Cross-breeding

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.