MANUAL FOR EVALUATION OF BREEDS AND CROSSES OF DOMESTIC ANIMALS
The purpose
of this manual is to suggest practical guidelines for evaluating the usefulness of available breeds or strains of
livestock species and their crosses under diverse world production-marketing
environments. For many years much attention has been focussed on performance comparisons among breeds and their
crosses (see indexes of Animal Breeding Abstracts). This emphasis is
justified because genetic differences among breeds or strains are large
relative to genetic variation within breeds, (e.g., Cundiff et al., 1986).
These differences are an important potential source of genetic improvement in
the efficiency of human food production
from livestock through 1) expansion of superior breeds, 2) gains in performance
from complementary breed effects and heterosis in crossbreeding, 3) development of superior new breeds from selected
combinations of several breeds, and 4) preservation of potentially
useful genetic stocks that are in danger of extinction.
The enormous number and variety of local breeds and strains of each
livestock species (Phillips et al., 1945; Cheng, 1984; OAU, 1985; FAO, 1986; Mason,
1988; Maule, 1989; Crawford, 1990) has
developed over very long periods of time. This diversity has developed partly
from both natural and artificial selection for performance (Mason, 1973) under
diverse world production-marketing environments (Phillips, 1961), and
partly from cumulative random changes in
gene frequency in relatively small local populations (Lush, 1946; Wright, 1948).
Patterns of
human and accompanying migrations have limited the sampling of potentially
useful breeds available in any given geographical region, (Stonaker, 1961), and
still do, in spite of recent improvements
in world-wide communication and transportation. Animal health and trade
restrictions also continue to be serious constraints on movement of livestock
between countries (CAST, 1984). However, improvements in diagnostic tests for
safe movement of animals or frozen semen and embryos are gradually easing these
limitations (O.I.E. 1985).
The other
serious limitation on effective current and future use of world animal genetic
resources is the lack of adequate information on relative performance abilities
of alternative genetic stocks under the
variety of existing and potential major world production-marketing
environments (ecosystems). Feasible improvements in poor production environments also need to be evaluated to allow
optimum use of genetic potential (Hammond, 1947, 1949; Dickerson, 1970; Donald, 1973). The problem is how to
identify those breeds, wherever located, that are best suited to future
needs of each region, and to learn how they may
be used most efficiently in producing animal products (Lee and Phillips, 1948;
Phillips, 1967; Dickerson, 1969, 1973; Duarte, 1989; Smith, 1989).
Most efficient use of world-wide
animal genetic resources in any given country or region would seem to require:
Careful
definition of performance objectives for the species, considering the economic effects of both production costs and market prices expected
under likely future production-marketing systems.
Identification
and adequate sampling of the more promising indigenous and exotic breeds,
based upon existing information.
Designing and
conducting experiments to determine which breeds and methods of using them are likely to permit greatest gain in efficiency under the
economically feasible production/marketing environments.
Implementing
live or cryopreservation of any endangered genetic stocks shown to be of
likely future value.
