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Understanding the EBVs, Selection Indexes and Accuracy
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 positive or negative
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 +30 kg for 400-Day Weight
is estimated to have genetic merit 30 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. The breed averages for the current
two year old animals are reported with the selected animals’ EBVs.
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,
British Simmental BREEDPLAN EBVs cannot be validly compared
with EBVs for any other breed.
EBVs are published for a range of traits covering fertility,
calving ease, milking ability, growth and carcase merit.
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.
(%) are based on calving difficulty
scores, birth weights and gestation length information.
More positive EBVs are favourable and indicate easier calving.
= 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.
= Daughters' Calving Ease - The EBV for
daughters' calving ease indicates how easily that sire's
daughters will calve at two years of age.
(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 a tendency for easier calving and increased
growth after birth.
(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.
(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.
(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.
(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.
(kg) is based on the cow weight
when the calf is weighed for 200 days, 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.
(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 own milking
ability.
(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.
(kg) is based on abattoir carcase records
and is an indicator of the genetic differences in carcase
weight at the standard age of 650 days.
(sq cm) is calculated from measurements
from live animal ultrasound scans and from abattoir carcase
data, adjusted to a standard 300 kg carcase. This EBV estimates
genetic differences in eye muscle area at the 12/13th rib
site of a 300 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.
(mm) are calculated from measurements of subcutaneous
fat depth at the rib (from live animal ultrasound scans
and from abattoir carcases) and are adjusted to a standard
300 kg carcase. This EBV indicates the genetic difference
in fat distribution on a standard 300 kg carcase. Sires
with a low, or negative, fat EBV are expected to produce
leaner progeny at any particular carcase weight than will
sires with higher EBVs.
(%) indicates genetic differences
between animals for retail yield percentage in a standard
300 kg carcase. Sires with larger EBVs are expected to
produce progeny with higher yielding carcases.
(%) is an estimate of the genetic
difference in the percentage of intramuscular fat at the
12/13th rib site in a 300 kg carcase. Depending on market
targets, larger more positive values are generally more
favourable.
The British Simmental Cattle Society has developed the
Terminal Production Index. Based on the BREEDPLAN performance
recording technology, the index is calculated for commercial
cattle producers wishing to produce slaughter progeny from
larger framed, mature cows. The Index is reported as an
EBV, in units of relative earning capacity (£’s)
per cow mated for the given market and reflects the short-term
profit generated by a sire through the sale of his progeny.
- Estimates
the genetic differences between animals in net profitability
per cow mated for an example commercial herd targeting
the EU market with no marbling requirement. All progeny
are assumed to be marketed at around 550-630 kg live weight
(290-340 kg carcase weight) at approximately 14-15 months
of age.
Bulls are ranked by their genetic potential for the production
of prime steers and heifers for beef production. While
growth and carcase EBV’s are the main drivers used
in compiling this Index, some emphasis is also placed on
calving ease direct, which is an estimate of genetic difference
between animals ability to calve without assistance.
Typical production parameters, prices and costs underpin
this selection Index. Benefits and costs evaluated include
both those for the sale animal from birth to slaughter
and average running costs for a typical cow herd. Feed
is assumed a limited resource for a large part of the year
and any increase in feed requirement is a cost.
Bulls with a higher BSCS Terminal Production Index will
have a greater genetic potential to sire heavier progeny
at slaughter age. As there are no maternal traits included
in the calculation of the BSCS Terminal Index, bulls with
a high Index will not necessarily be suitable for breeding
heifer replacements.
The Society will develop an Index in 2005 which is more
suited to a beef production system that also produces replacement
females. Should you require a bull suitable for breeding
replacement females before the “self replacing” index
is developed, then you should consider the EBVs for maternal
traits in conjunction with the growth traits to help your
selection decisons.
The Indexes are being developed using BreedObject. More
information is available from the BreedObject web site.
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 relationships
(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, there is an equal
likelihood that an EBV will 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% |
Low accuracy. EBVs are preliminary and could change
substantially as more performance information becomes
available. |
| 50-74% |
Medium accuracy, usually based on the animal's own
records and pedigree. |
| 75-90% |
Medium-high accuracy. Some progeny information included.
EBVs may change with addition of more progeny data. |
| more than 90% |
High accuracy estimate of the animal's true breeding
value. |
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.
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