Selection is one of the important processes for any improvement in farm animals.
The breeding merit
animal is not often determined by a single character, but more often based on many characters
simultaneously. The purpose of selection is to produce elite breeding stocks which act as parents of
future generations. The system of selection allows the best animals to act as parents of future
generations and culling of undesirable animals from the herd. The animals retained have certain
acceptable traits which make them produce more. The breeding of animals is underneath human
control, and the breeders decide which individuals shall produce the subsequent generation [1, 2]. The
breeding of animals is based upon the fact that certain qualities are genetic; hence valuable qualities are
passed on from parents to offspring’s. Due to selection, the qualities of animals can be maintained or
improved in the next generation [3]. The purpose of selection is to enhance the frequency of desirable
alleles and reduce the frequency of unwanted alleles from the herd which in the long run consequences
genetic improvement in livestock. The overall performance of an animal is mainly influenced by the
genetic potential that is inherited from its parents and the environment which particularly encompass
feeding, health, management and so forth.
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2. Methodology
Breeding for increased productivity over the past few decades has been very successful in terms of
improvement of growth, production and reproduction traits; however, it has also had negative
consequences on behavior and welfare [4]. Breeding and genetics are playing an important role in the
improvement of domestic animals. Therefore, a broad approach is needed that encompasses both
production and welfare traits, even though welfare may not be a primary breeding goal of the selection
scheme. Now, in the era of genomics, breeders have lots of opportunities to collect more precise
information on the biological impact of certain breeding decisions. This might help breeders to make
more accurate decisions in their selection programs. Genomic tools could also facilitate selection for
complex traits, which are frequently not possible to measure on a large number of animals. Looking to
this the salient features about selection criteria and methods of selection have been discussed in this
chapter.
2.1 Types of selection1 Individual selection/performance testing
Individual selection is most commonly used as a basis for selective improvement in livestock. Individual
selection is based on the performance of individual or individual phenotypic value. These animals are
selected based on their own phenotype. Individual selection is more effective when the heritability of
traits is high, but the effect decreases with falling heritability. It is the simplest, more rapid and most
commonly used basis of selection. If proper performance records are maintained then, the traits like
body weight, growth rate, fleece production and other parameters of similar nature can be evaluated
directly from the performance of individual animals [10, 12].
2.3.1.1 Selection for qualitative traits
The animals are kept or rejected for breeding purposes based on their phenotype for a particular trait.
The progress made in selection depends on how closely genotype is correlated with the phenotype. The
phenotype of the individuals is often used to estimate the breeding value for qualitative traits such as
color and horned or polled conditions. Selection for such traits based on mass or phenotype is more
effective than others. For example, in Angus cattle, the coat color Red (rr) is recessive to dominant black
(BB) color. But it is practically difficult to distinguish and differentiate the genotype BB and Bb
phenotypically. Thus, selection based on individuality will be useful but not always completely accurate
[4, 13].
2.3.1.2 Selection for quantitative traits
These traits are controlled by many genes and are also affected by various environmental factors. There
is no sharp distinction among the phenotypes and affected by both additive and non-additive gene
action. No trait is 100 per cent heritable, because the environment always affects the phenotype to a
certain extent. Therefore, the phenotype of an individual for quantitative traits is not the true indicator
of genotype. The phenotypic merit of the individuals for quantitative traits is determined by comparing
the individual’s own phenotype with that of the average of all the individuals within a group from which
it is selected and is called trait ratio [14, 15, 16].
Trait ratio=record of individual foratraitGroup mean for the same trait×100
The trait ratio depends upon the accuracy of records or available data. The individual’s record is of little
value unless it shows where the individual ranked relative to others under similar conditions. The
environmental part of phenotypic superiority or inferiority will not be transmitted to the offspring or
next generation. Therefore, in general, there is a tendency for the average phenotype of the offspring of
a phenotypically superior individual will tend to regress toward the average of the population, whereas
the average phenotype of the offspring of phenotypically inferior individuals will tend to rise toward the
average of the population. Animals own phenotypic value of the character under selection is considered
to estimate the probable breeding value (PBV) [5, 13, 14] of that character for that individual.
