270 B
. Heringstad et al. Livestock Production Science 67 2001 265 –272 Table 4
4. Discussion
Genetic correlations between clinical mastitis variables based on sampling periods from 15 days prior to calving to the date of
The heritability estimates reported here are in
calving 0 or 30, 120 and 210 days after calving CM0, CM30,
agreement with previous results. Heritability esti-
CM120 and CM210, respectively above the diagonal, and the corresponding standard errors below the diagonal
mates of clinical mastitis based on Nordic field data analysed with linear methods on the observable scale
CM0 CM30
CM120 CM210
range from 0.001 to 0.06, with most values in the
CM0 0.94
0.90 0.84
interval from 0.02 to 0.03 reviewed by Heringstad et
CM30 0.014
0.98 0.94
al., 2000. A linear sire model is currently used for
CM120 0.017
0.003 0.99
CM210 0.020
0.007 0.002
genetic evaluation of mastitis in Norway and was therefore chosen for analyses, although a threshold
model would have been more appropriate for cate-
Table 5
gorical data.
Variance components and heritability of clinical mastitis estimated
Heringstad et al. 1999 showed that variance
with two data sets: CM120FIX, analysed using a model with a
components based on first crop daughters only or on
fixed effect to account for culling prior to 120 days after first calving; and CM120, where cows culled before the end of the
all daughters of a sire, given that his first crop
sampling period were excluded if classified as healthy
daughters were present in the data, resulted in very
CM120FIX CM120
similar results. Hence, only first crop daughters were
a
used in this study to minimise computational needs.
Sire variance 0.00096
0.00115
a
A preliminary study of the effect of the sampling
Residual variance 0.11573
0.12314 Total variance
0.11669 0.12429
period, using a subset of the current data, showed
Heritability 0.033
0.037
that using information on clinical mastitis before first
a
Standard error of the ratios variance component total vari-
calving resulted in higher heritability estimates than
ance: 0.0004.
if the period started at calving Heringstad et al., 1997. However, no major differences were found
Table 6
between starting 10, 20 or 30 days prior to calving.
Covariance components for clinical mastitis and culling from a
Therefore, only one starting point, 15 days before
bivariate analysis of data set CM120FIX
calving, was considered here, as it is also the one
Mastitis Culling
Mastitis
currently used for breeding value estimation in
and culling
Norway.
a b
Sire covariance 0.00100
0.00028 0.00018
Fig. 1 shows that the heritability estimates in-
a b
Residual covariance 0.11664
0.07876 0.00847
crease with longer sampling periods. This was
Total covariance 0.11764
0.07904 0.00865
expected since mastitis frequency increased from 11
Heritability 0.034
0.014
c
to 20 as the sampling end point increased from 30
Genetic correlation 0.334
to 210 days after calving Table 1, and heritability
a
Standard error of the ratios variance component total vari-
estimates are frequency dependent when applying
ance: 0.0004.
b
linear models to categorical data Gianola, 1982. To
Standard error of the ratios variance component total vari-
remove this dependency, heritability estimates were
ance: 0.0003.
c
Standard error: 0.037.
transformed to the assumed underlying scale De- mpster
and Lerner,
1950. The
transformed heritability estimates show flat curves Fig. 1,
pling period from 15 days before to 120 days after indicating that increased heritability estimates for
calving, heritability was 0.037 with CM120, while longer sampling periods are an effect of increased
CM120FIX, with culling as a fixed effect, yielded a mastitis frequency.
heritability estimate of 0.033 Table 5. The bivariate In analysing a subset of this data with a threshold
analysis of clinical mastitis and culling in Table 6 model, Heringstad et al. 1997 also obtained flat
also shows a reduced heritability estimate 0.034 heritability estimates for sampling periods ending at
compared with CM120. 30 to 240 days after first calving. Lund et al. 1999
B . Heringstad et al. Livestock Production Science 67 2001 265 –272
271
compared heritability estimates based on clinical The joint effect of the increased heritability and
mastitis from 10 days before calving to 50, 180 and the changed genetic correlation over time can be
305 days after calving, respectively, and found estimated as a correlated genetic response to selec-
higher heritabilities for longer sampling periods on tion by use of simple selection index theory. The
the observed scale, although, converted to the under- effect can be illustrated by calculating the expected
lying scale, heritability was highest for the shortest genetic superiority in, for example, the sire to son
sampling period. These results indicate that relatively path, assuming CM210 as the breeding goal trait.
little is gained with respect to size of heritability Under these assumptions the selection responses for
from sampling clinical mastitis data from a longer CM0, CM30 and CM120 were 67, 90 and 97,
time period than the first 1–2 months after first respectively, of the response obtained under direct
calving. selection using CM210.
