90
MORTALITY IN OSSIMI AND RAHMANI LAMBS [ Vol. XXIV, Part II,
The average birth weights for Ossimi and Rahmani lambs were 3.65 ± 0.6802
and 3.78 ± 0.4243 kilograms respectively [Asker, et al. 1952]. The mortality rates
for Rahmani and Ossimi lambs, that were heavier in their birth weights than the
breed's average, were 4.6 per cent and 4.4 per cent respectively. On the other hand,
lambs that were lighter than the breed's average, have a mortality rate of 13.3
per cent for the Rahmani and 15.1 per cent for the Ossimi.
The differences in mortality rates between lambs of heavier and lighter weights
than the breeds' averages were statistically significant. Phillips and Dawson [1940]
and Venkatachalam, el al. [1949] came to the same conclusion studying the effect
of birth weight of lambs on mortality rate.
Therefore, breeders should give more concern to birth weight when selecting
animals for breeding purposes. Ragab, et al. [1953] have shown that the heritability
of birth weight in the Ossimi sheep was 34 per cent. This estimate does not justify
the use of phenotypic selection alone for improving birth weight of lambs.
Selection based on the pedigrees of the animals as well as the performances of
their ancestors and collaterals would be more appropriate and helpful in this
respcet.
Effect of breed
Out of 860 Ossimi lambs which were born during the period from 1940 to 1952
only 99 lambs died before weaning age (four months). This brings the mortality
rate in this breed up to 11.5 per cent. In the case of Rahmani, 439 lambs were
born. The number of surviving lambs in this case was 398, and therefore the
mortality rate was 9.3 per cent. Although the Rahmani showed a somewhat less
mortality rate than the Ossimi, yet the difference was not statistically significant.
Some breeds of sheep are known to be more robust than others. Venkatachalam,
et al: [1949] studying mortality rate in Costwald, Oxford, Hampshire, Southdown,
Shropshire and Rambouillet breeds of sheep ; reported that heavy breeds of sheep
showed significantly higher losses than light breeds.
In the case of Ossimi and Rahmani sheep, the difference in birth weight between
the two breeds is not very great [Asker, et al. 1952]. This may be the reason why
no difference in mortality rate was found between the two breeds. Moreover, the
mature weight of both Ossimi and Rahmani ewes is more or less the same, thus no
significant differences in mortality rate between the two breeds would be expected.
It may be worth mentioning here that the mortality rate for those breeds of
sheep studied by Venkatachalam, et al. [1949] ranged between 18.0 and 42.2 per cent
with an average of 28.6 per cent. These are much higher than those of both Ossimi
and Rahmani. It could be suggested here, that higher mortality rate reported by
Venkatachalam, et al., were due to the severe prevailing conditions of Michigan
State where those foreign breeds were studied.
Type of birth
The mortality rate of single Ossimi lambs was 9.9 per cent compared to 8.3
per cent for the twins of the same breed. The Rahmani singles had. a mortality
rate of 11.5 per cent while it was 11.6 per cent for the twins. There were no signi-
ficant differences between mortality rates of single and twin lambs in the two