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, Santa
Gertrudis 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.
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 Eye Muscle Area EBVs are expected to produce better muscled and higher percentage yielding progeny at the same carcase weight than will sires with lower Eye Muscle Area 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 (marbling) at the 12/13th rib
site in a 300 kg carcase. Depending on market targets, larger more
positive values are generally more favourable.
Santa Gertrudis selection indexes are calculated for two market specifications, namely, Domestic and Export. These Indexes relate to typical commercial herds targeting these specifications.
Indexes are reported as an
EBV, in units of relative earning capacity ($’s) for a given market. They
reflect both the short-term profit generated by a sire through the sale of his
progeny, and the long-term profit generated by his daughters in a sustainable
cow herd, if a proportion of heifers are retained for breeding. A selection index combines the EBVs with
economic information (costs and returns) for specific market and production
systems to rank animals based on relative profit values. Note that
different types of animals can give similar profit values, so consideration
should be given to both the index and the component EBVs when selecting animals
for a particular production system. More information is available on using a selection index.
The Indexes are derived
using BreedObject technology. More information on this technology is
available from the BreedObject web site.
Breeding
Production Index ($)
– Estimates the genetic differences between animals in
net profitability per cow joined for a typical self-replacing commercial
herd in productive country with good management. Selected heifers
are retained for breeding and so maternal traits are of importance. Heavy export markets
are the target for all turn-off animals at weights above 520kg (260kg carcase
weight) and 10-15mm fat at 22-30 months of age. Given this is a breeding
index for herds where seasonal conditions, country type and herd management are
less likely to adversely affect herd productivity, a balance of growth, carcase
and fertility traits is adopted.
Fertility
Plus Index ($) – Estimates the genetic differences between animals in net
profitability per cow joined for a typical self-replacing commercial
herd in less productive country and/or the herd is run under lower
management intensity. Selected heifers are retained for breeding and so
maternal traits are of importance. Heavy export markets are the target
for all turn-off animals at weights above 520kg (260kg carcase weight) and
10-15mm fat at 22-30 months of age. Given this is a breeding index
for herds where seasonal conditions, country type and/or herd management may
regularly adversely affect herd productivity, fertility, growth and carcase
traits are considered with fertility being the trait of most influence.
Terminal Index ($) – Estimates the genetic differences between animals in net profitability per female joined in a typical commercial herd where all progeny are marketed. Heavy export markets are the target at weights above 520kg (260kg carcase weight) and 10-15mm fat at 22-30 months of age. Given this is a terminal index, growth and carcase traits are the focus, along with calving ease which helps ensure live calves are born. Fertility traits are not considered.
Note that $Index Values for
individual animals are sensitive to the assumptions used in the BreedObject
analysis to calculate the relevant selection index. More information is available on the
weightings used in the Santa
Gertrudis Selection Indexes.
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% |
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.
For further information
please contact ABRI Extension
Services or Santa Gertrudis BREEDPLAN
BREEDPLAN and GROUP BREEDPLAN
results are calculated using software developed by the Animal Genetics and
Breeding Unit, a joint venture of NSW Department of Primary Industries and the
University of New England, which receives funding for this purpose from Meat
and Livestock Australia Limited.