B. Heringstad et al., Variance components of clinical mastitis in dairy cattle - effects of trait definition and culling, LIVEST PROD, 67(3), 2001, pp. 265-272
First lactation clinical mastitis records for Norwegian cattle from 1978 on
wards were analysed. Variance components for clinical mastitis were estimat
ed with a linear sire model using records of more than 500,000 daughters of
2043 sires. Heritability increased slightly as the period for sampling hea
lth data increased, and the longest period analysed (from 15 days before ca
lving to 210 days after calving) gave the highest heritability estimates (h
(2) = 0.04). However, a sampling period from 15 days before calving to 30 d
ays after calving captured most of the genetic variation (h(2) = 0.03), and
showed a high genetic correlation (>0.94) with clinical mastitis sampled o
ver a longer period of first lactation. This implies that recording of clin
ical mastitis over a short time period around first calving can provide a m
easure 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 heritabili
ty 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. (C) 20
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