Page 10 - Okaka 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, NZ ANGUS 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.
CE % Direct = 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 % Dtrs = 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.
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