Download Part 4: Breeding

Cattle production
Part 4: Breeding
Introduction ............................................................................... 2
Hormones.................................................................................. 3
Female..............................................................................................4
Male................................................................................................10
Fertility ................................................................................... 13
Genetics .........................................................................................14
Nutrition ..........................................................................................15
Climate ...........................................................................................17
Disease...........................................................................................19
Management ..................................................................................20
Breeding systems.................................................................... 21
Objectives.......................................................................................22
Systems..........................................................................................23
Genetics .........................................................................................26
Summary................................................................................. 30
Suggested answers................................................................. 31
Exercise – Part 4 ..................................................................... 34
Part 4: Breeding
1
Introduction
Introduction
Efficient reproduction that produces the best quality and quantity of
offspring is central to many animal enterprises. Females that are slow to
fall pregnant or do not manage to produce offspring can be a significant
drain on farm resources. These animals continue to use resources such as
feed and veterinary chemicals and will often be in better condition than
others because they are putting energy into their own growth and
development rather than into production of offspring.
In this part you will examine the central role of hormones in the
reproductive process and their effect on animal behaviour.
You will explore the various factors that can influence the fertility of
animals and how they can be managed efficiently.
Then you will discover how farm managers decide which animals to use
for breeding and the breeding systems that can be used to best meet their
goals.
This part contributes towards an understanding of Outcome H2.2 from
the Agriculture Stage 6 HSC Course. The syllabus can be found on the
Board of Studies, NSW website at http://www.boardofstudies.nsw.edu.au
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Cattle production
Hormones
Hormones
Hormones are sometimes called the chemical messengers of the body. They
are chemicals that are produced in one organ of the body then transported to
act in another part of the body. Endocrine glands produce hormones that are
secreted directly into the blood.
hypothalamus
pituitary
pineal
adrenal
parathyroid
th yroid
ovary
(female)
placenta
(pregnant female)
pancreas
testes
(male)
Figure 4.1: Location of the major endocrine glands of a sheep.
In some cases artificial hormones have been developed that mimic the
actions of the natural hormones. These are used in some production
systems to manipulate reproduction, behaviour and growth of farm
animals.
Revise the anatomy of male and female reproductive tracts in Part 2 of
Looking at ewe. You will need to be able to identify the parts and their
functions in order to understand the actions of the reproductive
hormones.
Access an interactive version of the male and female reproductive tracts using
this link.
In this course you will be looking at some of the hormones that regulate
animal reproduction and behaviour. It is important for farm managers to
understand hormones in their animals to allow best management practice
and efficient use of breeding technologies.
Part 4: Breeding
3
Female
Oestrus behaviour
Oestrogen is an important female hormone secreted by the ovary. One
of the effects of oestrogen is to cause a pattern of behaviour called
oestrus, also referred to as heat, or being in season. This is the time when
the female is sexually receptive, or willing to mate.
Some reproductive characteristics for different farm animal species are
listed in the following table.
Figure 4.2: The reproductive characteristics of different farm animals.
(Reproduced from The Scientific Basis of Modern Agriculture Ed. Keith
O.Campbell, 2006, p240 © Sydney University Press.)
As you can see from the table, the optimum time for mating occurs
during or shortly after heat. This is when ovulation occurs. Farm
managers should become familiar with the behaviour patterns associated
with heat, so that they can arrange joining or artificial insemination and
their female animals will become pregnant.
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Cattle production
Karen Fitzgerald runs Murray Grey cattle on her property on the NSW
Northern Tablelands. She selects her best cows for an artificial insemination
program. The following transcript describes what Karen is looking for when
checking these cows for signs of oestrus.
“Throughout the period of time that joining takes place
I ride around my cows first thing each morning and last
thing before dark each evening. While I ride around I
watch my cows closely looking for signs of oestrus or
heat.
I use the general rule for artificially inseminating my
cows. That is, those cows that show signs of oestrus in
the morning are artificially inseminated in the late
afternoon or evening and those that show signs of
oestrus in the evening are inseminated first thing in the
morning.
To ensure that my cows have the best chance of
conception I need to be able to recognise when they are
in oestrus. I know my cattle pretty well, and so I am
aware of typical behaviour. Most cows show some or
all of the following signs.
The most obvious sign of oestrus is a restless cow, she
is more active, bellowing and she tends to wander
about mounting other cows. She may show signs of
having stood to be mounted herself. These signs
include mud on her flanks and hair off her tail. When
you get close up to her you notice that her vulva is
moist and swollen and is discharging clear mucous.
You always need to keep your notebook and pen
handy. You need to note down the cow's ear tag
number and go home and check your records.
