Íslenskar landbúnaðarrannsóknir - 01.03.1977, Side 25

SOURCES OF VARIATION IN WEIGHTS OF LAMBS 23
Table 10. Constants for the effect of age of ewe on weaning weight,
carcass weight and dressing percentage.
Age, year Weaning weight
2 -1,58 ± 0,05
3 -0,31 ± 0,04
4 0,44 ± 0,04
5 0,58 ± 0,04
6 0,58 ± 0,04
7 0,37 ± 0,05
8 0,24 ± 0,06
>9 -0,32 ± 0,07
Carcass weight Dressing percentage
-0,73 + 0,02 co 00 o 1 + 0,03
-0,14 + 0,02 -0,11 ’+ 0,03
0,21 + 0,02 0,05 + 0,03
0,25 + 0,02 0,05 + •0,03
0,26 + 0,02 0,10 + 0,03
0,17 + 0,03 0,04 + 0,03
0,10 + 0,03 0,05 + 0,04
O vH O 1 + 0,04 0,15 + 0,05
different in the two weighing gtoups on five
farms with 280—350 lambs each. It is evi-
dent that the best correction for the age effect
will be obtained by using coefficients esti-
mated for each flock and weight date group.
AGE OF DAM
The constants estimated for the effect of age
of dam are listed in table 10. Five and six
year old ewes reared the heaviest lambs.
These constants agree well with those found
by Jónmundsson (1971) and Hallgríms-
son (1971).
The youngest ewes produced lambs with
the lowest dressing percentage. The same is
found by Fimland et al. (1969) and Vaa-
benö et al. (1974) in crossbreeding experi-
ments in Norway. This effect is probably due
to less milk production by the youngest ewes
and their lambs therefore have more devel-
oped digestive organs.
CORRECTION OF THE DATA
The model used presupposes that the factors
affect weaning weight additively. As men-
tioned before the sex effect is multiplicative.
The type of birth and rearing is by far
the most important factor, and the form of
correction for this effect is therefore of the
greatest importance. In Norwegian studies
conflicting results are obtained depending on
whether this effect is multiplicative or ad-
ditive in nature (Sangolt, 1969; Eikje,
1971).
The regression of the difference in weaning
weight between singles and twins on the
total weight of these was calculated for 361
flocks as 0.795±0.158. This indicates that
effect of type of birth and rearing is mul-
tiplicative between flocks.
For testing this effect within flocks one
has tested both additive and multiplicative
factors for the effect of type of birth and
rearing. The calculated difference from the
flock average with standard deviation for