Livestock Production Science 67 2001 265–272 www.elsevier.com locate livprodsci
Variance components of clinical mastitis in dairy cattle — effects of trait definition and culling
Bjørg Heringstad , Gunnar Klemetsdal, John Ruane
˚
Department of Animal Science , Agricultural University of Norway, P.O. Box 5025, N-1432 As, Norway
Received 17 May 1999; received in revised form 28 March 2000; accepted 27 April 2000
Abstract
First lactation clinical mastitis records for Norwegian cattle from 1978 onwards were analysed. Variance components for clinical mastitis were estimated with a linear sire model using records of more than 500,000 daughters of 2043 sires.
Heritability increased slightly as the period for sampling health data increased, and the longest period analysed from 15
2
days before calving to 210 days after calving gave the highest heritability estimates h 5 0.04. However, a sampling
2
period from 15 days before calving to 30 days after calving captured most of the genetic variation h 5 0.03, and showed a high genetic correlation .0.94 with clinical mastitis sampled over a longer period of first lactation. This implies that
recording of clinical mastitis over a short time period around first calving can provide a measure of clinical mastitis with a substantial value in genetic evaluation. Using culling reason as an additional source of information about mastitis increased
heritability only slightly compared with using clinical mastitis records only. Excluding cows culled before the end of the sampling period from the data if they have not had mastitis resulted in a higher heritability of mastitis than both including a
fixed effect to account for culling in the model and a bivariate analysis of clinical mastitis and culling. Hence, culling affects variance component estimates of clinical mastitis.
2001 Elsevier Science B.V. All rights reserved.
Keywords : Dairy cattle; Clinical mastitis; Genetic parameters
1. Introduction Norway together with Denmark, Finland and Sweden
are the only countries with national recording sys- Breeding for increased mastitis resistance is one
tems for health data and the only countries which strategy to reduce mastitis, which is the most fre-
include clinical mastitis resistance directly in their quent and costly disease affecting dairy cattle. A
dairy cattle breeding programs Heringstad et al., reduced mastitis frequency will reduce costs of
2000. production, reduce the use of antibiotics and is
In genetic evaluation, all four Nordic countries use important for ethical and animal welfare reasons.
field records of veterinary-treated cases of clinical mastitis. Generally, mastitis is considered as an all-
or-none trait, cows being categorised as diseased if
Corresponding author. Tel.: 147-6494-8000; fax: 147-6494-
they had mastitis in a defined period of lactation and
7960. E-mail address
: bjorg.heringstadihf.nlh.no B. Heringstad.
as healthy if there was no report of mastitis in this
0301-6226 01 – see front matter
2001 Elsevier Science B.V. All rights reserved. P I I : S 0 3 0 1 - 6 2 2 6 0 0 0 0 1 9 9 - 8
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. 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.