The cows showing signs of oestrus, I bring in closer to
the yards ready for inseminating either late that
afternoon or first thing the next morning.”
Use information from the transcript to answer the following questions.
1
Identify four types of behaviour that might be shown by a cow in
oestrus.
_____________________________________________________
_____________________________________________________
Part 4: Breeding
5
______________________________________________________
______________________________________________________
2
Identify two features of the appearance of a cow in oestrus.
______________________________________________________
______________________________________________________
3
Compare the timing of artificial insemination by Karen with the
optimum timing given in Figure 4.2.
______________________________________________________
______________________________________________________
Check your answers.
The oestrous cycle
Oestrogen and other hormones work together to produce a cycle of events
called the oestrous cycle. They operate on a feedback system where the high
or low level of a hormone influences the release of other hormones.
If you look again at Figure 4.2, you can see that the timing and length of
the cycle varies with the species of animal. Within the breeding season a
healthy, mature female will continue to go through the cycle until she
falls pregnant.
Read through each of the following steps and compare them with Figure
4.3. This will help you understand the interrelationships between the
hormones.
Step 1 The pituitary gland releases follicle stimulating hormone
(FSH) which travels in the blood to the ovaries. In the ovary,
the FSH stimulates the growth of Graafian follicles on the
ovary. The follicle is like a fluid filled bubble containing a
ripening ovum. As the follicles develop they begin to release
oestrogen.
Step 2 Oestrogen levels in the blood increase to the point where the
pituitary gland reduces the secretion of FSH and increases the
secretion of luteinising hormone (LH).
Step 3 LH secreted by the pituitary gland causes the follicle to rupture.
The ovum is released, and if the animal is mated or inseminated
at the right time, the ovum may then be fertilised. The ruptured
follicle forms a corpus luteum (yellow body) on the ovary.
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Cattle production
Step 4 The corpus luteum secretes progesterone and oestrogen.
Progesterone depresses the production of FSH and prevents
development of ovarian follicles. Hence while the levels of
progesterone are high, oestrus and ovulation cannot occur.
If an animal becomes pregnant then the corpus luteum persists
for the duration of pregnancy, and the progesterone it produces
maintains the pregnancy.
Step 5 In the non pregnant animal the corpus luteum persists for 12–14
days in ewes and 16 days in cows. As the corpus luteum
regresses, the level of progesterone decreases. The pituitary
gland releases FSH and continues the oestrous cycle.
1. follicle stimulating hormone
pituitary
gland
fo llicle
ovary
2. oestrogen
ovum
ovary
co rp us
luteum
ovary
if animal
NOT p re gnant
ovary
corpus
luteum
regresses
Figure 4.3: Interrelating steps which bring about oestrus and ovulation.
Part 4: Breeding
7
Use the words supplied to complete the following passage.
corpus luteum, ovary, release, regressed, LH, FSH, feedback,
oestrus, decrease, progesterone, low
The pituitary gland secretes the hormone
which causes a follicle in the ovary to mature and ripen producing the
hormone oestrogen. Oestrogen is the hormone responsible for an animal
showing typical
behaviour. It is produced by
the
.
The production of oestrogen stimulates the pituitary gland to
the production of FSH and
the hormone LH.
,
produced by the pituitary gland, causes a growing follicle in the ovary to
is formed
rupture and an ovum to be shed. A
where the follicle has ruptured, producing the hormone
.
While the corpus luteum persists, levels of the hormone FSH remain
and another follicle cannot mature.
Once the corpus luteum has
, progesterone
levels drop and the pituitary gland releases FSH again, continuing the
cycle. This interrelationship between hormones, rising and lowering of
hormone levels in response to each other is called
.
Check your answers.
Pregnancy and birth
Behavioural changes that occur when an animal is pregnant are largely a
result of the high levels of progesterone in her body. Progesterone
stimulates the development of the wall of the uterus, which protects and
nourishes the embryo. As the gestation period progresses in some
species the placenta will take over progesterone production.
During pregnancy, progesterone and oestrogen stimulate growth of the
mammary gland in preparation for lactation.
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Cattle production
Parturition (birth) is brought about by interactions between the hormones
of the foetus, placenta and mother. As the mother's hormonal balance
changes, the passage through the cervix and vagina enlarges. Continuous
contractions of the uterine wall then force the foetus through the cervix
into the vagina and finally through the vulva. The placenta is then
expelled.
heat
gestation (late)
parturition
mating
gestation (early)
lactation
Figure 4.4: Pregnancy and birth
Part 4: Breeding
9
Male
A mature male animal will maintain relatively constant levels of
hormones. This means that the hormones of the male are easier to
understand than the complex cycle of the female.
