Sheep - AgRIS

infarm flocks. Twinning in sheep often is desirable, because a ewe that weans 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.
13.2.2 Growth rate & feed conversion efficency
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 age. Probably 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 same herd and then adding the difference to the weaning weights of lambs
from the younger ewes. If this is not possible, an adjustment may be made by adding 3.1 kg to
the weight of the lambs.
13.2.3 Type and Conformation
Desirable type and conformation have also recieved attention in sheep as in other classes of
farm animals. With this species, however, attention must also be paid to selection for wool
production in addition to mutton quality and rate and efficiency of gains.
Animis possessing vesry obvious defects, such as over-shot jaws, undershot jows, black
wool, wool-blindness, skin folds, shallow bodies, and poor mutton qualities, should be culled
from the flock. If animals with these defects are eliminated from the breeding flock and
selections are made on the basis of body weight and quantity and quality of wool, especially in
the selection of rams, perhaps this will be sufficient attention to type.
13.3 Factors affecting mutton Production
13. 3.1 Body weight
Body growth affects the composition of the lamb. It has been observed by many research
workers (Reid et al., 1968) that the 63 percent, 95 percent and 88 percent of variation in ash,
protein and fat, respectively are due to changes in body weight at slaughter.
13. 3.2 Heredity
The rate of growth varies from breed to breed, even if given same type of fodder and
concentrate to all the breeds. The growth rate is affected by the adult body weight (Dackson,
1974).
13.3.3 Sex
The sex hormones of males and females affect their growth rate. The ewe differ from the
ram in that it has slower growth rate, a more early maturing carcass and reaches a lower mature
size due to the effect of oestrogen in restricting the growth of long bones of the body.
13.3.4 Castration
The castration of male decreases their growth rate and they mature at an early age. The
appreciation of the central role of the sex hormones in growth and development as manifested
in the consequences of castration have led to extensive study of the effects of the administration
of natural or synthetic hormones to meat animals in order to influence performance.
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