SELECTION: ESTIMATED BREEDING VALUE
Introduction
The goal of animal breeder is rapid genetic improvement in traits of economic importance. How can this
goal be achieved? In order to achieve this goal, accurate predictions (or evaluations) of animals’ genetic
merit for any given traits are required. This information is used by the breeder to rank the animals and
cull those with the best evaluations (Van Vleck et al, 1987). Unfortunately, quantitative traits are also
influenced by the environment. It is therefore important for a breeder to understand and adjust
(remove) the influence of these environmental factors before predicting an animal’s genetic worth for a
given trait. Adjusting for environmental factors enables breeders to compare animals of different
production levels.
Objectives
1. Define the terms breeding value, transmitting ability and producing ability
2. State the difference between producing ability and transmitting ability
3. Discuss the importance of environmental factors in genetic evaluations
Estimated Breeding Value
Predictions of additive genetic breeding value and future performance are used so frequently in animal
breeding that they have been given specific names. The additive genetic value of an animal for a specific
trait is referred to as its breeding value (BV) for that trait. The BV estimated from various measures
(records) of performance is called estimated breeding value (EBV). The term BV is used to denote
additive genetic value because it is mostly the additive values of the individual genes possessed by an
animal which are transmitted directly to its offspring. An animal’s additive genetic value, therefore,
reflects its value or worth for breeding.
Estimated Transmitting Ability
In terms of the model of gene action, an animal’s BV for a trait is the sum of the additive effects of the
genes it possesses which affect that trait. However, since an animal will transmit half of its genes, its EBV
actually represents twice its contributions to any of his offspring. EBV is, therefore, often converted to
estimated transmitted ability (ETA). It is simply half of an individual’s EBV.
ETAs are referred to by specific names in livestock industries. ETAs for sires computed from the average
performance of their progeny are called Predicted Differences (PD) or Predicted Transmitting Ability
(PTA) in dairy cattle and Estimated Progeny Differences (EPD) in beef cattle. The term Cow Index is
applied to the ETA of a dairy cow, and is computed from its own performance and the average
performance of its paternal half-sisters (i.e. the PTA of her sire). A Pedigree Index is an ETA computed
for an individual from the performance of his ancestors.
ETA of beef sires (EPD) describes particularly well the interpretation of ETA. The word “progeny”
indicates that differences among individuals (sire in this case) in ETA will be reflected in the performance
of their progeny. ETA measures that part of genetic superiority or inferiority which is transmitted to the
progeny. If a group of sires is mated to random samples of females (neither particularly good nor bad)
the relative performance of their progeny (if they are managed equally) should correspond to their
ETAs. The word difference means that the ETAs are expressed as a difference from some base or “zero”
point. The base point represents the population average ETA at some point in time. The choice of a base
point or ‘genetic base’ is arbitrary (and may, in fact, be changed) so long as all ETAs are expressed
relative to the same point. The practical result of expressing ETAs relative to some average ETA defined
as zero is that ETAs maybe positive or negative. Those exceeding the base point are positive, while ETAs
less than the base point are negative. The term EPD indicates what the breeder really wants to know –
expected performance of an individual’s progeny relative to the progeny of other individuals.
Activity: What is EBV and why is it important in livestock improvement?
Producing Ability
Producing ability is the performance potential of an individual for a repeated trait. A repeated trait is a
trait which individuals commonly have more than one performance record. Prediction of future records
works in the same way as EBV. Here the breeder is interested in the future performance of an individual
in some repeated trait sometimes referred to as true producing ability, of the individual. This gives the
best estimate of future performance. The producing ability of an animal is the sum of its total genetic
value (G = A+D+I) and any environmental effects which are permanent to all records (PE). Producing
ability would therefore indicate exactly the next record, except for the unpredicted temporary
environmental effects (TE) which will be associated with that record.
Estimated producing abilities, like ETAs, have different specific names in different livestock species. In
beef cattle, pigs, and sheep, the term used is Most Probable Producing Ability (MPPA). In dairy cattle,
the most commonly used terms are Estimated Producing Ability (EPA) and Estimated Relative Producing
Ability (ERPA). Since MPPAs, EPAs and ERPAs are most often used to compare producing abilities of
individuals in the same herd; they are generally expressed relative to the herd mean. EPAs are therefore
positive (above) or negative (below) relative herd average, and it is necessary to add average EPA to
obtain the absolute level of the predicted future record.
Producing ability and transmitting ability are clearly related. Producing ability measures A+D+I+PE, while
transmitting ability includes only A, and the two measures have relatively different base points.
However, individuals having the highest estimated transmitting abilities in a herd often will have the
highest estimated producing abilities as well. In the few cases where these measures do not agree, the
breeder may use EPA to decide on the fate of the individual, and the Eta to determine the disposition of
the individual’s offspring
PREDICTION OF BREEDING VALUES
Introduction
With the understanding that breeding values are needed by animal breeders in order to make accurate
selection and culling decisions. Breeders rank animals based on their estimated breeding values, select
those with highest estimates as parents of the next generation and cull those with low breeding values.
Without the knowledge of breeding value estimates, selection will not be possible. If you are a livestock
breeder, you can use this knowledge to improve either one trait at a time (single trait selection) or more
than one trait at a time (multiple trait selection) depending on your selection goals.
Depending on the amount and source of information available, proper weighting may depend on the
heritability (h2) and repeatability (r) of the specific trait. The number of repeated records (n), the genetic
relationship (aij) between a relative and the individual, the number of relatives (m) of a specific type and
any environmental correlation (C2) due to extra common environmental factors shared by relatives.