Libido
The male sex drive, or libido, is a behaviour pattern influenced by the
production of testosterone the major male hormone. Libido refers to the
level of interest in females and ability to inseminate them.
Libido can be measured by a serving capacity test. This counts the
number of times a male animal mounts and inseminates females in a
given time in a yard. This test is only an indication, and does not always
correlate well with performance of the same sire in a paddock. Libido is
not necessarily related to the amount or quality of semen produced.
The male libido is responsible for the typical behaviour of a male animal.
Entire males are more aggressive and difficult to handle. They tend to
fight and try to establish dominance over other animals.
Sigh...
They’re always
fighting over me!
I love the quiet laid back life of a steer...
no pressure to perform...no competition!
Figure 4.5: Testosterone makes males aggressive.
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Cattle production
The behaviour of males is the major reason that farmers tend to castrate
male animals that are not required for breeding. It would be much more
difficult to keep large numbers of bulls than large numbers of steers.
Reproduction
The pituitary gland constantly secretes FSH and LH. FSH stimulates the
growth of seminiferous tubules, and the production of sperm. LH stimulates
the cells of Leydig in the testes to produce testosterone. Testosterone is
needed to mature sperm, and to develop the organs and glands of the male
reproductive tract. Testosterone also causes development of other male
body characteristics.
Part 4: Breeding
11
Use your knowledge of hormones to complete this crossword.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Clues
Across
Down
3
This hormone causes females
to go into season.
1
Yellow body, secretes
progesterone.
7
This hormone maintains
pregnancy.
2
Mate.
4
Look at this for the
identification number of the
cow on heat.
12 Male sex drive.
5
13 Hormone produced in the
testes.
The reproductive cycle of a
female.
6
14 Hormone that stimulates
development of follicles.
This animal gestates for about
5 months.
8
15 This is often present when a
female is on heat.
LH is secreted and then this
happens to the follicle.
9
Parturition.
10 Production of this is stimulated
in the male by FSH.
11 These animals produce the
hormone testosterone.
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Cattle production
Fertility
Fertility
Access an interactive version of Fertility using this link. Resume with this
material on page 22: Breeding systems.
The term fertility refers to the reproductive capacity of an animal. It is
usually measured in terms of the number of independent offspring produced
by an animal over a period of time.
If a male animal is very fertile he will have good semen production, with
sperm of high quality and quantity. He will also have the ability and
desire to mate with a large number of females.
A highly fertile female will have a regular oestrous cycle and will release
one or more mature ova in each cycle. She will have the capacity to fall
pregnant quickly and to carry the resulting offspring to term. The fertile
female will birth easily, bond strongly with her offspring, and lactate
sufficiently to nourish her offspring until independence.
There are a number of factors that can influence the fertility levels of
animals. As you read about these factors try to think of management
strategies that farm managers could use to overcome limiting factors, and
promote those that are positive. Some strategies are written down in the
text, while others you will have to think about yourself. Note down
management strategies in the space below so you can refer to them later.
A management strategy is an action that can be taken by the farmer.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 4: Breeding
13
Genetics
Between species
There are differences in the level of fertility between different species of
animal, for example, a sow may produce two litters of piglets each year
with 10 piglets per litter, while a cow will normally produce only one
calf per year.
Within species
There are also fertility differences within a species of animal. In some
sheep breeds there is a greater likelihood of multiple births than in others.
A flock of Border Leicester ewes are likely to have a lambing percentage
of 150, while fine wool Merino ewes will only have a lambing
percentage of 80.
Multiple births have a genetic influence. An animal that is born as a twin
is more likely to produce twins. This characteristic was used to produce
the Booroola Merino. This breed has the highest lambing percentage of
any sheep breed, with an average of 240 percent. Booroola Merino sheep
producing large litters were originally found on the Booroola property of
Dick and Jack Seears in Cooma. The CSIRO developed the Booroola
strain by selecting sheep for breeding that were multiples themselves,
and continuing to select for this characteristic over several generations.
The gene that causes these sheep to produce large litters has since been
identified and is now called the Booroola gene.
Figure 4.6: Booroola merino ram.
© http://www.ansi.okstate.edu/breeds/sheep
It is undesirable to develop strains of cattle that produce multiple births
because in 90 percent of twin cattle there is fusion of the blood supply
between the calves while in the uterus. If one calf is male and the other
14
Cattle production
female, the additional male hormones received by the female calf will
cause her to develop into a freemartin, that is, a sterile female. With
multiples there is also a greater likelihood of dystochia, that is problems
associated with giving birth. Dystochia can lead to the death of the
mother and/or offspring.
