Íslenskar landbúnaðarrannsóknir - 01.09.1976, Side 64

62 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
Table 2. Average generation interval.
Generation interval in years.
Sire Sire Dam Dam
Farm Son Daugther Son Daughter
Hvanneyri .............................. 2.16 2.34 5.18 4.81
Reykholar .............................. 2.07 2.37 4.49 4.51
Skriðuklaustur.......................... 3.18 2.47 4.94 5.15
Hólar .................................. 2.36 2.24 5.08 5.07
The whole material...................... 2.69 2.38 4.94 4.97
In this connection it should be noted that the
age distribution of ewes at the farm Reyk-
hólar differs from that on the other farms
due to the founding of the Reykhólar flock
in 1960 and 1961 trough purchase of lambs
only. The generation interval is short on
the male side but on the female side com-
parable to that found by Hallgrímsson
(1971).
When the phenotypic selection differintial
(S), the generation interval (L) and the heri-
tability are known, expected annual genetic
gain can be calculated as
which calculated as percentage becomes
A G • 100 = per cent genetic gain per year,
b
where u is the average corrected autumn
weight in the group in question.
The expected annual genetic gain due to
selection has been calculated within year,
farm and sex. The calculations were done in
the four following ways.
1. By using the estimated heritability within
year, farm and sex. Negative estimates
were equalled to zero.
2. By using the estimated heritability within
farm and sex.
3. By using the estimated heritability within
sex.
4. By using a heritability of 0.25.
The heritability estimates are described by
Jónmundsson (1976b).
The above methods have also been used
for estimating genetic gain on the basis of
potential selection.
For estimating the average genetic gain
within sex and farm the selection differential
(S.) has been weighted, using the number of
lambs kept for breeding (n.) as weights,
according to the expression:
2"i sih!
1,2....,6.
Here one encounters the problem of over-
lappin of generations. As shown by Turner
and Young (1969), the additive genetic
gain is cumulative. The theoretical assump-
tion made here will therefore probably not
ho!d, but within the short period of time of
just below 2 generations the error introduced
can hardly be of great magnitude. The re-
sults from these calculations are shown in
tables 3a and 3b. It has also been calculated
what percentage of the potential selection
has been utilized