Selective breeding of the animals

The performance of the animals is an important aspect of the breeding. It aims to the improvement of the results of a livestock, a population, a line or a race, and not only the exceptional result of an individual. This selection is carried out since the beginnings of the Domestication. It implies objectives and varied strategies. The practices of selection can be empirical, appuier on cultural representations or knowledge resulting from a rigorous step. It can be occasional or the object of complex organizations. One often notes, in the modern breeding and for species and races like the pigs or the poultries, a dissociation between the populations of production, and the lines on which is exerted the selection, those providing in reproducers or seeds owners.

Selection

Selection massale

The selection massale does not consist in selecting the animals according to their mass, but choosing reproducers among a whole of animals according to their own performances on one or more selected characters. It will be for example the fact of multiplying by swarming artificial the colonies of Abeilles having the strongest production of honey, of choosing like reproductive hens having the earliest laying, longest or the largest eggs.

Its limits rise from the concept of Héritabilité which measures and indicates to which point the advantage of an individual on a character compared to the average of its congeneric will be transmitted to its offspring.

The selection massale is simple to implement, and allows a good intensity of selection. However it is not applicable to all the case of figure: if the character cannot be measured like the dairy production for a bull, or the quality of the meat for an live animal. The heritability being lower than 100% and sometimes rather weak, other methods of selection can prove to be better

Selection on ascent

It consists in choosing the reproducers, or the individuals on which will carry a second turn of selection, according to the performance of their ascending: a bull will be evaluated for example on the dairy performances of his/her mother, like those of her sisters, who reveal the value of her father for the sought character, but also on the bearing or not of a nature to be controlled or remove: http://omia.angis.org.au/

This aspect is extremely important, because a line with high genetic value on a character can lose its interest because of other undesirable characters.

It implies a reliable knowledge of ascending animals. The estimate of the genetic value of the animals by this skew calls upon formulas which take account of the proximity of the ascending ones. It is noticed that the influence of ascending distant from some generations quickly becomes very weak.

Selection on descent

The descent of the reproducer is tested on one or more generations. One can evaluate the line capacity of a character.
Le time necessary with this evaluation can take several years: the heifers resulting from a bull does not start to show their capacity of dairy after three years. To cure that, the seed of the male is frozen. A male with recognized qualities, can thus reproduce well after its death.

Selection with the production

So in the traditional systems, the selection and the production of animals can be confused, the modern organization implies a separation between these two roles. One adds to it even in general an intermediate stage which is the multiplication. This one consists in being provided in selected animals, of number limited, and possibly irregular, for then providing the stockbreeders of production in reproducer of good quality according to their request. The advantage is to make it possible the selective breeding to free itself from the constraints of production and even of regular supply of reproducers, these functions peuvant to be contradictory with its objectives. The disadvantage is that the effective population of a race, that whose genes will be transmitted is very reduced, in spite of the total staff complements which can be important. One has of which the risk of a reduction of genetic inheritance.

Crossing of improvement

This one consists in operating crossings between a race known as improving and a race which one seeks to improve. One can thus obtain this progress more quickly than with a simple selection within the race. The improving race must then have a real advantage for the required characters and not to have defects importants.
The crossing is carried out in general on only one generation, and not inevitably on the whole of the reproductive population of the race. Indeed it is not a question to give up the whole of the characters of this one.

Included/understood thus, this strategy is thus distinguished from the crossing of production, where the products are not retained like reproducers, but also crossing of absorption which consists has to repeat the crossing on several generations.

A race which is the object of a crossing of improvement is modified but perdure. The crossing of absorption can however be confused with the crossing of absorption and cause the disappearance of little specialized regional races.

Crossing of absorption

This one consists starting from a race or animal population existing and available, to call on several generations upon male reproducers of another race, towards which one wants to make evolve/move the first. The percentage of blood, or the theoretical and average genotype resulting from the absorbing race is calculated then easily according to the number of generation, and will be the continuation: 50%; 75%; 87,5% (products of third generation); 93,75%,96,88; 98,44; 99,22 (products of 7th generation)… The number of generation required to consider that the products belong to the absorbing race varies according to the choices and designs of the organizations of breeding between three generations and the exclusion of this possibility whatever the number of generation. This diagram of breeding is used to replace a race by another considered to be more interesting, under economic conditions and of implementation easier than the direct replacement of the animals. It makes it possible in other cases to quickly diffuse the characters of a low number of reproducers. It was also used in the case of the Baudet of Poitou more quickly to reconstitute manpower of this threatened race, starting from she-asses of race to the relatively close characteristics.

Crossing of production

This one consists in choosing reproducers whose products are intended for consumption or the use, but which will not be a priori not preserved themselves like reproducers. The crossing of several races of a species, even of different species often has an interest for the production. The produced animals present in general have an advantage called hybrid strength or Hétérosis, i.e. performances appreciably better than those of the average of their parents on many characters. In the parental measurement or lines or races are homogeneous, their products are it also. Finally this practice makes it possible to combine saving in breeding, adaptation to mediums of breeding and good growths or general performances of the products; that when one causes production of rustic mothers of race (Mouton S, Poney S) in crossing with races " améliorées" , or of the only fact that the maternal line is moreover small size or less corpulent than the paternal line: the weight and the growth of the products will be thus higher than that of the young people of the maternal line of pure race.

The crossings utilizing several species are the Mulet, produced mare and ass, the duck mulard, produced Musk duck and Canard domesticates, as well as the crossing Chameau of Bactriane and Dromadaire.

The hen and the Porc are the subject of programs of crossing based on this principle, which can utilize 3 or 4 different lines, i.e. the lines on the level of the grandparents of the finished products, in order to obtain this effect and this economy not only on the level of those, but also of the reproductive females. Such practices imply well-on distinguishing the lines relationships clearly, (to which possibly the efforts of improvement by selection relate), and products of these crossings.

Historically, the distinction was not clear between the crossing of production and the crossing of improvement, even of absorption. In the case of rustic races, one practiced the crossing with improved races, which brought the quoted advantages. It is however a form of natural selection or the resistance of the stockbreeders vis-a-vis institutions convinced of the interest of the races improved which caused the conservation of an original local race, and the continuation of a crossing of production in fact while the project was to transform this local race or to substitute a news to him.

Currently, the countries and organizations which wish to preserve their genetic tank preserve a population in pure race. The individuals resulting from this breeding are then crossed with more powerful races for conjuger the productivity and the capacities of adaptations to a harsh environment.

The crossings of production are of a risky use if the maintenance of the races or lines relationships is badly ensured, and that the products of these crossings can in the long term replace them. It is interesting if it is simple and inexpensive of reprocurer of the subjects of the lines used and is particularly employed for the poultries and the pigs. The median value of the females is important in the value of a production equipment. Then, combination of females powerful and/or adapted well to a system of breeding and a medium (mountainous, arid, cold…) and the contribution of male reproducers of “industrial” lines makes the interest of the crossing of production.

A lighting example: the dairy capacity of a female depends on the harmony, taking into consideration these character, of genetic inheritances of the male and the female, because it depends on the hormonal rate produced by the placenta (emanation of egg) during gestation. If it can appear a success to obtain the new born ones of beautiful conformation, it is very important also that the mother can bring the quantity of desirable milk to them to their optimal growth.

See too

External bonds

genetic Evaluation and selection: Presentation
animal breeding 2004 - INA P-G

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