Íslenskar landbúnaðarrannsóknir - 01.03.1977, Síða 80

78 ÍSLENZKAR LANDBÚNABARRANNSÓKNIR
MATERIAL AND METHODS.
One ewe, Steinka 6600, born in 1966, on
a farm in East-Iceland, was known to have
returned on heat with an unusually high
frequency. The ewe never gave birth to twins,
and all its progeny were males, some of
them exceptionally heavy. One of its ram
lambs, Grettir 215, was used on a neighbou-
ring farm during the winter 1972/73 where
the return rate of the ewes it served was
exceptionally high. Steinka 6600 was bought
to the experimental farm Skriduklaustur in
autumn 1974.
In spring 1975 it gave birth to a male
lamb, Bolli 236, which was reared and used
the following winter and in 1976 the ram
lamb Loki 245.
On a farm in Southeast-Iceland a ram
called Draupnir, born in 1962, was used in
four successive years, from 1962/3 to
1965/66. The ewes mated to this ram had
and exceptionally high return rate, and low
twinning rate. Durgur, a son of Draupnir,
was used on the same farm in two seasons,
and showed the same high return rate and
low twinning rate as its sire.
In order to compare the fertility rates of
the rams in question, the following procedure
was adopted. The mating results of Grettir
215 have been compared to the mating
results of two rams used in the same sheds
on comparable ewes in the same season.
The majting results of Draupnir and
Durgur have been compared to the mating
results of al'l other rams used on the same
farm in the same seasons. Ewes which
aborted or became barren have been excluded
from the data. The mating results therefore
give the number of ewes coming back on
heat (R), those giving birth to a single lamb
(S) and those giving birth to twins (T).
Two ewes with triplets have been included
with the twin-ewes.
A comparison between normal rams and
the rams under investigation was done by
a x2-test.
In order to examine the nature of the
depressed fertility of the rams under investiga-
tion, it was assumed that the depression was
caused by an excess mortality of embryos
sired by the test rams. This excess mortality
rate was denoted by s (1.0>j>0). It was
further assumed that the proportions of ewes
in which 2 eggs were fertilized (p), 1 egg
fertilized (q) and no egg fertilized (1 -p-q)
were equal for the two ram groups.
The parameters p, q and s and their
standard erros were then estimated by the
method of maximum likelihood.
A detailed description of the estimation
procedure is given elsewhere (Adalsteins-
son, 1977).
RESULTS.
Written records on the reproductive history
of the ewe Steinka 6600 are shown in table
1. The records of the first years of the ewe’s
reproductive life have been lost, but the
previous owner of the ewe gave the informa-
tion on the first 5 years shown in table 1
from memory.
The return rate of Steinka 6600 during the
last 6 years on record has been 4/10 or
40%, which is far above average.
The observed sex ratio of its lambs in
table 1, 8 males: 0 females differs signi-
ficantly from the expected 1:1 ratio (P=
0.04).
The mating results for Grettir 215,
Draupnir and Durgur, together with the
corresponding mating results used for com-
parison in each case, are shown in table 2.
As can be see from the x2-values, the three
rams under investigation all differ from their