Page 11 - Brookwood Angus Sale 2021 ebook

UNDERSTANDING THE EBVS,
SELECTION INDEXES AND ACCURACY

EBVs
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, TRANSTASMAN ANGUS CATTLE
EVALUATION (TACE) 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.

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