Probable Breeding Value(PBV)=P¯¯¯+h2(Pi−P¯¯¯)
Where, P¯¯¯ is population mean, h2 is heritability, Pi is individual phenotypic value.
Comparison is made with the average of other individuals kept under similar environmental conditions
of same age and same time; thus, individuals are ranked relative to others under similar conditions. It is
also called as mass selection.
In individual selection, the best animals are selected from within a group of animals of the similar age
group that has been reared and treated similarly at the same time, i.e., contemporaries. In individual
selection, the breeder will be having a single record of each animal’s performance (performance test)
and hence an estimate of probable breeding value (PBV) [13] for a given trait is calculated as:
Probable Breeding Value(PBV)=P¯¯¯+h2(Pi−Pc¯¯¯¯)
Where, P¯¯¯ is population mean, h2 is heritability, Pi is individual phenotypic value, Pc¯¯¯¯is average of
contemporaries.
2.3.1.3 Advantages of individual selection
Information of individuals to be selected is easily available [3, 5].
Used when pedigree information not available, this is the only available guide for selecting the breeding
stock.
Used when generation interval is shorter than progeny testing.
It gives a direct estimation of BV and is more accurate when h2 is high.
Traits such as body type, growth rate, fleece production, horn pattern, color and others of a similar
nature can be evaluated if suitable records are available.
Useful for traits expressed in both sexes and performance of the individual is above average for
breeding, regardless of the merit of near relatives.
In the absence of pedigree and progeny records, this is the only available guide for selecting the
breeding stock.
2.3.1.4 Disadvantages of individual selection
Not applied for sex-limited traits such as milk production, egg production, maternal abilities, semen
production and litter size, etc. [3, 13].
Not applied when traits are expressed in later life/after the death of individual.
Not applied when traits have low heritability, then the individual selection is the poor indicator of
breeding value such as reproductive characters.
Not possible for traits expressed only after sexual maturity, because selection has to be delayed till
maturity resulting in waste of time and money.
The easy appraisal of appearance often tempts the breeder to overemphasis this evaluation in selection.
It is concluded that the individual selection is based on individual’s phenotype (appearance) and
performance. Individuals are selected solely in accordance with their own phenotypic values. This is the
simplest and yields more rapid response. It is the most commonly used method for selective
improvement of livestock. Undoubtedly, most of the progress in livestock improvement can be credited
to individual selection. Traits such as body type, growth rate, fleece production and other of similar
nature can be evaluated directly from the performance of the individual animal, if suitable performance
records are being kept; such evaluations are usually available by the time initial selection of breeding
stock has to be made. In contrast, only a few can be progeny tested.
2.3.2 Pedigree selection
When the genetic worth or breeding value of animals is determined based on the performance of their
ancestors or pedigree information is called as pedigree selection. Pedigree may be a record of an
individual’s ancestors associated with it through its parents. Therefore, the selection is based on the
information of the ancestors of individuals that are related to it. Performance records from ancestors
can provide useful information about the potential genetic worth or the breeding value of the
individuals in question. This will give useful information before the animal is old. An estimate of calf’s
potential milk yield could be determined based on the milk yield of its mother until such time as the calf
is grown up and can be milked. When adequate information on the merit of the individual is not
available, then attention is given on pedigree information for selection of individual. From the selection
point of view, knowledge of the different economic traits of the ancestors is essential [17].
It’s usual to expect offspring of outstanding parents to be of superior genetic value than the mean of the
individuals of the herd. Each parent transmits only sample halves of its genes to every offspring and just
one quarter of genes from each grandparent. So, parents never provide the maximum amount of
information about the breeding value of a single individual as individual’s performance itself would
produce. Unless the performance of the ancestor is well known, selection based on pedigree is
meaningless. Distant ancestors of an individual give even less genetic information about the individual’s
breeding value especially for production traits. The pedigree is often classified into two as direct and
collateral [2]. Collateral means those descended from same ancestors.
Selecting a cow based on the performance of its great grandparent is as good as random selection
because the relationship is (0.5)2 = 0.125, i.e., only 1/8th of the superiority can be expected in the
progenies. It will not do much good to go beyond three generations into pedigree due to the halving
process of the chromosomes in each generation [9].