Sometimes fertility levels of an animal can be affected by inherited
genetic abnormalities. These diseases may prevent implantation, cause
abnormal development or cause death of newly born offspring.
Nutrition
The level of nutrition of an animal is important in enabling the animal to
become sexually mature; permitting reproductive function, and
developing the offspring to independence.
The onset of puberty in cattle is determined more by the bodyweight of
the animal than by age, so nutrition level is a significant factor in
determining when the animal will begin reproduction. Heifers must
reach a critical mating weight of around 280 kg prior to joining, and
continue to gain weight over the joining period.
Figure 4.7: Body weight is important in determining onset of puberty.
Part 4: Breeding
15
Poor nutrition
In some situations, for example drought conditions, there is an
insufficient level of nutrition. This can have important effects on
fertility. Mature female animals on a low plane of nutrition and below
normal body weight will have irregular or absent oestrus, lower ovulation
rates and decreased fertility. If these females become pregnant then the
offspring will be smaller and weaker, so less likely to survive. The
mother will produce little milk and may not show maternal instinct,
further decreasing the survival chances of the offspring.
Low levels of nutrition in heavily pregnant cows can result in a condition
called pregnancy toxaemia. This can cause the death of the cow unless
she is treated.
Good nutrition
Many sheep farmers provide a high plane of nutrition to their flock
immediately before joining. This is known as flushing. Flushing can
cause ewes to develop and shed a higher number of ova, and may also
increase the number of sperm produced by rams. This results in an
increased lambing percentage.
Steaming up is the management practice where the mother is provided
with a high level of nutrition shortly before parturition (birth). This
improves the milk production of the mother, and increases the size of the
offspring. A very small newborn animal has a lower chance of survival,
particularly if born during colder weather. Producers must be careful,
however, as very large offspring can result in dystochia.
Diet
Animals also need to eat balanced
levels of different nutrients to be at
their healthiest. If a diet is too low
or high in a particular nutrient, it
can cause illness, including some
effects on fertility. For example,
protein deficiency, and copper
deficiency have both been shown
to reduce fertility of females, and
Vitamin A deficiency can prevent
normal sperm production in males.
Figure 4.8: Eating a balanced diet.
16
Cattle production
Plant type can also affect fertility. Some varieties of clover contain
particularly high levels of a form of oestrogen. Subterranean clover
varieties with this problem include Dwalganup, Yarloop and Dinninup.
Ewes grazing on this clover can develop a condition known as clover
infertility disease. Plant breeders have developed low oestrogenic clover
varieties to use in situations where this is a problem.
Climate
Temperature
Cold temperatures, especially in combination with wind, can lead to
weakening or death of newborn animals. Newborn lambs are particularly
susceptible.
If you listen carefully to weather reports in winter and early spring you
will sometimes hear that a ‘grazier’s alert’ has been issued. This is to
warn managers that cold windy weather is expected, so they can put their
susceptible sheep into sheltered paddocks.
High temperatures can cause reduced birthweight, or cause death of the
embryo or foetus.
High temperatures can affect the fertility of male animals by decreasing
libido, and reducing the production of normal fertile sperm. To ensure
fertility during hot summers, managers must make sure there is adequate
shelter and plenty of water available.
Daylength
Some animal species, such as cattle and pigs, are able to breed
throughout the year. These species are called polyoestrous breeders
(poly = many). Other animal species, such as sheep, goats and poultry,
have a restricted season in which they will breed, and are called
seasonally polyoestrous breeders. Dogs have a single oestrous cycle in
the year, that is not related to season, and are called monooestrous
breeders (mono = one). Deer have a single cycle at a particular time of
year, and are called seasonally monoestrous breeders.
Part 4: Breeding
17
Those animal species that are seasonal breeders have developed the
timing of their breeding season by natural selection so that offspring will
be born in Spring when conditions are optimum, and they are most likely
to survive.
In sheep the breeding season is triggered by decreasing daylight hours.
The changing patterns of daylight are registered by the retina in the eye,
and a message is relayed to the pituitary gland, which is stimulated to
produce FSH.
Figure 4.9: Effect of daylength on fertility. (Reproduced from Introduction to
Agriculture by J.A. Sutherland, 1980, p 237 © McGraw-Hill Book Company)
1
Predict when the breeding season of sheep will occur. Look back at
Figure 4.2 and use the length of gestation to assist you.
______________________________________________________
2
Poultry only need a few weeks from mating until eggs are hatched.
If the eggs hatch in Spring, what will be happening to the daylight
hours at the time they are mating?
