Íslenskar landbúnaðarrannsóknir - 01.09.1977, Side 91

ARFGENGI MJÓLKURFRAMLEIBSLUEIGINLEIKA 89
SUMMARY.
A study of data from the cattle hreeding
associations
II. The heritability of the traits and their
correlation.
JÓN VlÐAR JÓNMUNDSSON
Agricultural Res. Institute,
Keldnaholt, Reykjavík
Ólafur E. Stefánsson
and
Erlendur Jóhannsson
Agric. Society
Bcendahöllin, Reykjavík
Production figures from the records of the
cattle breeding associations were used to
calculate heritability, repeatability of pheno-
typic and genetic correlations between milk
yield, milk fat and maximum daily yield.
Coefficients are evaluated in reíation to half-
sib correlation.
Production figures are corrected for age
and calving time of cows. In the calculations
information on 17792 recorded years for
11589 daughters of 181 bulls were used.
Repeatability for the traits in question were
for milk yiéld 0,40, for milk fat 0,36, for
fat percentage 0,42 and for maximum daily
yield 0,37. Heritability in the same order
was found to be 0,16, 0,09, 0,20 and 0,10.
Genetic correlation between milk yield and
milk fat was 0,88 and phenotypic correlation
for the same traits 0,91. Genetic correlation
between milk yield and fat percentage was
—0,61 and phenotypic correlation for the
same was —0,04. Maximum daily yield was
genetically positively correlated with milk
yield (0,86), but negatively correlated with
fat percentage (—0,48).
Possible bias factors are discussed, and
results, based on a part of the materiál, are
included in the paper, an attempt being made
to evaluate any bias caused by a number
of factors.
Heritability was evaluated at different
herd levels. For this purpose herds were
divided into three groups according to their
levels of corrected production. The results
are shown in table. No great difference was
found in heritability in the various groups
of herds thus classified. It seems, however,
that heritability tends to increase with higher
production.
Heritable difference between herds was
evaluated by the regression of the production
of daughters of certain bulls on herd averages.
Calculations used for this purpose are based
on records for 7272 cows between three
and five years old which were the daughters
of 154 bulls. The heritability of the herd
level found in this way was for milk yield
0,027, for milk fat 0,043, for fat percentage
0,130 and for maximum daily yield 0,037.