Sheep - AgRIS

ii) Independent Culling Method
In this method, selection may be practised for two or more traits at a time, but for each trait
a minimum standard is set that an animal must meet in order to be selected for breeding
purposes. The failure to meet the minimum standard for any one trait causes that animal to be
rejected for breeding purposes. Let us assume that Pig A was from a litter of 9 pigs weaned,
weighed 77 kg at 5 months, and had 1.3 inches of backfat. For Pig B, let us assume that it was
from 7 iitter of five weaned, weighed 94 kg at five months, and had 0.95 inches of backfat at 84
kg. If the independent culling method of selection were used, Pig B would be rejected, because
it was from a litter of only five pigs. However, it was much superior to Pig A in its weight at
five months and in backfat thickness, and much of this superiority could have been of a genetic
nature. Thus in practice, there is liklihood to cull some genetically very superior individuals
when this method is used.
The independent culling method of selection has been widely used in the past, especially in
the selection of cattle and sheep for show purposes, where each animal must meet a standard of
excellence for type and conformation regardless of its status for other economic traits. It is also
used when a particular colour or colour pattem is required. It is still being used to a certain
extent in the production of show cattle and sheep. It does has an advantage over the tandem
methood, when selection is practiced for more than one trait at a time. Sometimes, it is also
advantageous, because an animal may be culled at a young age for its failure to meet minimum
standards for one particular trait, when sufficient time to complete the test might reveal
superiority in other traits.
iii) The selection lndex
This method is based onthe separate detemlination of the value for each of the traits
selected for and the addition of these values to give a total score for all the traits. The animals
with the highest total scores are then kept for breeding purposes. The influence of each trait on
the final index is detemlined by how much weight that trait is given in relation to the other
traits. The amount of weight given to each trait depends upon its relative economic value, since
all traits are not equally important in this respect, and upon the heritability of each trait and the
genetic associations among the traits.
The selection index is more efficient than the independent culling method, as it allows the
individuals which are superior in some traits to be saved for breeding purposes even though
they may be slightly deficient in one or more of the other traits. If an index is properly
constructed, taking all factors into consideration, it is a more efficient method of selection than
either of the other two described earlier, because it should result in more genetic improvement
for the time and effort made its use.
Selection indexes seem to be gaining in popularity in livestock breeding. The kind of
index used and the weight given to each of the traits is determined to a certain extent by the
circumstances under which the animals are produced. Some indexes are used for selection
between individuals, others for selection between the progeny of parents from different kinds of
matings, such as line-crossing and crossbreeding, and still others for the selection between
individuals based on the merit of their relatives, as in the case of dairy bulls, where the trait
carmot be measured in that particular individual.
6. 2. 2.1 Breeding native Sheep among them
Information available on some of the important carpet wool breeds in the country is given
in Table 6.1 . A major portion of good carpet wool comes from breeds in Rajasthan, Punjab,
Haryana and Gujarat states. The common breeds are Magra, Jaisalmeri, Marwari, Pugal, Nali,
Malpura and Sonadi in Rajasthan, Nali in Punjab and Haryana, and Patanwadi in Gujarat. Of
these Chokla, Nali and Patanwadi produce finer wool. The available literature (Table 6.1)
indicates that the average fibre diameter for 6 monthly clip of Magra, Nali and Pugal is
32.45,34.22 and 35.13 microns; medullation percentage 48.29,30.74 and 61.86 and staple
length 5.81, 8.31 and 5.17 cm respectively. These values indicated much higher medullation
and kemp percentage than recommended, as also the coefficients of variation in excess of 50%
for both medullated and non-medullated fibres.
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