______________________________________________________
Check your answers.
18
Cattle production
There can be advantages to a farmer who breeds animals outside their
normal breeding season. Prices for prime lamb vary through the year, and a
farmer may try to get better prices by manipulating the breeding season and
so altering when lambs are born and then marketed. Housed animals, such
as poultry, can easily be manipulated by changing the pattern of lighting in
their sheds.
Disease
Disease can affect fertility by causing loss of stamina and vigour in affected
animals. Females will be less likely to cycle or conceive and males less
likely to mate. Any disease that produces a fever will reduce the fertility of
males as sperm can be damaged by high temperatures.
Animals can also be affected by diseases specific to the reproductive
tract (venereal diseases), which can result in lack of implantation,
abortion, and in death and absorption of the foetus. Usually these
diseases are spread when the animals are mating. Male animals are often
symptomless carriers of the disease. Venereal diseases of domestic
animals include brucellosis, vibriosis, listeriosis, trichomoniasis and
leptospirosis.
Brucellosis
In 1970 a campaign was established in Australia to eradicate brucellosis
from cattle. The $840 million campaign was funded from a transaction
levy that is imposed by the government when cattle are sold. In 1989
Australia was declared free of brucellosis in cattle.
Leptospirosis
Leptospirosis is a disease of cattle, pigs and humans that causes abortion.
The pathogen is found in the kidneys of affected animals and can be
spread by contact with urine.
Humans can become infected by contact with urine or through contaminated
water, soil, or vegetation. Affected people suffer from severe aching of the
muscles, and chills that are followed by rapidly rising temperatures. Other
symptoms include nausea, vomiting and anorexia. Pregnant women may
abort. Treatment of leptospirosis is successful if started in the early stages
of the disease.
Part 4: Breeding
19
Prevention is possible by vaccinating cattle and by good management
practices such as wearing surgical gloves and using disinfectant when
handling animals with dystochia.
If you visit a pig or cattle farm you should check with the farmer to find out
if the animals have been vaccinated.
Management
If management does not provide good conditions for joining, the result may
be poor levels of fertility. Factors such as overlarge paddocks and too few
bulls can result in cows not being mated.
If the breeding of animals relies on an artificial insemination program, it
is essential that there is good detection of heat.
Newborn animals can die because of predator attack, mismothering and
weather extremes. Appropriate management can try to prevent these
deaths.
Use your list of management strategies to help you complete Exercise 4.1.
You will need to describe and evaluate the major management strategies of
those you have identified.
20
Cattle production
Breeding systems
Breeding systems
The selection of particular types of animal for breeding within agricultural
production systems is critical to the successful sale of produce from a farm.
Selection is an important tool for making a particular trait, or feature, occur
more often in a herd. The following diagrams show some traits that are
important in farm animals.
strong, straight back
wid e pin bones for
easier birth
ability to produce one
calf each year
high udder
straight teats
sound legs
and hooves
high milk production
Figure 4.10: Important traits of dairy cattle.
high fertility
high yield of lean
me at backfat
high growth rate
litter and pre weaning survival
Figure 4.11: Important traits of pigs for pork production.
meat production
egg production
carcase quality
age at first egg
body weight
number of eggs
laid to set age
feed conversion
efficiency
egg size and quality
eg texture and shape
Figure 4.12: Important traits of poultry for meat and egg production.
Part 4: Breeding
21
Artificial selection involves actively selecting the best animals to breed
from rather than leaving this to random breeding in the paddock.
Animals are chosen for breeding because they are individuals with
superior performance.
Objectives
When breeding animals for a particular market, for example, export beef,
superfine wool, it is essential to have very clear breeding objectives.
The breeding objectives for the Murray Grey cattle on Ondiong are to
produce animals with:
•
high fertility
•
sound structure
•
good growth rates
•
superior carcase qualities.
Bull selection
The photograph below shows a bull from Ondiong that has played an
important role in their breeding program in the past. The bull is called
The Glen Wallaby.
Figure 4.13: The Glen Wallaby. (Photo: Sue Francis)
The Francis family made use of The Glen Wallaby as a sire in the past
because he passed onto his progeny his good carcase qualities such as a
large percentage of saleable marbled tender meat.
22
Cattle production
Ondiong enters cattle every year at the Royal Easter Show. In 1995 the
stud entered a steer, called 2F P84, that won the Heavy Weight Steer
class. This steer’s father was The Glen Wallaby. The family tree for this
steer is shown below.
