largest part of the
proteins in the egg.
- Proteins in the albumen ovalbumin (most common), ovomucoid, ovomucin and ovoglobulin.
- Proteins in the yolk mostly vitellin and livetin. They often form bonds with lipids lipoproteins.
- Fat (only in the yolk) biggest part of the yolk, main source of nutrition of the embryo. They are
mostly triglycerides (only with saturated fatty acids), but also phospholipids and sterols (mostly
cholesterol).
Quality of eggs (see figures on practical part)
External quality:
- Size and weight There are 7 weight classes in the NL; the 3 class is the most common (60-64g).
- Shape and appearance shape index = width/length * 100. The ideal index is 72-76 because they
can easily fit in the boxes. If it is < 72/74 elongated egg; > 76 spherical egg.
- Shell strength it has to be strong to avoid cracking during transportation. Thickness is measured
at the widest part of the egg, where the shell is thinnest this allows to see how strong it is in the
thinnest part; it can decline due to high temperatures, calcium deficiency in the feed, stress,
disease, medications. Abnormalities in the shell very rough surface; lots of pores.
White or brown shell depending on the earlobes of hens; Dutch prefer brown eggs for direct
consumption, while white eggs are mostly used for other products. Shell color does not influence
egg quality. Shell quality can be improved by feeding hens with limestone.
- Size and shape are economically important because if the eggs do not fit in the boxes, they can be
easily damaged during transportation.
Internal egg quality:
- Yolk color it actually tells nothing about the quality; ideal color depends on consumer’s wish.
Xanthophyll substance that determines the color intensity of the yolk depending on its
concentration in the feed.
- Blood and meat spots undesirable.
- Egg white thin white and thick white can be distinguished in a quality egg. Height of the thick
white is measure of quality, and depends on the freshness of the egg and the stiffness of the white
itself. The quality is measured in Haugh units HU HU = 100 * log (H -1.7*G0.37 + 7.6), where H is
height of thick white in mm and G is egg weight in g.
Number of HU:
o > 72 very good quality
o 60-72 good quality
o 31-60 non-standard quality
o < 31 very non-standard quality
Formation of the egg (only discussed for hens)
- Egg fertilisation is not necessary for laying (one fertilization is enough and the hen will produce eggs
for a long time) we consume unfertilised eggs
, - Egg production depends on hormones FSH promotes yolk formation and growth of follicles in
the ovarium; LH ensures ovulation when a follicle (egg cell) is ripe.
- After ovulation the egg cell enters the oviduct and is surrounded by the albumen and shell
membranes; the shell is formed in the uterus (also called shell gland). The formation of an egg
takes about 21 hours (1 egg a day).
- A chicken shows no oestrus during its cycle, and no corpus luteum is formed after ovulation.
Laying the egg
- Oviposition lasts a few minutes, under the action of the oxytocin hormone. It occurs during the day
(it is largely influenced by light).
- 30 minutes after an egg has been laid, a new ovulation occurs. However, if an egg is laid at the end
of the daylight, the next ovulation will be 16-18 hours later so there is an unproductive day end
of a laying series.
laying series number of eggs that a hen lays on subsequent days. Good layers have series of 4-
200 eggs.
Laying cycle:
- Total egg production (E) of a hen is determined by the length of the laying cycle in days (T) and the
intensity of lay (I) E = T * I
Intensity of lay number of productive days per laying cycle.
- Starting time of the lay is a measure of the sexual maturity of a hen. Light and genetic differences
are the main factors influencing the start of the lay.
- The end of the laying cycle can be influenced by:
o Broodiness if the eggs are not removed from the nest, hens might become broody
(hanno voglia di covare/diventare madri)
o Moulting (fare la muta, influenzato da tiroide) it is a form of physiological pause from
laying. It can be induced by not giving food, water and light for one day. Currently, hens can
be kept without moulting for more than 100 weeks.
o Shortage of essential nutrients for egg production due to shortage of reserves in the body
(especially calcium)
- Laying cycle of a hen is divided into:
1. Start of the reproductive activity at 23 weeks ; the first phase lasts 2-3 weeks. Laying is irregular
more than one egg/day, wind eggs (without yolk or shell), double yolks, long intervals
between eggs.
2. Longest period, from 25th to 74th week; laying is regular so has most influence on total
production.
3. End of the cycle, short period. Production of abnormal eggs like in first period.
- Laying hens are usually kept for only one laying cycle, because after the first cycle hens start to
produce less until it stops.
Differences between eggs
Caused by:
- Hereditary predisposition
, - Age of the hen egg size increases over the laying cycle. After moulting, the hen lays small eggs
again.
- Location in the laying series first egg is usually the heaviest. Then they become lighter and
lighter.
- Body size big hen = big egg
- Intensity of lay high egg production = smaller eggs
- Temperature high temp. = smaller eggs
- Feed nutrients can influence egg size, e.g. high protein = heavier egg.
- Small eggs have in proportion more yolk and less egg white if compared to big eggs.
Practical
- Quality of the egg and HU units
- The number of eggs and their weight are economically important.
- The largest cost in egg production is the feed.
- Egg candling (putting a light source behind the egg to see details through the shell) when eggs
are incubated, they are candled on the 6 th day to see if they have been fertilised and if the germinal
disc has developed well. If it is fertilised, they have a dark spot on the yolk (germinal disc?); if the
spot is attached to the shell, the germinal disc is dead.
Eggs are candled also on the 18 th day: if the chick is well developed, the egg is completely dark
except for the air chamber.
- Eggs are lighter after boiling them, than before boiling.
CASE 2A – SPERM (PIG)
- Spermatozoid male gamete; small and very mobile; continuously produced in very large
quantities.
- Oocyte female gamete; larger and immobile; produced intermittently at regular intervals.
Ovarian cycle
- Ovulation release of one or more oocytes from the ovaries (NB: cows do not ovulate during
heat)
- Oestrus cycle period between two ovulations. It lasts 21 days in sows. Four phases:
1. Prooestrus: lasts 3-4 days. Period when the antral follicles develop into Graafian follicles.
Ovaries are under the influence of the hormone FSH (Follicle Stimulating Hormone).
2. Oestrus: 2-3 days; the female is in heat, shows oestrus behaviour and allows mating (if you let a
boar walk in front of the sows, they will stand up and alert so-called “standing response”). At
2/3 of oestrus, ovulation of the follicles occurs under the influence of the hormone LH
(Luteinizing Hormone) and the released oocytes can be fertilised.
3. Metoestrus: 2-3 days. the remains of the Graafian follicles luteinize because of LH, and form
the corpora lutea that produce progesterone (hormone needed for pregnancy).
4. Dioestrus: 11-13 days. Functional corpora lutea are present in the ovaries. If the female does
not get pregnant, the regression of corpora lutea occurs and the prooestrus starts again.
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