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Title: Impact of Genetic Merit for Milk Somatic Cell Score of Sires and Maternal Grandsires on Herd Life of Their Daughters

item Miller, Robert
item Norman, H
item Wright, Janice
item Cole, John

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2009
Publication Date: 5/1/2008
Citation: Miller, R.H., Norman, H.D., Wright, J.R., Cole, J.B. 2008. Impact of Genetic Merit for Milk Somatic Cell Score of Sires and Maternal Grandsires on Herd Life of Their Daughters. Journal of Dairy Science. 92(5):2224-2228.

Interpretive Summary: The results of selecting AI bulls based on their breeding values for milk SCS were studied for Holstein and Jersey cows. Breeding value estimates of sires and maternal grandsires were combined and related to productive life, percent culled for mastitis and SCS of daughters. Consistent benefits were achieved as bulls' breeding values for SCS changed in the favorable direction (lower SCS). Productive life increased 5.70 and 4.73 mo, percent culled for mastitis decreased 7.0 and 5.8%, and first lactation SCS decreased 0.95 and 1.04. Genetic selection to lower SCS produces cows with fewer mastitis-related losses.

Technical Abstract: A retrospective study of the relationship between Productive Life (PL) of Holstein and Jersey cows and the estimated breeding values of their sires and maternal grandsires for SCS was conducted. Data included records from 2,626,425 Holstein and 142,725 Jersey cows. Data included cows' PL, whether culled for mastitis, and first-lactation mature equivalent (ME) SCS. Cows' sires and maternal grandsires were required to have been available through artificial insemination and to have PTA SCS evaluations based on 35 or more daughters. A weighted function (WPTA) of sire and maternal grandsire PTAs for SCS was used: (sire PTA + 0.5 MGS)/1.5. The model included effects of herd, birth year, and WPTA (WPTA was categorized into groups: <2.70, 2.70-2.79, ..., 3.20-3.29, >=3.30). For analysis of SCS, calving year and calving month were substituted for birth year. Differences among WPTA groups were highly significant: as WPTA increased, PL decreased while percent culled for mastitis and first-lactation SCS increased. The range in PL from lowest to highest WPTA was 5.70 mo for Holsteins and 4.73 mo for Jerseys. Corresponding differences for percent culled for mastitis were 7.0% and 5.8% and for SCS were 0.95 and 1.04 (for Holsteins and Jerseys, respectively). Although phenotypic studies suggest that cows with extremely low SCS are less resistant to mastitis, our results show consistent improvements in PL, percent culled for mastitis, and SCS of daughters when bulls are chosen for low PTA SCS.