An animal model evaluation of Canadian Holsteins for a lactation measurement log2 transformed somatic cell count (SCC) was completed based on data from Ontario and Quebec. About 360,000 multiple lactation LSCS of over 200,000 cows with first lactation information in the data, based on two or more test-day records were processed. Weighting factors were computed from single test-day records and assigned to each observation according to the number of test- day records in LSCS. Herd-year-season and age by parity fixed effects were included in a repeatability animal model for genetic evaluation. The additive relationship matrix included two generations of ancestors. Unknown animals were assigned to 23 phantom groups according to sex and year of birth. Solutions for age by parity showed a highly significant increasing trend, which was stepping up in subsequent parities. No genetic trend was detected for cows, while sire EBV showed a favorable but small decreasing trend from the mid eighties. Average inbreeding coefficients of daughters were plotted against their sire EBV for LSCS. The linear regression coefficient indicated a small but significant increase of SCC (about 1,000) per 1% of increase in inbreeding.

G. Pagnacco, F. Miglior, W. C Zhang, Jack CM Dekkers, E. B Burnside

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 17. Genetics and breeding of dairy and beef cattle, swine and horses, , 93–96, 1994
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