When the pedigree data provides information on the phenotypic and genotypic merit of the ancestors
then it is called performance pedigrees. If the selection differential for the ancestor could be presented
in the pedigree or if the performance record of the ancestor could be expressed as a percentage of the
average contemporaries (Trait ratio), tPedigree selection
When the genetic worth or breeding value of animals is determined based on the performance of their
ancestors or pedigree information is called as pedigree selection. Pedigree may be a record of an
individual’s ancestors associated with it through its parents. Therefore, the selection is based on the
information of the ancestors of individuals that are related to it. Performance records from ancestors
can provide useful information about the potential genetic worth or the breeding value of the
individuals in question. This will give useful information before the animal is old. An estimate of calf’s
potential milk yield could be determined based on the milk yield of its mother until such time as the calf
is grown up and can be milked. When adequate information on the merit of the individual is not
available, then attention is given on pedigree information for selection of individual. From the selection
point of view, knowledge of the different economic traits of the ancestors is essential [17].
It’s usual to expect offspring of outstanding parents to be of superior genetic value than the mean of the
individuals of the herd. Each parent transmits only sample halves of its genes to every offspring and just
one quarter of genes from each grandparent. So, parents never provide the maximum amount of
information about the breeding value of a single individual as individual’s performance itself would
produce. Unless the performance of the ancestor is well known, selection based on pedigree is
meaningless. Distant ancestors of an individual give even less genetic information about the individual’s
breeding value especially for production traits. The pedigree is often classified into two as direct and
collateral [2]. Collateral means those descended from same ancestors.
Selecting a cow based on the performance of its great grandparent is as good as random selection
because the relationship is (0.5)2 = 0.125, i.e., only 1/8th of the superiority can be expected in the
progenies. It will not do much good to go beyond three generations into pedigree due to the halving
process of the chromosomes in each generation [9].
When the pedigree data provides information on the phenotypic and genotypic merit of the ancestors
then it is called performance pedigrees. If the selection differential for the ancestor could be presented
in the pedigree or if the performance record of the ancestor could be expressed as a percentage of the
average contemporaries (Trait ratio), the ancestor’s records would be of greater predictive value [9, 10].
Figure 3 is showing the different basis of selection that are commonly used to estimate the probable
breeding for the selection of animals for a single trait.he ancestor’s records would be of greater
predictive value [9, 10]. Degree of relationship
If ancestors are more closely related to the individual (Parent: 12/, Grandparent: 14/ and Great
grandparent: 18/) should receive most emphasis in pedigree assessment [16].
In pedigree selection, the PBV [3] of an individual is estimated on the basis of the performance of his
ancestors.
PBV=Pc¯¯¯¯+bAP(P¯¯¯i−Pc¯¯¯¯)
Where, P¯¯¯ is population mean, bAP is regression of additive genetic value or breeding value on
phenotypic value, Pi is phenotypic average of individual, Pc¯¯¯¯ is average phenotypic value of individual
contemporaries,
The selection criteria based on the ancestor’s performance is called as the pedigree selection. For
pedigree selection, more recent ancestors consider rather than distant.
2.3.2.2 Difficulties in pedigree selection
Ancestral records are not always available.
Recording may be faulty due to stray mating
Most of the characters have low heritability.
The accuracy of pedigree selection when only single information is available for ancestor has been
summarized in Table 1.
Ancestor b Accuracy of selection (n = 1)
Dam 0.5 h2D 0.5 hD
Sire 0.5 h2s 0.5 hs
Mean of both parents 1/√2 h2 1/√2 h
One grand parent 0.25 h2 0.25 h
Table 1.
Accuracy of selection (n > 1) [3].
Table 1 summarizes the accuracy of pedigree selection when only single information is available for
ancestor (n = 1) [3]. The accuracy of selection based on individuals own record increases, when
ancestors’ information (parents and grandparents) is combined with an individual’s own records.
When information of more than one ancestor are available the accuracy of selection increases, which is
described in Table 2.
Ancestor b Accuracy of selection (n > 1)
Dam 0.5 h2D [n/1 + (n-1)r] 0.5 √[nh2 D/1 + (n-1)r]
Sire 0.5 h2s [n/1 + (n-1)r] 0.5 √ [nh2 s/1 + (n-1)r]
Mean of both parents 1/√2 h2 [n/1 + (n-1)r] 1/√2 √[nh2/1 + (n-1)r]
One grand parent 0.25 h2 [n/1 + (n-1)r] 0.25 √ [nh2/1 + (n-1)r]
Table 2.
