Íslenskar landbúnaðarrannsóknir - 01.03.1977, Page 23

SOURCES OF VARIATION IN WEIGHTS OF LAMBS 21
in a study of data from two breeding as-
sociations, but Adalsteinsson (1966) and
JÓnmudsson (1971) found a corresponding
difference of approximately 7 kg. Twins at
birth but reared as singles were found to be
3.7 kg heavier than twins. This is a smaller
difference than that found by JÓNMUNDSSON
(1971), but in his study the lambs were
reared as singles from shearing at end of
June, but in this material information was
not available about the date at which the
opposite twin was lost.
The singles had 2.18 percent units higher
dressing percentage than twins. This dif-
ference is of the same order as found by
Rönningen et al. (1971) in a crossbreeding
experiment in Norway.
SEX
The sex differences found in this smdy are
given in table 7. The difference in weaning
weight is 2.82 kg which is in close agree-
ment with earlier Icélandic studies. Adal-
STEiNSSON (1966) found sex differences of
3.6 kg for singles and 2.45 kg for twins,
Hallgrímsson (1971) 2.6 kg and JÓN-
mundsson (1971) 2.94 kg. In Norwegian
studyes there are found to be greater sex
differences in Spælsau (Gjedrem 1965;
Sangolt 1969; Eikje 1971).
Table 7.
Ewe lambs have a 0.82 percent unit higher
dressing percentage than ram lambs. This
agrees closely with other studies (Donald
et al., 1970; Rönningen et al., 1971;
Bradford and Spurlock, 1972).
The regression coefficient of sex difference
in 10176 pairs of unlike sex on the total
weight of both members of each twin pair
was estimated 0.077±0.005 kg/kg. This
shou'ld indicate that the sex difference is
multiplicative rather than additive in nature.
The multiplicative constant for ewe lambs
to correct their weaning weight to ram lambs
in this material is estimated 1.08, based on
the constants estimated.
For testing the effect of the sex partner
in twin pairs one has calculated the difference
in weight of twins that have a partner of
lilce and unlike sex. The harmonic number
in the two groups is used as a weighing
factor. These differences are shown in table
8. There is found to be a significant effect
of the partners’ sex, in such a way that ram
lambs in mixed pairs are favoured and ewe
lambs disfavoured. This effect is so small
that it will be of little practical interest.
Such effect was found in birth weight by
Donald and Purser (1956), but Hall-
grímsson (1966) found no such effect in
weaning weight.
Difference between ram and ewe larnbs.
Trait Difference
Weaning weight 2,82 kg
Carcass weight 1,08 kg
Dressing percentage -0,82 kg