Agriculture Reference
In-Depth Information
One approach is to include functional traits
that fall under the general heading of 'sustaina-
bility' in BO, using methodology developed for
deriving profit-maximizing selection indexes.
Traits that might fall into this category include
disease resistance, reproduction, longevity and
well-being. These functional traits are related to
animal health and welfare and should arguably
have been included in the BO from the beginning
as they ultimately affect the profitability of an
enterprise. Where these traits have declined as a
result of selection it is generally due to their
absence in the BO. There are several reasons why
they have not been included in the BO up to this
point; low heritability (the proportion of
observed variation that can be attributed to
inherited genetic factors in contrast to environ-
mental ones), lower MV than production traits
and/or negative genetic correlations with pro-
duction traits. A negative genetic correlation
between two traits means that selection for one
trait (e.g. production traits) results in an
unwanted change in the second trait (e.g. func-
tional traits). Some examples include single trait
selection for milk yield resulting in reduced
resistance to mastitis and fertility in dairy cows,
selection for increased growth rate resulting in
increased leg problems and ascites (pulmonary
hypertension) in broiler chickens, and selection
for lean meat production resulting in reduced
stress tolerance in pigs (Rauw
et al
., 1998).
Including functional traits or other traits in
BO will reduce the selection pressure (and hence
slow genetic progress) in production traits.
Production traits typically possess both high
heritability and high MV, and so low heritability,
low MV functional traits need to be given an
inflated emphasis in the selection index to
achieve the same rate of genetic progress, or at
least to minimize their decline. Animal breeders
are always negotiating trade-offs among com-
peting goals, especially when it comes to breed-
ing for sustainability (Gamborg and Sandøe,
2005). Typically it has been difficult to obtain
records on functional traits, and the old adage
that 'you can't manage what you do not meas-
ure' is particularly true for animal breeders. This
raises an important point as it relates to breeding
for sustainability. Animal breeding programmes
that involve complex traits such as robustness,
animal well-being, or disease resistance in the
selection objectives require well defined phenotypes
(records) upon which to base selection decisions.
Identifying a phenotype that can be observed
with high repeatability (test-retest reliability)
and which can be used as selection criteria to
quantify complex functional traits in the BO can
be difficult, and ideal traits may be very expen-
sive or impractical to measure. In that regard,
an important advance will be development of
objective, quantifiable measures of welfare,
which could be used as selection criteria for
breeding decisions (Hume
et al
., 2011). Functio-
nal genomics has the potential to provide bio-
markers, which can be used to define complex
traits such as behaviour, stress, or disease in unique
quantifiable ways (Kadarmideen
et al
., 2006).
Some important examples where func-
tional traits have been added to BO include the
incorporation of fertility and disease resistance
traits into dairy cattle selection indexes, and the
inclusion of leg traits into poultry breeding.
Table 5.3 shows the ten dairy traits that are cur-
rently included in the US dairy selection index
Table 5.3.
Year that genetic rankings began and emphasis placed on dairy traits in 2010
US national dairy selection indexes.
Trait
Year begun
Emphasis (%)
1. Milk
1935
0
2. Milk fat
1935
16
3. Milk protein
1977
19
4. Calving ease/stillbirth
1978/2006
5
5. Udder shape and support
1983
7
6. Feet and leg conformation
1983
4
7. Body size/weight
1983
−6
8. Productive life/longevity
1994
22
9. Mastitis resistance
1994
10
10. Daughter pregnancy rate/fertility
2003
11
