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 ($’s) for a given market. They reflect both
the short-term profit generated by a sire through the sale of his progeny, and
the long-term profit generated by his daughters in a sustainable cow herd, if a
proportion of heifers are retained for breeding.
The Indexes are derived using
BreedObject technology. More information on this technology is available
from the BreedObject web site.
Domestic Production
Index ($) – Estimates the
genetic differences between animals in net profitability per cow joined for an
example self replacing commercial herd (run in a temperate environment)
producing steers for the Domestic trade. This Index assumes that the
steer progeny are pasture grown and finished and marketed at 500 kg live weight
(280 kg HSCW and 12 mm P8 fat depth) at 16 months of age. Daughters are
retained for breeding. More information is available on the EBV weightings
for the Domestic Production Index.
Export Production Index
($) – Estimates the genetic
differences between animals in net profitability per cow joined for an example
commercial herd (run in a sub-tropical environment) producing steers for the
Export trade. This Index assumes that the steer progeny are pasture grown
and finished and are marketed at 620 kg live weight (340 kg HSCW and 15 mm P8
fat depth) at 28 months of age. Daughters are retained for breeding.
More information is available on the EBV
weightings for the Export Production Index.
Note that $Index Values for individual
animals are sensitive to the assumptions used in the BreedObject analysis to
calculate the relevant 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 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 Tropical Beef Technology Services
or Santa Gertrudis BREEDPLAN