Íslenskar landbúnaðarrannsóknir - 01.09.1976, Side 66

64 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
plained by the fact that external appearance
is emphasized in the selection of ram lambs,
and it is easier to find ram lambs of the
desired type among singles than among twins.
It was found, however, that application of
potential selection would have reduced the
selection intensity against tan fibres. The
relatively intense selection against tan fibres
is most likely one of the main reasons for
rhe relatively limited utilization of the poten-
tial selection.
Discussion.
The validity of the assumption that selection
for autumn weight of lambs is mass selection
only may be disputed. The information which
is available about the parents has no doubt
been used to some extent and may have af-
fected the genetic gain.
It should also be kept in mind that due to
deliberate selection for desired wool colour
in the present material, the scope for appling
intensive selection to autumn weight has been
appreciably reduced.
Only a limited amount of information is
available on the effect of the application of
selection in sheep bredding. Pattie (1965)
showed rhat mass selection for weaning
weight of lambs resa.il ted in a response,
which, during the first generations of selec-
tion was close to expectation. At the same
time milk yield of ewes in the group selected
for high weaning weight increased (Yates
and Pattie, 1970).
Peters et.al. (1961) studied the applied
selection intensity for a few characters in the
Romnelet breed in Canada. They found the
highest applied selection intensity for autumn
weight of lambs. Only just above 50 per cent
of potential selection for this character had
been applied, a higher value being found for
males than for females, which is in agree-
ment with the results in the present study.
Botkin and Stratton (1967) have shown
how rapidly the selection intensity for ecana-
mic charecters decreases when deviations
from type, which are of minor importance
economically, are taken into accont. Chap-
man (1967) states that in Wisconsin, a rela-
tively large difference has been found be-
rween potential and applied selection, wit-
hout explaining the reason for this difference.
It has been shown that mass selection for
autumn weight of Icelandic lambs should
result in a relatively rapid genetic gainforthis
character. In order to achieve this it is ne-
cessary to make use of correction factors for
adjustment of environmental variation. At
the same time it should be avoided to pay too
much attention to characters of low economic
value.
A cknowledgements.
In connection with this work I have received
assistance from several people and institutes.
Thanks are due to dr. Stefán Aðalsteinsson
of the Agricultural Research Instimte in
Iceland for making available to me the data
which the study is based on. Thanks are also
due to Ivar Ramstad for exellent program-
ming work. Special thanks are due to dr.
Trygve Gjedrem for the many valuable dis-
cussions in connection with the study.