The Glen
Oslo
Th e Glen
Do ris 22nd
Ca della Park
Panther
Th e Glen
Wallaby
Ondiong Evelyn
3rd
Ra vensworth
Apache
Ondiong Jennifer
5th
Ondiong Jennifer
18th
Ondiong
Everafter
Ondiong
Jennifer
27th
= females
Ondiong
2F P84
28/6/94
= bulls brought in
from other studs
Figure 4.14: Family tree of the steer known as Ondiong 2F P84.
Systems
In the breeding of an animal such as Ondiong 2F P84 there are different
breeding systems that can be used.
Inbreeding
This is the mating of individuals more closely related than the average of
the population to which they belong. Close inbreeding mates very
closely related animals such as brothers and sisters, mothers and sons.
Look at the family tree for the Ondiong steer in Figure 4.14. There are no
examples of close inbreeding on this family tree. Suggest two animals
which, if crossed, would be an example of close inbreeding.
_________________________________________________________
_________________________________________________________
Check your answer.
Part 4: Breeding
23
Inbreeding has the advantage of producing animals that are very similar.
Desired traits are brought out in the herd very quickly. Uniform groups
can be produced for marketing. A major disadvantage of this form of
breeding is that undesirable recessive traits have a high chance of
appearing.
Linebreeding
Linebreeding is a specialised form of inbreeding. This system is used so
that a particular superior animal, such as The Glen Wallaby, can be used
over several generations of mating. This very intense form of inbreeding
is shown in the example below.
bull A
X
cow B
bull A
X
heifer C
bull A
X
heifer D
heifer E
Figure 4.15: Linebreeding. Note that heifer C, D and E are all offspring from
bull A.
Look at the family tree from Ondiong in Figure 4.14. Suggest which animal
you could mate Ravensworth Apache with to give an example of
linebreeding.
_________________________________________________________
Check your answer.
24
Cattle production
Outbreeding
Outbreeding involves the breeding of completely unrelated animals. This
means new genes are brought into the herd. There are two main forms of
outbreeding:
•
outcrossing
•
crossbreeding.
Outcrossing
Outcross breeding is where unrelated animals of the same breed are
mated. A common example of this is when a producer purchases a bull
from a stud to use over his cows of the same breed.
Crossbreeding
Crossbreeding is the most extreme form of outbreeding, this involves the
crossing of different breeds of animal.
The following photograph shows a steer produced from a cross between
a Murray Grey bull and a Hereford cow.
The offspring from this cross:
•
are calm in temperament
•
reach slaughter weight
relatively quickly
•
are suitable for finishing in a
local feedlot which sells meat to
Japan.
Figure 4.16: Murray Grey and
Hereford cross. (Photo: Murray Grey
Beef Cattle Society Inc.)
Part 4: Breeding
25
Crossbreeding allows you to combine the best from two breeds, so that
animals can be produced that are better suited to the local environment
and production requirements of the property. There are a number of
advantages in crossbreeding.
•
Ability to overcome a weakness in the breed, for example, Hereford
cattle are very susceptible to eye cancer. By using a Bos Indicus
cross the levels of eye cancer can be reduced.
•
Combination effect of the good characteristics of different breeds,
for example, a Hereford X Friesian will combine the milking ability
of a Friesian with the carcase characteristics of a Hereford to
produce a good vealer mother.
•
Hybrid vigour. There are claims there may be an increase of up to
14% in the weaning weight of crossbreed calves. Crossbred calves
are also stronger and more likely to survive, so the weaning
percentage can be 3% higher.
In Australia there are over 40 breeds of beef cattle that breeders can
select from.
Look at the Ondiong family tree in Figure 4.14. Are there any examples of
outbreeding?
_________________________________________________________
_________________________________________________________
Check the answer section.
Genetics
Genes are found in every cell of an animal. They contain a code that tells
the cell how to develop, and what to produce. A normal body cell has two
copies of each gene. Ova and sperm are produced by halving the normal
number of genes. When a male animal is mated with a female animal they
both contribute genetic material to the offspring. Half of the genes are
provided in the sperm of the sire and half in the ovum of the dam. This
means that the offspring will end up with two copies of each gene, one from
each parent.
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Cattle production
Figure 4.17: A calf will get one copy of a gene from each parent.
Specific genes code for particular characteristics. Through a process
known as mutation a gene can become altered. Sometimes this
alteration can result in improved characteristics, and at other times it can
result in abnormal function in the animal. Altered genes that code for the
same characteristic are called alleles. These alleles are responsible for
the variation that exists in a population. If an animal has two different
alleles it is called heterozygous for that gene (hetero = different). If the
animal has two alleles the same it is called homozygous for that gene
(homo = same).