Accuracy of selection (n = 1) [3].
Table 2 summarizes the accuracy of pedigree selection when information of more than one ancestor (n >
1) [3] is available. This increases the accuracy of selection. The pedigree selection is basically only useful
to select the individual before its own records is available.
2.3.2.3 Merits
It is less costly and allows selection at a younger age and provides first-hand information [3, 17].
It helps in multistage selection and is also useful for sex-limited traits.
It is useful when two individuals have similar performance.
The pedigree should be used only as a minor ancestry to individual selection. It may be used to tip the
balance between two individuals who are very close on individual merits.
The selection based on pedigree is only useful than of individual selection only when heritability is
moderate or low.
2.3.2.4 Demerits
All the animals from an inferior pedigree are culled in spite of the fact that an individual may be of good
merit and free from recessive alleles [3, 17].
Some pedigrees get favored irrespective of the true merit of the individuals in the population.
Pedigree records are from different environmental conditions.
Pedigree selection provides no basis of selection among the descendants of the same ancestor.
2.3.3 Family selection
Family is a group of individuals that descended from the same ancestor. Family represents a group of
animals having common genetic relationship. In family selection, it is presumed that the ancestor has
outstanding merit. In animal breeding, generally, the family is a group of animals having a common
genetic relationship. In animal population under random mating, generally half sibs (HS) and full sibs (FS)
are the most common collateral relatives, whose records are often used to estimate the breeding value.
When individual’s performance is also included in calculating the sibs average performance, it is called
family selection. Family selection is very useful in case of traits with low heritability [1, 3].
2.3.3.1 Sib selection
The selection of individual based on the sibs performance not including individuals own performance.
Based on its sib performance it is of 3 types:
Full sibs, Maternal half sibs, Parental half sib - cousins, uncle/aunt, nephew/niece.
Sib selection is performed when the measurements on the individual are not available. For example,
Slaughter traits; Sex limited traits; Threshold traits like disease resistance.
HS selection is preferred over FS selection:
HS are easily available in more number
The rate of inbreeding can be kept low in HS mating as compared to FS
FS selection is more likely to be increased by c-effects
Breeding value of sib selection:
PBV=Pc¯¯¯¯+rh2n1+(n−1)t(Ps¯¯¯¯−Pc¯¯¯¯)
Where, P¯¯¯¯ - Average of contemporaries, Ps¯¯¯¯¯ - average of sibs, n- number of sibs, r- coefficient of
relationship (12/ for FS and 14/ for HS), t- intra-class correlation (rh2) among sibs (12/ for FS and 14/ for
HS), h2- heritability of the traits, h2s heritability of sibs = nh21+(n−1)t. [18]
Breeding value of Family selection:
PBV=Ps¯¯¯¯+h2∣∣1−r1−t(P1−P¯¯¯s)1+(n−1)r1+(n−1)t(Ps¯¯¯¯−Pcs¯¯¯¯¯)∣∣
Heritability of the trait:
Accuracy of family selection is more for the traits of low heritability [5, 19]
Accuracy of sib selection based on one FS is 0.5 h and based on one HS is 0.25
2.3.3.2 Advantages of family selection
Improve the character of low heritability in species with high reproductive rates
It does not allow generation interval to increase.
It supports individual selection because it is better to select an individual from a superior family.
2.3.3.3 Disadvantages of family selection
It is costly
It requires a large family size depending on the genetic relationship which is only possible in prolific
breeder.
It results in inbreeding and limits the genetic diversity.
Its accuracy depends upon the genetic relationship among the family members.
2.3.4 Progeny selection
The selection criteria for evaluating an individual based on his progeny performance is known as
Progeny selection or progeny testing. Progeny testing is the most important and one of the best criteria
of selection. It is regarded as a form of family selection since progenies are the family members of each
other. Progeny selection is very useful in the case of sex-limited traits. Such traits are milk yield and fat
percentage in cattle and goat, litter size and litter weight at weaning in pigs and egg production in
poultry etc. [20]. Progeny selection is also useful for the evaluation of an individual for carcass quality
traits which could only be recorded after slaughter. The various functions or equations (sire indices) are
used for the estimation of breeding value of individual. The accuracy of progeny testing is depending on
number of progenies tested, heritability of traits and the environmental correlation between the records
of different progeny [5].
