Íslenskar landbúnaðarrannsóknir - 01.03.1977, Blaðsíða 37

EWE PRODUCTION TRAITS 35
Táhle- 4. Constants for the effect of the age of the ewe on the number
of lambs at birth and at weaning.
Age Number Number of lambs Number of lambs
at birth at weaning
5825 -0,172 + 0,006 -0,160 + 0,006
6219 -0,030 + 0,006 -0,026 + 0,006
5882 0,047 + 0,006 0,054 + 0,006
5838 0,059 + 0,006 0,064 + 0,006
5391 0,050 + 0,006 0,047 + 0,006
4086 0,039 + 0,007 0,038 + 0,007
2557 0,018 + 0,008 0,010 + 0,008
2035 -0,011 + 0,009 -0,027 + 0,010
but the lowest number by two year old
ewes. There is little difference in the number
of lambs born from 4 to 7 years of age but
then the lambing rate declines. The difference
in the number of lambs born to two and five
year old ewes is 0.23 lambs. Hallgrímsson
(1966) found the greatest number of lambs
born to five and seven year old ewes.
In many investigations in other counrries
it is found that the highest number of lambs
is born to ewes from 4 to 7 eyars of age
(Eikje, 1971; Johanson and Hansson,
1943; Sharafeldin, 1960; Forrest and
Bichard, 1974a, Vakil et al., 1968; Dicker-
son and Glimp, 1975)
The estimates of the age effect can be af-
fected both by culling and genetic gain.
Relatively little culling of ewes on basis of
performance records is practised in sheep
breeding in Iceland. Hallgrímsson (1966)
found no culling for fertility except for
barren ewes. No estimate of genetic gain is
available for the trait number of lambs born,
but the effect of this factor is probably
negligible.
The age-effect on the number of lambs at
weaning shows the same tendency as that
found for the number of lambs at birth.
There is a tendency for relatively fewer lambs
being present at weaning that at birth in the
case of the oldest ewes. The difference of
the constants for the number of lambs at
birth and weaning is therefore a combined
effect of type of birth and age of dam on
losses of lambs from birth to weaning.
The increase in the multiple correlation
coefficient by including weight of ewe or
date of mating in the model in addition to age
of ewe is shown in table 5. There it was found
that the ewe body weight in January accounts
for 2.9% in the variation of number of lambs
born in addition to age. The weight effect
is therefore greater than the age effect. In
table 6 the regression coefficients are shown.
There is found to be a curvelinear effect of
body weight on the number of lambs born
and the greatest number of lambs is obtained
by ewe body weight of 75 kg in January.
HallgrÍmsson (1966) found only a line-
ar effect of body weight on the number of
lambs born, but he estimated the regression
within age classes whereas here the regression
is estimated simultaneously with the age effect.
In biological terms curvilinear effect is resona-
ble. The linear regression found here is hig-
her than that found by Hallgrímsson
(1966) who found 0.0096 compared with
0.0145 here.