R.H. Miller1, H.D. Norman, J.R. Wright, and J.B. Cole
Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350
2008 J. Dairy Sci. (?)
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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.
(Key words: selection, mastitis, culling)