Íslenskar landbúnaðarrannsóknir - 01.03.1977, Blaðsíða 30

28 ÍSLENZKAR LANDBÚNABARRANNSÓKNIR
or more progeny in both groups. From this
covariance the correlations are estimated:
Weaning weight of ewe lambs:
Carcass weight of ram lambs 0.79
Weaning weight of ewe lambs:
Dressing percentage of ram lambs —0.17
These results show little effect of selection.
GENETIC VARIATION
BETWEEN FLOCKS
As mentioned before the variance between
flocks in weaning weight was 16.3% of
the total variance. After correction this had
risen to 33.6%, but then all the lambs had
been corrected to a similar age.
The ram lambs are often selected across
flocks and one must therefore know how
much of the flock difference is genetic in
origin to get correct ranking.
The genetic difference between sheep
flocks is little studied. The main reason for
this is the breeding structure, each ram only
being used in one flock and therefore con-
founded with the flock.
The method proposed by Robertson and
Rendel (1954) for estimating the genetic
difference in milk yiéld between herds by
regression of daughter yield on herd mean
can be used in this material for lambs sired
by the rams used in artificial insemination.
There were a total 2212 of progeny by 28
sires in this material. The heritability of the
flock average was estimated from the regres-
sion as 0.03±0.05. Eikje (1974b) has esti-
mated this heritability as 0.03 in Norway, by
a method of regression of the progeny test
results on flock average of the flock in which
the ram was born.
The lambs sired by the rams used in
artificial insemination are often born before
the normal lambing season starts in the
flock and may therefore receive special treat-
ment. It is therefore difficult to determine
how good an estimate of the genetic dif-
ference between flocks the data can give.
DISCUSSION
This study shows that environmental factors
studied here account for 38.8% of the total
variation in weaning weight. One can there-
fore get a substantial gain in accuracy of
selection by correction for these factors. Such
correction factors can be very useful when
selecting the lambs kept for replacement
in the autumn. Selection is undertaken when
the lambs are weighed and one must there-
fore have correction factors avilable before-
hand. In progeny tests the correction factors
can be estimated from the data.
The greatest problem is the correction of
the age effect of lambs because of the dif-
ferent treatments the lambs in the same
flock often get in the autumn.
The genetic variation in these traits is so
great that resonable genetic gain should be
obtained by selecting for weight of lambs.
Because of the negative genetic correlation
between weaning weight and dressing percen-
tage the use of mass selection only for
weaning weight is not recommended.
Organized progeny testing is not common
in the breeding work in Iceland. The relati-
vely low heritability of carcass weight and
the fact that one must álso select for meat
quality characters which are only observable
after slaughter should favour the use of
progeny testing in the breeding work. Because
of the large size of many flocks one can
practise progeny testing on a within farm
basis. In areas with smaller average flock
size, the Norwegian ram circle system is an
alternative.