Agriculture Reference
In-Depth Information
Genetic Variation
result, a much smaller population of dairy cows
supplies the US market. The US dairy cattle pop-
ulation peaked in 1944 at an estimated 25.6 mil-
lion animals with a total annual milk production
of approximately 53.1 billion kg. In 1997, dairy
cattle numbers had declined to 9.2 million ani-
mals and total annual production was estimated
at 70.8 billion kg. The advent of frozen semen
also dramatically curtailed the number of natu-
ral service dairy bulls on farms, which further
lessened the inputs required to produce a unit of
milk (Capper
et al
., 2009).
There is a trade-off associated with the rapid
dissemination of genetics through populations
by AI, and that is a reduction in genetic diversity.
A good example of the reproductive potential of an
elite dairy bull comes from a bull named Eleva-
tion, born in 1965. He had over 80,000 daugh-
ters, 2.3 million granddaughters and 6.5 million
great-granddaughters (VanRaden, 2007). Such
extensive use of small numbers of sire families has
reduced the genetic diversity of the Holstein popu-
lation. Intense selection leads to rapid genetic
improvement, but it also reduces the relative num-
ber of parents or the effective population size.
Worldwide, estimates of effective population size
in Holsteins range from 100 to 150, despite the
fact there are more than 3.7 million Holstein cows
enrolled in milk recording in the USA. Reduced
genetic diversity can cause a reduction in mean
phenotypic performance as a result of inbreeding
depression. This term refers to the decrease in fit-
ness and vigour that results from the breeding of
related individuals. One of the primary concerns
related to inbreeding is reduced reproduction and
fertility. It has been observed that dairy cow fertil-
ity has been declining at 1% per annum for several
decades. For example, daughter pregnancy rate,
a measure of how quickly cows become pregnant
after having a calf, declined from 33% to 23% over
the period from 1960 to 2007. As with many con-
siderations associated with sustainability, some
balance needs to be reached between the inherent
conflict of accelerating the rate of genetic gain by
increasing the intensity of selection on superior
lines of cattle, and minimizing the rate of inbreed-
ing. Statistical methods have been developed that
allow optimization of the long-term response to
selection and restricted rates of inbreeding by
selecting the best animals while minimizing the
average relationship among the selected animals
(Meuwissen, 1997).
Of the 18 mammalian species and 16 avian spe-
cies commonly consumed for food, six mammals
(cattle, buffalo, sheep, goats, pigs and horses) and
four birds (chickens, ducks, turkey and geese) are
widespread. One common feature of large-scale
animal production systems is that they are based
on a few breeds and so reduce, rather than
increase, genetic diversity. As human popula-
tions have experienced growth and increased
their demand for dietary animal protein, highly
productive breeds have replaced local breeds
throughout the world. This has led to concern
about maintaining the genetic diversity present
in low-production breeds as a source of genetic
variation in traits of interest to future breeding
programmes. Genetic improvement programmes
must always conserve genetic diversity for future
challenges, both as archived germplasm (such as
frozen eggs and sperm) and as live animals
(Blackburn, 2004). According to the FAO, 20%
of the roughly 7600 breeds reported worldwide
are at risk and 62 breeds became extinct within
the first 6 years of this century (FAO, 2007).
Breeding animals for conservation differs from
breeding for production purposes in that the
focus is not on making genetic progress in cer-
tain traits, but rather on maintaining genetic
variation in breeds with a low population size.
The availability of DNA marker information,
used to inform GS for selective breeding, will also
help inform optimum management programmes
to maintain genetic diversity. Ironically, some of
the techniques that have resulted in a reduction
in genetic diversity in agricultural populations
(such as AI, embryo transfer and cloning) offer
opportunities to preserve endangered breeds
through gene banks. Several authors have
described approaches to select which animals
should be cryopreserved to maximize the genetic
variation stored (Caballero and Toro, 2002). The
frozen storage of genetic material is not without
problems, including suboptimal fertility/viability
due to freezing and thawing and the continued
environmental degradation or population pres-
sures that resulted in the breed becoming endan-
gered in the first place.
Another approach to increasing genetic
diversity is to introduce new variability using
genetic engineering (GE). GE refers to the pro-
cess of introducing recombinant DNA (rDNA)