2.3.4.1 Genetic principle of progeny testing
Each progeny of an individual inherits half of the genes. Hence, the breeding value of the parent is twice
the mean deviation of the progenies from population mean.
Points to be considered in Progeny testing:
Test as many as sires as possible (5 to 10 would be minimal) [5, 13]
Make sure that dams are mated to sires at random, within age group as possible.
Produce as many progenies per sire as possible (10 to 15 progenies of either sex for growth traits but up
to 300 to 400 progeny is required for traits like calving difficulty and fertility).
No progeny should be culled until the end of the test.
Offspring that are being tested are not a select group.
The performance of an adequate sample of an animal’s progeny under normal environmental conditions
will give a true indication of its genotype than any knowledge of individuality or pedigree.
To involve a large number of individuals, Progeny testing should be followed in associated herds.
Five males should be tested to select one Progeny testing breeding bull
Ten female progenies of each bull should be performance tested.
One set of bulls should be completed in two years
Facility for recording performance of progenies
Constraints in Progeny testing:
Small Population Size
Unplanned Mating
Breeding value of Progeny testing
PBV=Pc¯¯¯¯+2nh24+(n−1)h2(Pi¯¯¯−Pc¯¯¯¯)
Accuracy of Progeny testing
rGP¯¯¯=rAP¯¯¯=0.5nh21+(n−1)t−−−−−−−√
Where, r is Coefficient of relationship between sire and his progeny, n is Number of progenies, Pi is
Mean performance of progenies of ith sire, Pc is Mean performance of contemporaries of progenies of
ith sire, h2 is Heritability of the trait
The choice of records for the optimum breeding program for low and high heritable traits has been
summarized in [5, 14, 21] (Table 3).
Restriction on Records Heritability
Low (h2 < 0.20) High (h2 > 0.40)
For selection of Males:
1 None Progeny Own
2. Females only (i.e., MY) Progeny Progeny, maternal relative
3 Relative only (i.e., carcass traits)Progeny Sibs, progeny
For selection of Females:
1 None Own, Pedigree Own
2. Males only (i.e., Semen Production) Sibs, Pedigree Sibs, Pedigree
3 Relative only (i.e., carcass traits)Sibs, Pedigree Sibs
Table 3.
Choice of records for the optimum breeding program [4, 9].
Table 3 [4, 9] summarizes the appropriate criteria for selection male and female for optimum breeding
program under different restrictions of records for low and high heritable traits.
Advantages of Progeny testing:
It is the better method for sex-limited traits, the traits with low heritability and slaughter traits.
The bulls carrier of recessive gene can be identified by mating with its progenies.
It evaluates carcass traits that demands sacrifice of animal.
Progeny testing increases selection intensity.
Its accuracy increases with an increase in the number of progenies.
Limitations/disadvantages of progeny testing:
High cost and time are required.
It increases the generation interval and due to longer generation interval genetic gain per year is low.
It requires an adequate number of progenies to be tested on a bull.
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3. Systems of breeding
Systems of breeding can be classified into twoSystems of breeding
Systems of breeding can be classified into two major groups: Inbreeding and Out breeding [5, 22].
3.1 Inbreeding
Inbreeding is the mating of animals more closely related to each other than the average relationship
with in the population concerned. The mated individual should have one or more common ancestors in
their pedigree up to 4–6 generations. Inbreeding includes mating like parent-offspring, brother–sister.
Inbreeding is classified into two types: close Inbreeding and line breeding [22].
Close inbreeding is mating between sibs or between parents and progeny to achieve inbred lines with a
relatively high degree of homogeneity. Most of the time, we use the full sib mating method. The same
effect can be achieved by consistently back crossing the progeny to the younger parents. Line breeding
is a milder form of close inbreeding is in which the relationships of mated individual is kept as close as
possible to some ancestor. As a general rule sire is not mated to its daughters but half sib mating is
made among the offspring of the particular sire. Line breeding was used extensively in the past in
development of British breeds of cattle such as Angus, Hereford and Shorthorn [3]. Line breeding should
be practiced in purebred populations of the high degree of excellence, after identifying outstanding
individuals and it can be advocated to form a new breeds.
