In Norway, somatic cell count (SCC) has been recorded on a bimonthly basis since 1978, but so far not been used in indirect selection against mastitis. The data used in this analyses were recorded in the period from 1978 to 1995. As the data were collected over a long time span, the trait may have changed genetically over time. This could result from a changed environment due to, for example, new treatment strategies for mastitis. Treatment of mastitis would normally result in reduced somatic cell scores (SCS) for diseased cows, and thereby reduced phenotypic and genetic variance. If so, the genetic variance at specific days in milk (DIM) may have changed, and the correlation between sires’ transmitting abilities at specific DIM in different time periods may therefore be smaller than unity. The aim of this study was to account for this, by using a random regression model (RRM) with heterogeneous genetic variance across calving years.

J. Odegard, J. Jensen, G. Klemetsdal, P. Madsen, B. Heringstad

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 2002. Session 13, , 13.28, 2002
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