Íslenskar landbúnaðarrannsóknir - 01.03.1977, Síða 42

40 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
estimates of this correlation in the literature.
The genetic correlation of the number of
lambs and the ewe body weight is found
to be about 0.10. In studies in other countries
this correlation is also found to be positive
but shows wide variation (Purser, 1965;
Chang and Rae, 1972; Eikje, 1970;
Shelton and Menzies, 1968; Young et
al, 1963).
DISCUSSION
The prolificacy of ewes is the most important
trait in the sheep breeding in Iceland. Adal-
STEINSSON (1971) gives the economic weight
of 10,5:1 for number of 'lambs and carcass
weight. The heritability estimates found here
indicate that considerable genetic gain could
be obtained for this character by selection.
The selection response is lowered because
this is a sex limited character and one must
therefore use pedigree selection or progeny
testing for the trait. If using progeny testing
one must plan progeny testing on a large
scale if one is to have some selection intensity
for the daughter production-traits that will
be obtained at the second stage of the progeny
testing. The progeny test will also give longer
generation interval. In this material it is
found that the selection intensity in pedigree
selection, measured as the prolificacy of the
mother in the year the lamb is kept for
breeding, is low, especialiy for ram Iambs
(Jónmundsson, 1975b).
The weight of lambs produced by the
ewe is affected by the mothering ability of
the ewe of which milk production is the
most important factor. The lamb weight is
also affected by genes for growth rate which
the lamb has inherited partly from its mother,
and the covariance between these two factors.
The weight of lambs produced by the ewe
is therefore an expression of the genes of
two related individuals in two different
generations. The genetic covariance between
direct and maternál effects will influence
the genetic gain in weaning weight. As shown
by Hanrahan (1976) the progeny test for
weaning weight will give a rather low gain
in this trait if this covariance is negative
and the heritability of maternal effect is high
compared with the direct effect. Then the
performance test is a better selection alterna-
tive for weaning weight.
The body weight of a ewe influences the
cost of production due to the higher main-
tenance cost of the heavy ewe than the
light one. With selection both for fecundity
and score for lamb production the body
weight will be increased because of the
positive genetic correlation of these traits
with ewe body weight. As discussed by
Cunningham and Gjedrem (1970) the
body weight can be restricced and included
in a selection index. As they show it is not a
realistic alternative to restrict the body weight
unless it is recorded and included in the
index by a negative weighing. This will
slightly lower the genetic gain for score of
lamb production.
ACKNOWLEDGEMENTS
The data used in this study were obtained
from the Agricultural Society of Iceland.
I want to express my thanks to Dr. Halldór
Pálsson, Director of the Society, and Mr.
Sveinn Hállgrímsson, adviser in sheep bre-
eding, for making the data available to me.
Most of the study was done when I was
studying at the Institute of Animal Genetics
and Breeding, Agricultural University of
Norway. I want to express my thanks to
Professor Dr. Harald Skjervold and the staff
of the Institute for providing working
facilities, when I was staying there.