Inbreeding is that it makes more pairs of genes in the population homozygous irrespective of the type of
gene action involved. Inbreeding does not increase the number of recessive alleles in a population but
merely brings to light through increased homozygosity. When the animals are homozygous for several
traits, the regularity of inheritance is assured (i.e., it fixes the characteristics). Inbreeding reduces vigor
or it results an inbreeding depression.
Despite certain obvious disadvantages of inbreeding, there are certain instances where it may be used
as the advantage of livestock production. It is used to maintain genetic purity and thereby to increase
prepotency. It is also used to develop inbred lines and also to eliminate undesirable recessives from the
population. When a sire is mated to at least twenty of its daughters and does not produce any recessive
characters in the offspring, it may conclude that the sire is not heterozygous for recessive characters.
Inbreeding is to be practised only when the herd is better than the average, i.e., when the frequency of
desirable genes is more, herd has an outstanding sire, the breeder knows the merits and demerits of
inbreeding and the herd is not maintained for commercial purpose.
3.2 Out breeding
Out breeding is the mating of animals that are less closely related to each other than the average of the
population. Its general, effects are the opposite of those of inbreeding. Out breeding increases the
heterozygosity of the individual. The maximum practical usefulness of out breeding systems is the
production of animals for market. Out breeding is a form breeding where the mates are chosen based
on not being related. The following type of out breeding is used in animal breeding [3, 6].
3.2.1 Selective breeding
The selective breeding is used to maintain the purity of the breed along with their improvement. Figure
4 describes the brief information for genetic improvement of indigenous cattle breeds by selective
breeding is shown below:
Figure 4.
Schematic of selective breeding [3, 5].
Figure 3 is showing the different basis of selection which are commonly used to estimate the probable
breeding for selection of animals for single trait.
3.2.2 Out crossing
It is usually applies only to mating within a pure well-defined breed. If two lines within the same breed
are separated for 4 or 5 generations and the sire from one herd is used in another herd that accounts to
out crossing. It is used when the genetic variability and there is lack of selection response [3]. It
introduces new genes in the population with reference - color, horn type, etc.
3.2.3 Top crossing
It refers to the use of highly inbred sires to the dams of the base population or non-inbred population
within the same breed. It usually refers to the best sire in a pedigree. It also refers to the continued use
of sires to different families within a pure bred, same breed or different breed [3, 5].
3.2.4 Up-grading
Grading up or upgrading is the repeated use of pure breed sire (or sires) over females of non-descript
population. There is a noticed improvement in crosses if sires from a particular breed (A) are repeatedly
back crossed to another breed/non-descript animals (B). Five generations are sufficient to raise the level
of inheritance of breed A to 96.9% (0.969) in the fifth generation. After five generations of repeated
back crossing to a particular breed, the animals after the end of fifth generation become eligible to be
registered as purebred. After 7 to 8 generations of continuous grading up the non descript population
will be transferred into well defined purebreed [3, 4].
The level of inheritance (%) of pure-bred male and non-descript in different generation under
upgradation program is summarizes below in the [4, 5] (Table 4).
Generation Level of inheritance (%)
Pure breed male (A) Non-descript females (B)
First generation 50.00 50.00
Second generation 75.00 25.00
Third generation 87.50 12.50
Fourth generation 93.75 6.25
Fifth generation96.87 3.13
Sixth generation 98.44 1.56
Seventh generation 99.24 0.76
Eighth generation 99.62 0.38
Table 4.
Up-gradation of non-descript population [4, 5].
Table 4 [4, 5] summarizes the change in the per cent level of inheritance of pure-bred male and non-
descript female in different generation under upgradation program. By the successive backcrossing from
one population into another population over generation after generation (7–8 generations), the non-
descript population can be substituted by pure bred population.
The representative model for upgrading the local cattle by frozen semen and nucleus breeding unit is
summarized in [4, 5] (Figure 5).
Figure 5.
Representative model for upgrading the local cattle by frozen semen and nucleus breeding unit [5].
