Íslenskar landbúnaðarrannsóknir - 01.03.1977, Side 34

32 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
MATERIAL AND METHODS
The data used in the present study were
obtained from the Sheep Recording As-
sociations in Iceland for the years 1970 and
1971. A description of the sheep recording
system is given in a previous paper. (JÓN-
MUNDSSON 1977).
In the material there were a total of 38349
ewes. Of these 516 were barren, 14245 ewes
had one, 23304 had two lambs and 284
had three or more lambs.
Only the ewes which reared lambs until
weaning in the autumn were included in
the analysis. This was a total of 37833 ewes.
The recording of barren ewes is probably
faulty in some flocks and they were therefore
excluded from the analysis. Whether the
ewe drops a lamb or not and the number of
lambs per ewe lambing are, at any rate, two
different traits.
In this material 4.93% of all lambs born
were lost from birth to weaning. The percen-
tage of losses is highest with the youngest
and oldest ewes and higher for twins than
singles (JÓNMUNDSSON, 1975a). Since the
loss was so small it should not bias the
estimates of the age-effect that ewes which
lambed but failed to rear lambs to weaning
are excluded from the analysis.
The age-effect on the number of lambs
born and number of lambs weaned was esti-
mated by the following least square model:
Y. .
13
For ewes that had recorded body weight
in October and January this was added to
the model as a regression vaiable and the date
of mating too. Both first and second order
regressions were tested.
For the ewe body weight the age effect
was tested by the same model as described
for number of lambs.
The means and standard devitions for the
number of lambs born and weaned and ewe
body weight in October and January are
shown in table 1.
The mothering ability of the ewe is evalu-
ated by the score of lamb production as
described by Adalsteinsson (1966). This
score (P) is calcualted as:
P = k + (X. - X) / s .
Where Xj is the weight of lambs of the
ith ewe corrected for environmental factors as
described by Jónmundsson (1977 X is
the mean weight of the lambs per ewe in the
flock calculated seperately for ewes with
singles and twins and s is the standard
deviation for the group. The k is a constant
giving the mean of the score and equals 5
here. This score was calculated both on carcass
weight of lambs (score 1) and weaning weight
of lambs (score 2).
The heritabilities and genetic and pheno-
typic correlations were calculated by half-sib
correlations from the components estimated
Here Y^ denotes number of lambs.
y = The overall mean
■a^ .= the effect of the ith age group, i — 2,..., 9.
e^ =r a random element.