Íslenskar landbúnaðarrannsóknir - 01.09.1977, Síða 75

ÁHRIF ALDURS OG BURÐARTÍMA 73
SUMMARY
A study of data from the cattle breed-
ing associations.
I. The influence of age and calving time
of cows on their production.
JÓN VlÐAR JÓNMUNDSSON
Agric. Res. Institute,
Keldnaholt, Reykjavík
Ólafur E. Stefánsson
and
Erlendur Jóhannsson
The Agric. Society
Bíendahöllin, Reykjavík
The paper describes the results of an
investigation of the production records of
the cattle breeding associations for the years
1974 and 1975.
The information covered 25017 product-
ion years of a total of 1763 herds/years.
The traits investigated were milk yield (kgs),
milk fat yield (kgs), fat percentage and maxi-
mum daily yield during the calender year.
The means for the traits and standard
deviations within herd (in brackets) were
as follows: milk yiéld 3678 (765), milk fat
yield 152 (33,5), fat percentage 4,12 (0,36),
maximum daily yield 21,7 (3,7). Cows
three years and older were included in this
study.
An interaction between the influence of
age and calving time was found and it
appears that partly this is because the annual
production is divided between parts of the
first two lactations of heifers. Figures from
material used show that age and calving time
are responsible for 18% of the variation in
milk yield, 14% in milk fat, 3% in fat
percentage and 23% in maximum daily
yield.
Cows reach their maximum production
at the age of six to seven years. Age influ-
ence on fat percentage is fairly small and fat
percentage becomes lower with higher age.
Cows of seven years of age exeed the maxi-
mum daily yiéld of three years olds by 3,4
kgs.
Cows calving in January give the highest
annual production where as cows calving
during the summer months (between June
and August) together with those calving
in December give the lowest annual yield.
The difference in annual yield between cows
calving in January and July is 705 kgs. The
influence of calving time on fat percentage
is small and so is the influence of calving
time on maximum daily yield, whereas cows
calving in spring and summer (between
April and August) reach the highest maxi-
mum daily yield.
The influence of age and calving time
on production appears to be proportional
rather than additive. Coefficients for correct-
ion of production figures are shown in tables.
There seems to be only a small difference
between different parts of the country as
influence of age on production is concerned,
whereas the influence of calving time on
production appears to vary more in different
parts and years.
The results are discussed in connection
with earlier investigations made in Iceland
as wéll as in other countries.