An animal's breeding value is its
genetic merit, half of which will be passed on to its progeny. While we will never know the exact breeding
value, for performance traits it is possible to make good estimates. These estimates are called Estimated Breeding
Values (EBVs).
In the calculation of EBVs, the
performance of individual animals within a contemporary group is directly
compared to the average of other animals in that group. A contemporary group consists of animals of
the same sex and age class within a herd, run under the same management
conditions and treated equally. Indirect
comparisons are made between animals reared in different contemporary groups,
through the use of pedigree links between the groups.
EBVs are expressed in the units
of measurement for each particular trait.
They are shown as + ive or - ive differences between an individual animal's genetics
difference and the genetic base to which the animal is compared. For example, a bull with an EBV of +50 kg for
600-Day Weight is estimated to have genetic merit 50 kg above the breed base of
0 kg. Since the breed base is set to an
historical benchmark, the average EBVs of animals in each year drop has changed
over time as a result of genetic progress within the breed.
The absolute value of any EBV
is not critical, but rather the differences in EBVs between animals. Particular animals should be viewed as being
"above or below breed average" for a particular trait.
Whilst EBVs provide the best
basis for the comparison of the genetic merit of animals reared in different environments
and management conditions, they can only be used to compare animals analysed
within the same analysis. Consequently,
NZ ANGUS BREEDPLAN EBVs cannot be validly compared with EBVs for any other
breed.
Although EBVs provide an
estimate of an animal’s genetic merit for a range of production traits, they do
not provide information for all of the traits that must be considered during
selection of functional animals.
In all situations, EBVs should be used in conjunction with visual
assessment for other traits of importance (such as structural soundness,
temperament, fertility etc). A
recommended practice is to firstly select breeding stock based on EBVs and to
then select from this group to ensure that the final selections are otherwise
acceptable.
EBVs are published for a range
of traits covering fertility, calving ease, milking ability, growth, carcase
merit and feed efficiency. When using
EBVs to assist in selection decisions it is important to achieve a balance
between the different groups of traits and to place emphasis on those traits
that are important to the particular herd, markets and environment. One of the advantages of having a
comprehensive range of EBVs is that it is possible to avoid extremes in
particular traits and select for animals with balanced overall performance.
Calving Ease
EBVs (%) are based on
calving difficulty scores, birth weights and gestation length
information. More positive EBVs are favourable and indicate easier
calving.
CE % Direct = Direct Calving Ease - The EBV for
direct calving ease indicates the influence of the sire on calving ease in
purebred females calving at two years of age.
CE % Daughters = Daughters' Calving Ease - The EBV for
daughters' calving ease indicates how easily that sire's daughters will calve
at two years of age.
Gestation
Length EBV (days) is an
estimate of the time from conception to the birth of the calf and is based on
AI and hand mating records. Lower
(negative) GL EBVs indicate shorter gestation length and therefore easier
calving and increased growth after birth.
Birth Weight
EBV (kg) is based on the measured birth
weight of progeny, adjusted for dam age.
The lower the value, the lighter the calf at birth and the lower the
likelihood of a difficult birth. This is
particularly important when selecting sires for use over heifers.
200-Day Growth
EBV (kg) is calculated
from the weight of progeny taken between 80 and 300 days of age. Values are adjusted to 200 days and for age
of dam. This EBV is the best single
estimate of an animal's genetic merit for growth to early ages.
400-Day Weight
EBV (kg) is calculated
from the weight of progeny taken between 301 and 500 days of age, adjusted to
400 days and for age of dam. This EBV is
the best single estimate of an animal's genetic merit for yearling weight.
600-Day Weight
EBV (kg) is calculated
from the weight of progeny taken between 501 and 900 days of age, adjusted to
600 days and for age of dam. This EBV is
the best single estimate of an animal's genetic merit for growth beyond yearling
age.
Mature Cow
Weight EBV (kg) is based on the cow weight when the
calf is weighed for weaning, adjusted to 5 years of age. This EBV is an estimate of the genetic
difference in cow weight at 5 years of age and is an indicator of growth at later
ages and potential feed maintenance requirements of the females in the breeding
herd. Steer breeders wishing to grow
animals out to a larger weight may also use the Mature Cow Weight EBV.
Milk EBV (kg) is an estimate of an animal's milking ability. For sires, this EBV indicates the effect of
the daughter's milking ability, inherited from the sire, on the 200-day weights
of her calves. For dams, it indicates
her milking ability.
Scrotal Size
EBV (cm) is calculated from the
circumference of the scrotum taken between 300 and 700 days of age and adjusted
to 400 days of age. This EBV is an
estimate of an animal's genetic merit for scrotal size. There is also a small negative correlation
with age of puberty in female progeny and therefore selection for increased
scrotal size will result in reduced age at calving of female progeny.
Days to
Calving EBV (days) indicates the fertility of the
daughters of the sire. It is the time
interval between the day when the female is first exposed to a bull in a
paddock mating to the day when she subsequently calves. A negative EBV for days to calving indicates
a shorter interval from bull-in date to calving and therefore higher fertility.
Carcase Weight
EBV (kg) is based on abattoir carcase
records and is an indicator of the genetic differences in carcase weight at the
standard age of 750 days.
