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, BREEDPLAN EBVs for Namibian Brahman cattle 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 (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
visually 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 % Dir = 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 650 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 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.
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 300 kg carcase. These EBVs are indicators of the genetic
differences in fat distribution on a standard 300 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 300 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
300 kg carcase. Depending on market
targets, larger more positive values are generally more favourable.
There is one standard selection
index calculated for Namibian Brahman animals: Extensive Range Grazing Index.
This index has been developed for a specific production / market scenario.
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.
Brahman
Rangeland Grazing Index
(R) - Estimates the genetic differences between animals in net profitability
per cow joined for an example commercial herd targeting steers for Export
markets. Steers are assumed to be pasture grown & finished at around
500 kg live weight or 250 kg carcase weight at 30 months. The index also has
moderate emphasis on calving ease and maternal traits. Replacement cows are
generally sourced from within the herd and daughters are largely retained in
the industry for breeding. The index is also suited to using Brahman sires over
mixed breed cows.
Brahman Wean
Index (R) - Estimates
genetic differences between animals in net profitability per cow joined for an
example self replacing commercial herd producing
weaners for sale at about 7 months of age off pasture. The index has emphasis
on calving ease and maternal traits while acknowledging that these animals will
be grown on to be finished and slaughtered at an older age. This index is
suited to select sires and dams where more emphasis on cow traits is required.
However, you are strongly encouraged
to consider selecting replacement animals using both the Wean Index as well as
a later finishing index (Rangeland or Feedlot) at the same time.
Brahman
Feedlot Index (R) -
Estimates genetic differences between animals in net profitability per cow
joined for an example self replacing commercial herd
targeting steers to turn off at 12 months of age. There is some emphasis on
calving ease and maternal traits while finishing steers at around 400 kg live
weight (230 kg carcase weight) after 120 days on feed. This index is also suitable to using Brahman
sires over mixed breed cows.
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 assumptions and weightings used in
the Namibian
Brahman selection index.
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 40% |
Very low accuracy. EBVs should be considered a preliminary
estimate. They could change
substantially as more performance information becomes available. |
|
40-65% |
Low accuracy, usually based on the animal's
own records and pedigree. Useful for
screening "best bet" animals.
Still subject to substantial changes with more information,
particularly when the performance of progeny are analysed. |
|
65-80% |
Medium accuracy and includes some progeny
information. Becoming a more reliable
indicator of the animal's value as a parent. |
|
80-95% |
High accuracy. Some progeny information included. Unlikely that the EBV will change much with
the addition of more progeny data. |
|
more than 95% |
Very 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.