It is likely that a sampling period covering 15 days Including culling reason as an additional source of
before first calving to 30 days after calving captures information about mastitis increased the heritability
a large part of the genetic variance, since more than estimates only slightly. This was because the inclu-
50 of the first cases of clinical mastitis, in a data sion of data on culling reason provided very little
set of 1.7 million cows, occurred within the first 30 extra information on the occurrence of clinical
days after calving Heringstad et al., 1999. For the mastitis. Taking the whole period up to 210 days
data set analysed in this study, the frequency of after calving Table 1, only 2739 cows 0.6 were
clinical mastitis up to 30 days after calving CM30 identified as culled for ‘high SCC mastitis’ and not
was 11.2, which is not greatly inferior to the recorded for clinical mastitis treatment. This is partly
15.9 incidence that would be analysed in the due to the fact that 37 of the cows were culled
current sampling period used in Norway Table 1. A before 1989 and ‘high SCC mastitis’ was only
further increase of the sampling period past 30 days included as culling reason from that year onwards. It
would include more cases of mastitis, but might also may also suggest that the health recording system is
introduce a bias due to culling of cows. Lund et al. very efficient or, alternatively, that the culling reason
1999 found heritability on the underlying scale to is being inaccurately recorded by the farmers. It may
be highest in early lactation and suggested that also be a problem that, in the Norwegian recording
clinical mastitis in the beginning of lactation, when of culling reason, high SCC and mastitis are put
the cows’ physiological demands are high, is more together as one category, while SCC and clinical
related to the cows genetic resistance to mastitis than mastitis are two different traits with a genetic
¨ ¨ ¨
later in lactation. correlation far from unity e.g., Poso and Mantysaari,
These results have important practical conse- 1996; Lund et al., 1999.
quences. So far, only the four Nordic countries have With Swedish data, Koenen et al. 1994 found a
national health recording systems. Although there is much larger effect when including culling reason.
considerable interest, it is often considered that Heritability on the observable scale increased from
systems for recording clinical mastitis treatments 0.018 to 0.083. The importance of utilising culling
would be difficult to establish in other countries e.g., reason as additional information in breeding against
Colleau and le Bihan-Duval, 1995. However, for mastitis will obviously vary between countries and
those countries wishing to include clinical mastitis in will depend on recording systems of both health data
dairy cattle breeding programs, recording mastitis and reason for culling. Both in this study and in
over a very short period of the first lactation could be Koenen et al. 1994 the heritabilities transformed to
an alternative that would not be too expensive and the underlying scale were enhanced by utilising
that would provide a measure of clinical mastitis culling reason information. This indicates that the
with a reasonable heritability value. This argument is increased heritability was not only an effect of
further strengthened by the high genetic correlation increased mastitis frequency in the CMCR data sets
0.98 between clinical mastitis observed over 30 but rather that CMCR defines a trait with a slightly
days CM30 and over 120 days CM120 of the first higher heritability than CM.
lactation Table 4. The bivariate analysis of clinical mastitis and
272 B
. Heringstad et al. Livestock Production Science 67 2001 265 –272
culling showed that culling, defined as a categorical but both strategies gave lower heritability estimates
trait, had an estimated genetic correlation of 0.33 than simply excluding cows culled before the end of
with clinical mastitis Table 6. Hence, including the sampling period if they did not have mastitis.
culling as a fixed effect the univariate analysis Hence, culling affects variance component estimates
reduces the genetic variance and the heritability of of clinical mastitis.
clinical mastitis compared with a bivariate analysis of the two traits Tables 5 and 6. The increased
heritability of mastitis indicates that a bivariate
Acknowledgements
analysis accounts better for culling than including a fixed effect of culling in the model. However, simply
The Norwegian Dairy Cattle Recording System excluding cows culled before the end of the sampling
Husdyrkontrollen NML is acknowledged for pro- period that were not classified as diseased CM120
viding data and GENO Breeding and A.I. Associa- resulted in a higher heritability than the two other
tion for providing pedigree information on bulls. approaches. The advantage of this strategy is that
mastitis can be analysed by a univariate model. The disadvantage is that there is some selection in the
data.
References
The problem of using information from a longer period of first lactation is bias due to culling of cows,
Colleau, J.J., le Bihan-Duval, E., 1995. A simulation study of selection methods to improve mastitis resistance of dairy cows.
and the results presented here demonstrate that
J. Dairy Sci. 78, 659–671.
culling affects the variance component estimates.
Dempster, E.R., Lerner, I.M., 1950. Heritability of threshold
Using information from a longer period of first
characters. Genetics 35, 212–235.
lactation may be advantageous if a method that
Gianola, D., 1982. Theory and analysis of threshold characters. J.
properly accounts for the effect of culling is de-
Anim. Sci. 54, 1079–1096. Heringstad, B., Karlsen, A., Klemetsdal, G., Ruane, J., 1997.
veloped. Otherwise, using only a short sampling
Preliminary results from a genetic analysis of clinical mastitis
period will avoid most bias due to culling of cows,
data. In: Proceedings of an International Workshop on Genetic
since culling rate is low in the first part of lactation.
Improvement of Functional Traits in Cattle; Health. Interbull Bulletin No. 15, pp. 45–49.
Heringstad, B., Klemetsdal, G., Ruane, J., 1999. Clinical mastitis in Norwegian cattle: frequency, variance components, and
5. Conclusions