Table 1: Factors considered in weighting performance records to estimate breeding value and
producing ability
Observed Performance
Individual
Estimated Breeding Value
Estimating Producing Ability
One record
n record
Single Relatives
h2
h2, r, n
r
r, n
One record
n records
m Relatives
h2,aij
h2, aij, r,n
One record each
n records each
h2, aij, m, C2
h2, aij, n, r, m, C2
Objectives
1. To estimate an individual’s breeding value from its own records or performance
2. To predict an individual’s breeding value from performance of relatives (progeny, parents, halfsibs)
Estimating Breeding Values from individual Performance (own records)
Individual performance (one record)
Individual performance is simply an animal’s own record. For example, if a dairy cow produces 7000kgs
of milk, the yield 7000kgs is the cow’s individual performance. In terms of breeding value estimation
there is need to know how much of the yield is due to genetics. In order to do this a heritability value of
the trait need to be known. Heritability represents the proportion of differences in performance which is
due to differences in the additive effects of individual genes, or due to additive genetic variance.
Heritability also represents the proportion of differences in performance which is due to differences in
the breeding values of individuals. In practice the heritability has been estimated from performance
records which have been adjusted for major environmental sources of variation (age, cow weight,
feeding regime, lactation length etc) and expressed as deviations from the average performance in some
environmental conditions. It also indicates the proportion of differences in adjusted records which is
due to differences in breeding value.
An estimate of the individual‘s breeding value is given by;
EBV = Ấ = h2 (P - µ)
=h2 (Individual record – herd average)
Where h2
=
COV (a,p)/σp2
=
σA2/σp2
and indicates the average (for the trait, in the population) number of units change in breeding value per
unit change in performance. The denominator and the numerator are the genetic variance and the
phenotypic variance respectively. The accuracy with which EBV predicts true value depends on the
magnitude of heritability and the definition of ‘accuracy’. Accuracy of EBV is the square root of
heritability (h).
Thus for a single record, the accuracy (r (a, y)) is given by;
𝐶𝑜𝑣 (𝑎,𝑦)
𝛿𝑎𝛿𝑦
=
𝛿2𝑎
𝛿𝑎𝛿𝑦
𝛿𝑎
= 𝛿𝑦
=
2
√ℎ
=
h
EXAMPLE: Estimating breeding value from an animal’s own record
Question:
A Mashona bull at the University farm gains 1.4kg/day on a feeding trial in which
average gain is 1.0kg/day. All animals are fed and managed identically, and gains are
adjusted for beginning age and weight. If heritability of weight gain is 0.45, what is the
animal’s EBV, and the accuracy of that estimate?
Solution:
Relative to the performance of other individuals under similar conditions, gain of the
individual is;
(P - µ) = (1.4 – 1.0) = 0.4 kgs/day
Therefore EBV =
Ấ
The accuracy of this EBV
=
h2 (P - µ)
=
0.45 (0.4)
=
0.18 kgs/day
=
√h2
=
√ (0.45)
=
0.67 (67%)
Individual performance: n records
An estimate of breeding value based on average performance of several records, logically will be more
accurate than EBV from a single record. Therefore when several records are available, all should be used
to estimate breeding value. The adjusted average performance is weighted by the following coefficient
which can be thought of as heritability of average performance or more precisely the regression of
breeding value on average performance in n records.
The b coefficient is;
b
=
𝑛ℎ2
1+(𝑛−1)𝑟
where ;
b
is the weight
h2 is the heritability
n
is the number of records in the average performance P
r
is the repeatability of the trait
This b coefficient, which can be thought of as heritability of average performance, is used in estimating
breeding value from average performance (mean of n records) in exactly the same way heritability is
used with single records. Specifically,
EBV
=
Ấ
=
𝑛ℎ2
(1+(𝑛−1)𝑟 ) (Ṗ-µ)
In this case P is the average of the n records and it is adjusted for the environmental factors. The
accuracy of EBV from the average performance is the square root of the b coefficient. Note that when n
= 1, the b coefficient becomes the heritability as shown in the previous case. From the above formulas,
one can also conclude that as the number of records increases the accuracy of the estimated breeding
values also increases.
Table 2: Weights (b coefficients) and accuracy of estimating breeding value (rAP) from average
performance
2
h
0.10
0.25
0.35
0.50
n
2
3
4
5
10
2
3
4
5
10
2
3
4
5
10
2
3
4
5
10
b
.15
.19
.21
.23
.27
.38
.47
.53
.57
.68
0.30
rA,P
.39
.43
.46
.48
.52
.62
.68
.73
.75
.82
b
.13
.15
.16
.17
.18
.33
.38
.40
.42
.45
.47
.53
.56
.58
.64
.67
.75
.80
.83
.91
REPEATABILTY
0.50
rA,P
.37
.39
.40
.41
.43
.58
.61
.63
.65
.67
.68
.72
.75
.76
.80
.82
.87
.89
.91
.95
NB* h2 = heritability, n = number of records in the average
b
.12
.13
.14
.14
.15
.30
.33
.34
.35
.36
.42
.46
.47
.49
.51
.61
.65
.68
.69
.73
0.55
rA,P
.35
.36
.37
.37
.38
.55
.57
.58
.59
.60
.65
.68
.69
.70
.71
.78
.81
.82
.83
.85