Cross-breeding
There are very many situations in which the ideal
producing animal is some intermediate between a tropical adapted and an
improved temperate breed. This
applies
particularly to dairy cattle kept under intensive conditions in the humid
tropics and in many parts of the wet-dry tropics and to beef cattle under range
conditions in the wet-dry tropics and the subtropics. This intermediate can
most easily be formed by crossbreeding.
A large portion of the world's goat population is
maintained under free ranging conditions in which controlled mating are not
possible. Thus, the mating system may approach random mating. However, to the
extent that only one or two breeds are prevalent in a general area, it also
might be considered as pure breeding.
The amount of inbreeding which occurs would be largely
a function of size of the population. True crossbreeding would be possible only
if controlled mating were possible with distinct types. Extensive use of
crossbreeding could be practical only with the dairy goats, in that with other
types only a single genotype is often available. as with the Barbari for meat
production (under production conditions), crosses between the Barbari and
meat-type goats often occur, but more by accident than intent, as the result is
a serious disruption to the primary function of both types and should be
avoided. For the most part, these crosses have been initial steps in creation
of new types and not to exploit the phenomenon of heterozygosis. The results of
the initial crosses appear to be intermediate in type of meat produced, much as
might be expected.
Maintaining
a crossbred population
When it has been established that in a given
environment (including climate, disease situation, feeding and management
systems) the most productive animal is one containing a proportion of local
blood and a proportion of imported genes, then an appropriate breeding system
must be chosen which will maintain this intermediate type after the initial
crossing. There are several possibilities.
New
breed formation
This is also called the formation of a gene pool. After
the first cross, F1 animals are mated together to form an F2 followed by an F3,
and so on. If the desired proportion of Outside blood is 75 percent then a
backcross is made before the inter sex mating starts. The advantage of the
gene-pool approach is that the population is self-replacing and after the
initial cross no further outside blood is needed.
People are afraid that breeding from crossbreds will
lead to excessive variation, and therefore that formation of a new breed is
only possible if immense resources of animals are available. In fact, the
increase in variation on breeding an F2 generation has been much exaggerated.
To begin with, it is only the genetic variation which is increased, and with
characters of low heritability this is only a small part of the total
variation. Secondly, the obvious effects are due to segregation of color, color
pattern and morphological characters (e.g. horns) which are controlled by
single genes. The increase in variation between F1 and F2 for such quantitative
characters as growth rate and milk yield will be very small and, in view of
other sources of variation, may not be detectable.
Certainly there is a danger that if the initial
crossing is not done on a large enough scale then problems of inbreeding may be
encountered. This is particularly the case if the breed is based on a single
herd or on only one or two imported bucks. Inbreeding leads to decline in
fertility, viability and growth rate to a greater or lesser extent. This has
often not been realized and inbreeding has been deliberately employed in order
to concentrate the blood of outstanding bucks or to produce uniformity in a new
breed. But uniformity is an elusive goal, a will-o'-the-wisp. It must not be
sought at the expense of productivity. Indeed it is essential at the outset to
have as much variation as possible in order to allow scope for selection. The
aim of crossbreeding is to combine the high yield of the outside breed with the
resistance of the local breed, and intense selection is needed to find the few
animals combining both characters. If a breed is to be formed in a single
experimental herd it is therefore essential to use a large number of unrelated
bucks (at least 10–15) in the formative stages. As early as possible the
program should be extended to other herds and bucks should be exchanged between
herds. Above all, animals should not be selected according to color,
conformation or other fancy points but for important economic characters.
Systematic
crossbreeding
The essence of a systematic crossing system is that
breeding animals of both pure breeds involved (local adapted and temperate
improved) are used in each generation. There are two main categories - terminal
crossing and rotational crossing.
1.
Terminal crossing
In this type of cross the progeny are not used for
breeding. It is therefore not suitable for dairy animals but is important for
meat animals. There are several varieties:
a.
Re-creation of thein each generation
Since it is the commercial F1 animals which are
produced every generation this is also called “commercial crossing”. Both sexes
are slaughtered (for meat) without being used for breeding.
b.
Three-way crossing
Commercial crossing exploits only the hybrid vigour in
the growing animal. In order to cash in also on that in maternal ability a
second stage of crossing is needed so as to breed from a crossbred dam.
This is very important because a major proportion of
the total advantage may come from the use of crossbred dams. The second cross
may be a backcross to the buck breed or a cross to a third breed.
This system has its maximum advantage if the crossing
is stratified according to the environment. In Britain, for instance, local
breeds of sheep are maintained on the mountains; they are crossed with a hardy
improver breed and the crossbreds are kept in the hills; the crossbred ewes are
crossed with a second improver breed and the second-cross lambs fattened in the
lowlands.
A similar scheme could be applicable in the tropics
with the environment being superior in each phase in terms of rainfall, temperature
or feeding level rather than altitude.
2.
Rotational crossing
In crisscrossing (or reciprocal backcrossing) bucks of
the temperate and tropical breeds are used in alternate generations.
Crisscrossing
(reciprocal backcrossing) between temperate and tropical breeds
At equilibrium two-thirds of the hybrid vigour of the
F1 is retained. In alternate generations progeny will be two-thirds tropical or
two-thirds temperate in their genotype. This system is very flexible; if more
than an average of 50 percent temperate blood is needed then there can be two
generations of crossing with the temperate breed alternating with one cross
with the tropical breed. In this case at equilibrium succeeding generations
will have 3/7, 5/7 and 6/7 of the temperate blood or the tropical breed can be
replaced by a half bred - either a new intermediate breed or a half-bred buck.
Rotational
crossing between temperate and tropical breeds
A similar result can be achieved by using three
different breeds in the crossing. It then becomes rotational crossing and 87
percent of the maximum heterosis is retained.
This system is also flexible in the face of changing
market demand - the type of crossing breed can be changed immediately according
to the product needed.
Systematic crossbreeding systems need a source of
crossing bucks. The tropical bucks should come from an improved local breed
under selection. The temperate bucks should also come from nucleus herds
selected in the local environment rather than from selection programs overseas
in view of the phenomenon of genotype x environment interaction.
3.
Combination of terminal crossing and rotational crossing
A meat sire can be used on the crossbred females which
emerge in a crisscrossing or rotational crossing system. It is terminal because
the offspring of this sire are not used for breeding but are all slaughtered
for meat.
Use
of crossbred bucks
In this scheme half bred bucks are produced in each
generation and used to grade up the local females. It is thus suitable for
situations in which it is desired to stabilize a 50:50 mixture of local and
exotic strains. The half bred bucks would have to be bred in a special herd and
supplied to the local farmer either on a loan system (so that each was not used
too long in one herd). The female populations will in a few generations
approach 50 percent outside blood. The first cross on to the local females is
by a pure outside buck and the specially bred F1 bucks are then used on these
F1 does. The population is then 50 percent outside from the first generation.
It must be emphasized once more that all these schemes
require a source of temperate bucks which should preferably be selected in an
environment similar to that in which they are to be used. They also require a
selection program for the indigenous breed which is used in the crossing
program.