Eye Muscle
Area EBV (sq cm) is
calculated from measurements from live animal ultrasound scans and from abattoir
carcase data, adjusted to a standard 400 kg carcase. This EBV estimates genetic differences in eye
muscle area at the 12/13th rib site of a 400 kg dressed carcase. More positive EBVs indicate better muscling
on animals. Sires with relatively higher
EMA EBVs are expected to produce better-muscled and higher percentage yielding
progeny at the same carcase weight than will sires with lower EMA EBVs.
Rib Fat and
Rump Fat EBVs (mm) are
calculated from measurements of subcutaneous fat depth at the 12/13-rib site
and the P8 rump site (from live animal ultrasound scans and from abattoir
carcases) and are adjusted to a standard 400 kg carcase. These EBVs are indicators of the genetic
differences in fat distribution on a standard 400 kg carcase. Sires with low, or negative, fat EBVs are
expected to produce leaner progeny at any particular carcase weight than will
sires with higher EBVs.
Retail Beef
Yield EBV (%) indicates genetic differences between
animals for retail yield percentage in a standard 400 kg carcase. Sires with larger EBVs are expected to
produce progeny with higher yielding carcases.
Intramuscular
Fat EBV (%) is an estimate of the genetic
difference in the percentage of intramuscular fat at the 12/13th rib site in a
400 kg carcase. Depending on market
targets, larger more positive values are generally more favourable.
Docility EBV (%) is an estimate of the genetic differences between
animals in temperament. Docility EBVs
are expressed as differences in the percentage of progeny that will be scored
with acceptable temperament (ie. either “docile” or “restless”).
There are currently three selection indexes calculated for New Zealand
Angus animals. These are Self-Replacing, AngusPure
and Heifer/Dairy Terminal. Each
selection index describes a different production/market scenario and relates to
a typical commercial herd in
Index values are reported as
EBVs, in units of relative earning capacity ($) for a given market. They reflect both the short-term profit
generated by a sire through the sale of his progeny, and the longer-term profit
generated by his daughters in a self-replacing cow herd. More information is available on using
a selection index.
The Index values are derived
using BreedObject technology. More
information is available from the BreedObject web site.
Self-Replacing
Index ($) - Estimates
the genetic differences between animals in net profitability per cow joined in
an example self-replacing
commercial herd that targets the production of grass finished steers. Steers are assumed marketed at approximately
480 kg live weight (265 kg carcase weight and 7 mm fat depth) at approximately
16 months of age. Selected heifers are retained for breeding and the balance
marketed at approximately 16 months weighing 415 kgs (230 kg carcase weight and
8 mm fat depth) As some daughters are retained, maternal traits are also of
importance.
AngusPure Index
($) – Estimates the genetic differences between animals in net
profitability per cow joined in an example
self-replacing commercial Angus herd that targets the production of grass
finished steers for the AngusPure programme. Steers are assumed marketed at 530 kg live
weight (290 kg carcase weight and 10 mm fat depth) at approximately 20 months
of age. Selected heifers are retained for breeding and the balance marketed at
approximately 20 months weighing 450 kg (240 kg carcase weight and 10 mm fat
depth). A significant premium for carcase quality was assumed and, as some
daughters are retained, maternal traits are also of importance.
Heifer/Dairy
Terminal Index ($) –
Estimates the genetic differences between animals in net
profitability per female joined in an example
herd where all progeny are marketed. All progeny are marketed at approximately
510 kg live weight (280 kg carcase weight and 7 mm fat depth) at approximately
24 months of age.
Note that $Index values for
individual animals are sensitive to the assumptions used in the BreedObject
analysis to calculate the selection index.
More information is available on the weightings used in the New
Zealand Angus Selection Indices.
Accuracy (%) is based on the amount of performance information
available on the animal and its close relatives - particularly the number of
progeny analysed. Accuracy is also based
on the heritability of the trait and the genetic correlations with other
recorded traits. Hence accuracy
indicates the "confidence level" of the EBV. The higher the accuracy value the lower the
likelihood of change in the animal's EBV as more information is analysed for
that animal or its relatives. Even
though an EBV with a low accuracy may change in the future, it is still the
best estimate of an animal's genetic merit for that trait. As more information becomes available, an EBV
is just as likely to increase in value, as it is to decrease.
Accuracy values range from
0-99%. The following guide is given for
interpreting accuracy:
Accuracy
range |
Interpretation |
less
than 50% |
EBV
should be considered as a preliminary estimate. It could change
substantially as more performance information becomes available. |
50-74% |
Medium
accuracy, usually based on the animal's own records and pedigree. Still
subject to substantial changes with more information, particularly when the
performance of progeny are analysed. |
75-90% |
Medium
- high accuracy and includes some progeny information. Becoming a more
reliable indicator of the animal's value as a parent. |
more
than 90% |
High
accuracy estimate of the animal's true breeding value. It is unlikely
that the EBV will change much with the addition of more progeny data. |
As
a rule, animals should be compared on EBVs regardless of accuracy.
However, where two animals
have similar EBVs the one with higher accuracy could be the safer choice,
assuming other factors are equal.
For further information please
contact NZ Angus or BREEDPLAN.