266 B
. Heringstad et al. Livestock Production Science 67 2001 265 –272
period. The relevant period varies between countries, Recording System, from 1978 onwards described by
covering the period of 10 days before to 180 days Ruane et al., 1997. As in Heringstad et al. 1999,
after calving in Denmark, 7 days before to 150 days data extraction was carried out by combining in-
after calving in Finland, 15 days before to 120 days formation in a pedigree file with information on
after calving in Norway and 10 days before to 150 clinical mastitis and culling. In building up the data
days after calving in Sweden Heringstad et al., sets, a record was included if first calving was
2000. To avoid bias due to culling of cows, between 1 September 1978 and 31 December 1995,
information from only a short period of lactation is age at first calving was between 450 and 1200 days
used. In Danish clinical mastitis data, the highest and the lactation started with a normal calving i.e.
heritability was found when using data from 10 days lactations starting with an abortion or with a calving
before to 50 days after first calving Lund et al., in another herd were omitted. To ensure participa-
1999 tion in the health recording system, only data from
For genetic evaluation in Finland and Sweden, herds with at least one disease record during the year
culling due to udder health problems reported by the the cow calved were accepted.
farmer is used as additional information about First lactation clinical mastitis information, from
mastitis Heringstad et al., 2000. Koenen et al. 15 days before calving to 210 days after calving or
1994 found significantly higher heritability esti- to date of culling if less than 210 days after calving,
mates for mastitis in Swedish data when such was stored by use of two variables; dates of the last
information was utilised. In Norway, culling reasons mastitis observation before calving and the first
have been reported by farmers since 1978, but were mastitis observation after calving. Additionally, cul-
initially not sufficiently defined to be of value in ling date and culling reason were kept for cows
genetic evaluation. Since 1989, however, ‘high culled during the first lactation, from 1989 onwards.
somatic cell
count SCC mastitis’
has been
‘High SCC mastitis’ was not a category of culling specified as a category of culling reasons, and this
reason for cows culled before 1989 37 of cows. information can be used in genetic analyses of
Finally, information was limited to daughters of mastitis. Culling reason potentially makes it possible
young bulls progeny tested in the years 1978 to to identify cows with mastitis that are culled instead
1995. Only first crop daughters were used, and of being treated.
records of A.I. sires with less than 20 daughters were The first aim of this study was to study the effect
deleted. The resulting data file included a total of of length of the sampling period of health data on the
549,995 first lactation daughters bred by 2043 sires, heritability of clinical mastitis and to estimate the
from 253,371 different herd by year classes. genetic correlations between clinical mastitis from
different sampling periods. The second aim was to 2.2. Trait definition
examine whether inclusion of culling reason as an additional source of information about mastitis af-
Eight different data sets were created to study the fected the heritability estimate. The third aim was to
effect of the length of the sampling period of health perform an introductory study of the effect of culling
data on the heritability of clinical mastitis. Clinical of cows before the end of the sampling period on
mastitis CM was defined as a binary trait, on the variance component estimates.
basis of whether or not the cow had had clinical mastitis in the period from 15 days before first
calving to date of calving 0 or to 30, 60, 90, 120,
2. Material and methods 150, 180 or 210 days after calving.
Eight other data sets were constructed to analyse 2.1. Data
the effect of including culling reason as additional information CMCR in variance component estima-
Data were extracted from a research database tion. The same eight sampling periods were used,
comprising phenotypic information on an individual and the mastitis variable was again defined as a
cow basis, as stored by the Norwegian Dairy Cattle binary trait. However, cows culled before the end of
B . Heringstad et al. Livestock Production Science 67 2001 265 –272
267
the relevant period were classified as diseased, if the all cows were classified as culled or not before 120
culling reason reported by the farmer was ‘high days after calving. The second strategy to account
SCC mastitis’, even if the cows did not have a for culling is to define culling before 120 days after
record of mastitis treatment. calving as a binary trait, and to use data set
In all, 16 data sets, data on cows culled before the CM120FIX in a bivariate analysis of clinical mastitis
end of the relevant period, were included if they and culling 05not culled; 15culled. The structure
were classified as diseased. If not, they were ex- of the CM120FIX data set is given in Table 2, which
cluded, since cows culled before the end of the also gives information on CM120 defined in Section
period were not considered to have had a sufficient 2.2 for comparison purposes. In CM120, the 35,967
chance to express the trait. The number of records cows culled before 120 days after calving and
and mastitis frequencies in the 16 data sets are given classified as healthy are excluded, while they are
in Table 1. CMCR data sets have at most 2739 included in CM120FIX.
additional records compared to their CM equivalents. Estimation of genetic correlations between clinical
2.4. Sire pedigree file mastitis from different sampling periods was limited
to CM0, CM30, CM120, and CM210. A sire pedigree file was constructed by tracing the
pedigree of the 2043 bulls appearing as sires in the 2.3. Culling of cows before the end of the
data files as far back as possible. The pedigree file sampling period
contained a total of 2159 bulls, the first born in 1940. In total, 50 bulls had unknown sires, and were
Instead of simply omitting data on cows that were assumed to represent the base population.
