Sheep - AgRIS

The formula for calculating the simple coefficient of correlation between two variables is:
XY - X) – (Y)
_________________________
r = X 2 - X 2 Y 2 - Y 2
n
n n
Where X is each individual observation for variable X, Y is each individual observation for
variable Y, n is the number of observations for each variable, and the Greek symbol () means
the summation of all items for each variable or pair of variables.
5. 6.1 Growth rate/Body weight
The age at which lambs are weaned varies under different conditions, but the age of 180
days is often used for selection purposes.
Lambs can be weighed as they reach 180 days, or can be corrected to this age. the
correction is done by multiplying the average daily gain from birth by 180 and adding this
product to the birth weight.
The age of the ewe may have considerable influence on the weaning weight of her lambs.
Two-year-old ewes wean lambs that are from 2 to 4 kg lighter than those from mature ewes.
Production of ewes usually increases to four or five years of ageRrobably the most important
adjustment is that for weaning weights of lambs from ewes that are two years of age. This
adjustment can be made by comparing the production of the two-year-old ewes with that of
mature ewes in the sarne herd and then adding the difference to the weaning weights of lambs
from the younger ewes. Or, if this is not possible, an adjustment may be made by adding 3.1 kg
to the weight of the lambs.
Postweaning Gain is also very highly repeatable. Heavier yearling ewes also wean
heavier lambs and produce heavier fleeces, so selection for this trait would probably be
effective and desirable.
5. 6.2 Reproduchon and Fernlity traits
Sheep in India, and other parts of tropics in the world breed through the year, but are
seasonal breeders in Euporean countries and in most areas of the United states the breeding
season is in the fall. Some breeds, such as the Merino and Dorset Horn, may be bred under
some conditions to produce two lamb crops per year. Most brecds produce just one crop per
year, although rams produce sperm throughout the year. Some rams are suceptible to high
summer temperatures, and may be infertile or of low fertility in late summer during the first
part of the breeding season.
The number of lambs raised per ewe is one of the most important factors determining the
efficiency of production. Lamb production varies a great deal under different conditions and
with different breeds. Ewes under farm conditions are usually more prolific ffian those
produced on the range. This is probably due to a higher level of nutrition generally found in
farm flocks. Twim ing in sheep often is desirable, because a ewe thatweans twins produces
from 10 to 15 kg more lamb than the ewe that weans a single lamb.
Fertility in sheep is lowly heritable, with an average heritability and repeatability estimate
of 7 to 13 per cent. These estimates are in agreement with those for other classes of livestock.
This indicates that fertility in sheep is not greatly affected by additive gene action and could be
improved very little by selection. Most of the phenotypic variation, therefore, is due to
environmental factors and attention to these should improve the lamb crop.
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