Sometimes an animal only needs one copy of a particular allele to
produce a characteristic. Other alleles can remain hidden and the
characteristic only appears if the animal has two copies of the allele (is
homozygous for the gene).
When animals are related to each other they tend to have a lot of alleles
the same. The closer the inbreeding that occurs the more likely it is that
for any gene that the animal will have two copies of the same allele.
These animals can be described as having a high level of homozygosity.
This can allow a concentration of superior genes in an animal.
All animals have some detrimental alleles in their genetic code. Often
these cannot be seen unless an individual is homozygous for the allele.
Homozygosity tends to make individuals less productive. They are
generally slower growing, more prone to disease and less fertile.
When a crossbreeding system is used it uses the more homozygous pure
breeds as parents. When these different breeds are crossed together the
resulting offspring have a high level of heterozygosity. This is also
called hybrid vigour. Animals with hybrid vigour have high production
levels, they are faster growing, more fertile and disease resistant.
Part 4: Breeding
27
Which breeding system?
Deciding which breeding system to use in an enterprise depends on the
desired outcome of the enterprise. Stud breeders tend to concentrate on
producing superior animals with a high level of homozygosity. These
superior stud animals are intended as parents for commercial animals. The
best way to achieve this is to have a limited gene pool, such as you would
find within a pure breed, and continually select superior animals to breed
with. Often particularly good individual animals are used in a linebreeding
or close inbreeding program to produce superior strains within the breed.
Commercial producers aiming for maximum production should try to
achieve a high level of heterozygosity in their animals. These producers
should use a crossbreeding system using animals from different breeds as
parents to get maximum hybrid vigour. For many commercial cattle
producers this system is impractical as it means the purchase of breeding
cows rather than using their own heifer calves as replacements. Most
commercial cattle producers purchase unrelated bulls of the same breed
from a stud and follow an outcross breeding program.
Producers of prime lambs (lambs for meat) have developed a production
system that routinely uses crossbreeding to produce a fast growing meaty
lamb, covered in Part 2 of Looking at ewe. Different producers specialise
in a single part of the production system. There are some people in the
beef cattle industry who believe that they should adopt a similar system
to get the most advantage from hybrid vigour.
Genetic disease
When an allele has a detrimental effect on an animal, it is called a genetic
disease. These types of disease can be passed on from one generation to the
next. Sometimes these disease alleles need to be homozygous before any
symptoms can be seen in the animal.
During the 1960s in Australia, an American Holstein Friesian bull, GrayView Crisscross, was widely used as a sire in artificial insemination
programs. What was not known was that the bull was heterozygous for a
genetic disease called citrullinemia.
Citrullinemia affected calves are unable to manufacture an enzyme that
converts ammonia into urea. This means they are unable to remove
ammonia from the body, and suffer from progressive brain damage
leading to death within one week of birth.
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Cattle production
Gray-View Crisscross did not show any symptoms of the disease. His
calves were also free of symptoms as they carried only one copy of the
disease causing allele. Symptoms of the disease only started to become
apparent when offspring of the bull were used in breeding programs and
were crossed with related animals.
A laboratory test was developed that detects carriers of citrullinemia by
identifying in their DNA the mutation responsible for the disorder.
Gray-View Crisscross had been so widely used that more than 10 % of
tested Australian Holstein Friesian cattle were discovered to be carriers
of this condition.
Research by NSW DPI is being conducted at the Elizabeth Macarthur
Institute to try and cure citrullinemia by injecting healthy DNA into
affected calves. It is hoped that the research will also benefit humans
with the disease.
Normal
Carrier
Affected
176 bases
98 bases
78 bases
Figure 4.18: In the test for citrullinemia, gene fragments are separated by an
electric field, and the patterns are analysed. Normal animals have two short
segments. Affected animals have one long uncut segment because the
mutation prevents the cut. Unaffected carrier animals (heterozygotes) have
three segments (two normal segments and one long mutated sequence).
Illinois Research, Spring/Summer, 1991, Vol 33. No1/2
The example of citrullinemia demonstrates the dangers associated with
inbreeding, even with animals that appear superior. Many other genetic
diseases have been identified in cattle, most are associated with particular
breeds. The use of crossbreeding programs means a reduced incidence of
genetic diseases.
Part 4: Breeding
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Summary
Summary
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30
Hormones are produced by endocrine glands and act on other parts
of the body.
Oestrogen is an important female hormone. One function is to cause
oestrus behaviour, when the female is sexually receptive.
Oestrogen and other hormones work together to produce a cycle of
events called the oestrous cycle. Fine control is maintained by a
complex feedback system.