culled before the end of the sampling period and that were classified as healthy, two strategies for includ-
2.5. Model ing the data were considered. The first is to include a
fixed effect of culling two classes in the model, as Variance components for mastitis were estimated
currently done in genetic evaluation of mastitis in using the following linear sire model:
Norway. To examine the effect of this approach on variance components, a data set CM120FIX was
Y 5 A 1 M 1 HY 1 S 1 E
ijklm i
j k
l ijklm
created. As before, clinical mastitis was considered as a binary trait, on the basis of information in the
where Y is an observation of mastitis 05no
ijklm
period from 15 days before to 120 days after first mastitis; 15mastitis, A is the fixed effect of the ith
i
calving. Culled cows were kept in the data set, and age at calving in 15 classes, where ,20 months is
Table 1 Number of records and mastitis frequencies in the 16 data sets used to study the effect of the sampling period on variance component
a
estimates for traits defined on the basis of clinical mastitis only CM or, in addition, using information on culling reason CMCR End of sampling
Clinical mastitis CM Clinical mastitis and culling reason CMCR
period, days after Data
No. of Mastitis
Data No. of
Mastitis
b
first calving set
records frequency
set records
frequency CM0
549,995 3.5
CMCR0 549,995
3.5 30
CM30 537,902
11.2 CMCR30
538,714 11.4
60 CM60
529,430 12.9
CMCR60 530,596
13.1 90
CM90 521,918
14.4 CMCR90
523,341 14.7
120 CM120
514,028 15.9
CMCR120 515,754
16.2 150
CM150 505,087
17.3 CMCR150
507,136 17.7
180 CM180
494,967 18.7
CMCR180 497,371
19.1 210
CM210 484,351
20.0 CMCR210
487,090 20.5
a
For both CM and CMCR, cows culled before the end of the sampling period were excluded if classified as healthy.
b
For all data sets the sampling period started 15 days prior to first calving.
268 B
. Heringstad et al. Livestock Production Science 67 2001 265 –272 Table 2
Summary statistics of the data set CM120FIX used for the bivariate analysis of mastitis and culling and to examine the effect of accounting for culling as a fixed effect on variance component estimates of clinical mastitis. The sampling period was from 15 days before to 120 days
after first calving, and the reference data set CM120 was defined as in Table 1
a
CM120FIX CM120
Number of records 549,995
514,028 Number of sires
2043 2043
Number of herd3year classes 253,371
246,960 Mean number of daughters per sire
269 252
Mean number of records per herd3year 2.2
2.1 Mastitis frequency
14.9 15.9
Year of calving 1978–1995
1978–1995
a
A total of 8.7 of the cows were culled before 120 days after first calving.
2
the first class and .32 months the last class, and the reasonably high h 5 0.033 and 0.034. It is also
other classes are in single months, M is the fixed interesting to note the heritability of 0.016 when the
j
effect of the jth month of calving in 12 classes, HY sampling period ended at the day of first calving.
k
is the fixed effect of the kth herd by year class, S is Heritability estimates for CM120, CM150 and
1
the random effect of the cth sire, and E is a
CM180, which correspond to the sampling periods
ijklm
random error term. used in Norway, Finland and Sweden, and Denmark,
An additional fixed effect was included in the were 0.037, 0.039 and 0.040, respectively. Heritabili-
univariate analysis of data set CM120FIX: C is the ty estimates transformed to the underlying scale were
fixed effect of culling or not before 120 days after fairly constant, between 0.084 and 0.092. Estimates
calving, in two classes 05not culled; 15culled. In of heritability from CMCR were slightly higher than
the bivariate analysis of clinical mastitis and culling, from CM on both the observable and underlying
and in the bivariate analyses of clinical mastitis from scale.
different sampling periods, the same model as in the Table 3 shows the estimated variance components
univariate analyses was assumed for both traits. for clinical mastitis CM and the mastitis variable
An additive relationship matrix containing the where the trait is defined on the basis of clinical
relationship between sires was included in the analy- mastitis and culling reason CMCR, for different
ses. Variance components for random effects were length of the sampling period. Both the sire and the
estimated with REML, using the program VCE4 residual variance components increase with increas-
Neumaier and Groeneveld, 1998. Heritability esti- ing sampling period. Sire variance increases rela-
mates were transformed from the observable 0 1 tively more than the residual variance, which results
scale to the assumed underlying scale using the in higher heritabilities. The genetic variance was
classical formula of Dempster and Lerner 1950. roughly twice as high for sampling periods up to 210
days CM210 and CMCR210 as for sampling periods ending 30 days after calving CM30 and
3. Results CMCR30.