3.2.5 Cross breeding
Cross breeding is mating of two individuals from different distinct breeds. In recent years, crossbreeding
has been used for development of new breeds or synthetics strain. For example: Santa Gertrudis,
Jamaica Hope, Norwegian Red and White, Australian Milking Zebu, Hissardale, Karan Swiss, Sunandini,
Taylor breed etc. [4, 5]. The representative diagram for genetic improvement of non-descript zebu cattle
by crossbreeding is shown in Figure 6.
Figure 6.
Representative cross for genetic improvement of non-descript zebu cattle by crossbreeding unit [5].
Crosses of animals from different breeds result in offspring whose level of production is above that of
the average of the parents. The increased production may be due to increased fertility, increased pre
and post-natal viability, faster and more efficient growth, improved mothering ability, etc. The increased
level of performance as compared to the average of the parents is known as heterosis or hybrid vigor.
Heterosis is due to non-additive gene action. The breeds or lines with good nicking ability or combining
ability are crossed to exploit heterosis.
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4. Conclusions
In any species of livestock, the primary aim of breeder is to improve the production traits. In these
traditional methods of selection along with intervention of recent molecular techniques has paved the
way to exploit the genetic potential of animals upto certain limit. However, in this rapid race of
genomics ethical and environmental issues should be taken into consideration.
Mating system
There are two types of mating of animals for production of the progenies. These are
Natural propagation/mating : Mating of animals by natural means.
Artificial insemination (AI) : Mating is done through artificial means by collecting semen from male and
the inseminating the females. This method help in use of outstanding males for mating of a large
number of females thereby production of large number of highly productive and performing progenies.
System of breeding
Basically, there are two methods of breeding which are as follows:
Inbreeding : Breeding of the related animals as sire (male) and dam (female) are known as inbreeding.
Out breeding : Out breeding of unrelated animals as male and female is known as out breeding.
Inbreeding:
Inbreeding i.e. mating of related individuals often results in a change in the mean of a trait.
Performance of inbred animals lowers down the reproductive efficiency and may have several disorders.
Hence, this method of breeding is not practiced for livestock improvement.
However, inbreeding is intentionally practiced to create genetic uniformity of laboratory stocks and to
produce stocks for crossing (animal and plant breeding).
But inbreeding is unintentionally generated by keeping small populations for breeding and during
selection.
There are again two types of inbreeding which are:
Close breeding : This is most intensive breeding where animals are very closely related and can be
traced back to more than one common ancestor. Examples: Sire to daughter Son to dam Brother to
sister.
Line breeding : Mating animals that are more distantly related which can be traced back to one common
ancestor. Examples: Cousins Grandparents to grand offspring, Half-brother to half-sister. Line breeding
increases genetic purity amongst the animals of progeny generations.
Out breeding:
Out breeding system i.e. breeding of unrelated animals is generally of two types:
Crossbreeding: Crossbreeding is the mating of two animals of different breeds. Superior traits that
results in the crossbred progeny from crossbreeding are called hybrid vigor or heterosis.
Grading up:
Grading up is the breeding of animals of two different breeds where the animals of an indigenous
breed/genetic group is mated by an improved pure breed for several generations towards attaining the
superior traits of the improved breed.
Grading up is continuous use of purebred sires of the same breed in a grade herd. By fifth generation,
the graded animals may reach almost purebred levels.
By following out breeding system i.e., crossbreeding, selective breeding and grading up according to the
need, results in improvement in production performance of the crossbred/graded along with
improvement in growth rate, reproduction and production.
General Considerations Regarding Crossbreeding Systems
Good record keeping of the animals is required for selection good quality of male and females for
breeding. Records on milk production and age at first calving are two important characters besides
others.
Calving difficulties may increase when crossing large breed sires with small breed dams and hence
selection of animals/breeds should be done carefully.
Fewer calving problems if large breed dams are used. However, large breed dams have higher
maintenance costs.
Artificial insemination allows access to better bulls which may be practiced.
Selection of breeding animals
Selection of good quality improved animals for breeding is done by allowing some animals to have
offspring while preventing the animals with inferior quality from reproducing.
The process allows producing genetically superior animals. Emphasis has to be given on several traits
when selecting breeding stock