As follicles on the ovary develop, increased oestrogen levels block
production of FSH and increase levels of LH.
An important male hormone is testosterone. One function is to cause
typical male behaviours such as the desire to mate (libido) and
aggression.
Testosterone is needed to mature sperm, and to develop organs and
glands of the male reproductive tract. Testosterone also causes
development of other male body characteristics.
Fertility level of animals has a genetic component, so stock selection
is important.
For peak fertility animals must have an adequate level of nutrition.
High and low temperatures can decrease fertility levels of animals.
Animal species with a breeding season will have highest fertility
within that season.
Disease can decrease fertility, so it is important for managers to
prevent and control these where possible.
Managers should have clear breeding objectives for their animals, so
they can select breeding animals which meet these objectives.
Inbreeding and outbreeding are breeding systems that can be used to
achieve breeding objectives.
When animals have two genes that are the same for a characteristic
they are homozygous for that gene.
When animals have two genes that are different for a characteristic
they are heterozygous for that gene.
Heterozygosity causes hybrid vigour.
Managers wanting maximum productivity need to aim for maximum
levels of heterozygosity.
Homozygosity can lead to the expression of genetic diseases.
Cattle production
Suggested answers
Suggested answers
Oestrus behaviour
1
2
3
A cow in oestrus may:
•
be restless
•
be more active
•
bellow
•
wander about mounting other cows
•
stand to be mounted.
A cow in oestrus may show the following physical signs:
•
mud on her flanks
•
hair off her tail
•
moist, swollen vulva
•
discharge of mucus from the vulva.
The timing of artificial insemination by Karen, at 8–16 hours after
the onset of oestrus, is the same as the recommendation given in the
table.
The oestrous cycle
The pituitary gland secretes the hormone FSH which causes a follicle in
the ovary to mature and ripen producing the hormone oestrogen.
Oestrogen is the hormone responsible for an animal showing typical
oestrus behaviour. It is produced by the ovary.
The production of oestrogen stimulates the pituitary gland to decrease
the production of FSH and release the hormone LH. LH, produced by
the pituitary gland, causes a growing follicle in the ovary to rupture and
an ovum to be shed. A corpus luteum is formed where the follicle has
ruptured, producing the hormone progesterone.
While the corpus luteum persists, levels of the hormone FSH remain low
and another follicle cannot mature.
Part 4: Breeding
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Once the corpus luteum has regressed, progesterone levels drop and the
pituitary gland releases FSH again, continuing the cycle. This
interrelationship between hormones, rising and lowering of hormone
levels in response to each other is called feedback.
1
3
2
C
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P
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R O G E
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T
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S T
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B
J
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R O G E N
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T
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S P E R M
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U
F S H
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M U C O U S
Daylength
1
The gestation length for a ewe is approximately 147 days. This is
about 5 months. 5 months prior to Spring is April (Autumn), which
is when the breeding season for sheep is at a peak.
2
The breeding season for poultry is stimulated by increasing daylight
hours as Spring approaches.
Inbreeding
Mating of any animals that are closely related is termed inbreeding.
Examples from the Ondiong family tree include:
•
Ondiong Everafter X Ondiong Jennifer 27th
•
Ravensworth Apache X Ondiong Jennifer 18th
•
Ravensworth Apache X Ondiong Jennifer 27th.
Linebreeding
Ondiong Jennifer 18th X Ravensworth Apache is an example of
linebreeding with Ravensworth Apache. The calf produced would have
Ravensworth Apache as both father and grandfather.
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Cattle production
Crossbreeding
There are no examples of crossbreeding. Examples of outcrossing are:
•
Ravensworth Apache X Ondiong Jennifer 5th
•
Cadella Park Panther X Ondiong Evelyn 3rd.
Part 4: Breeding
33
Exercise – Part 4
Exercise – Part 4
Exercise 4.1
Name: _________________________________
Exercise 4.1
Describe and evaluate management techniques that can be used to
increase the level of fertility of domestic animals.
Hints:
The words describe and evaluate are key words used in the Board of
Studies syllabuses and examinations. These words have particular
meaning when used in HSC exam questions.
Describe – provide characteristics
and features
Evaluate – make a judgement
based on criteria; determine the
value of
BOS NSW 1999 The New Higher
School Certificate Assessment
support Document
•
Select a small number of the most important management techniques
and provide information on what is involved for each of these.
•
Include reasons why the management techniques improve fertility.
•
Write about the good and bad aspects of using the management
techniques on a commercial property, for example additional costs,
or extra income from sale of more offspring.
•
Make a judgement on the value of the management techniques.
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Cattle production
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Part 4: Breeding
35