Íslenskar landbúnaðarrannsóknir - 01.03.1977, Page 41

F,WE PRODUCTION TRAITS 39
mated the heritability of Blackface and Welsh
Mountain in Scotland at 0.14 and 0.16 re-
spectively and Forrest and Bichard
(1974b) found 0.14 for Clun Forrest ewes
in England. Sharafeldin (1960) found
heritability estimates of 0.17 and 0.05 for
Texel ewes in two provinces in the Nether-
lands. In New Zealand, Chang and Rae
(1970) found a heritability estimate of 0.05
for Romney ewes.
The estimates for Icelandic ewes are there-
fore among the highest found in the literature
for number of lambs born.
In other studies (Eikje, 1975; Bradford,
1972; Young et al., 1963) evidence is found
of variation in heritability depending on the
age of ewe. In this material there was no
evidence of such variation.
The heritability of the number of lambs
at weaning is found to be 0.13 or a little
lower than that for the number of lambs born.
The loss of lambs from birth to weaning is
mainly caused by environmental factors and
it is therefore natural that this heritability
estimate is lower than for the number of
lambs born.
The heritability for the score of lamb
production is 0.21 or of the same magnitude
as found for the weaning weight of lambs
(JÓNMUNDSSON, 1977). Adalsteinsson
(1971) has estimated heritability for score of
lamb production of the samesizeasfoundhere.
JÓNMUNDSSON (1971) found a heritability
estimate of 0.29 for maternal effects on the
weight of lambs estimated as described by
Gjedrem (1967). In Norway Gjedrem
(1967) and Eikje (1975) have estimated
the heritability of maternal effects as 0.12
As they point out the heritability of score
as calculated here will consist of both matern-
al and direct effects on the weight of the
lamb and covariance between these effects.
Negative estimates of this covariance are
found by Eikje (1975) and Chang and
Rae (1972).
The maternal influence may not be ex-
pressed in the same way in ewes which rear
singles and twins. For testing this the herita-
bility of the score for lamb production was
calculated seperately for ewes that reared
singles and twins. The results were the
following:
Ewes whit singles ............. 0.20±0.039
Ewes with twins ............... 0.21 ±0.031
There is therefore no difference in the
heritability of the score of lamb production
caused by number of lambs.
There is found a high heritability of ewe
body weight: 0.42 for weight of ewe in
October and 0.49 for weight in January.
The higher heritability of January weight
is most probably caused by more standardized
environment as discussed before.
Eikje (1970) found a heritability estimate
of 0.21 for autumn body weight of ewes in
Norway and Forrest and Bichard (1974b)
0.23 for Clun Forrest ewes in England. Hig-
her estimates are found by Chang and Rae
(1970) for Romney ewes in New Zealand
0.51 and Basset et al. (1967) 0.82 for
Rambouillet ewes in U. S. A.
No older estimates of such correlations in
Icelandic data are found except that Adal-
steinsson (1971) estimated the genetic
correlation between the number of lambs
and the score for lamb production as 0.06.
This correlation is here estimated as 0.15.
Few estimates are found of this correlation
in the literamre. In Norwegian data Eikje
(1975) estimated this correlation as —0.11
by half-sib analysis and 0.22 with regression
of daughter on dam.
Score of lamb production and ewe body
weight are positivély correlated, the genetic
correlation being 0.3—0.4. There are no