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Animal Improvement Programs Laboratory, ARS, USDA, Beltsville, MD 20705-2350
1 Corresponding author: millerrh{at}juno.com
A retrospective study of the impact of the estimated
breeding values of sires and maternal grandsires for somatic cell
score (SCS) on productive life (PL) of Holsteins and Jerseys was
conducted. Data included records from 2,626,425 Holstein and 142,725
Jersey cows. The sires and maternal grandsires of cows were
required to have been available through artificial insemination and
to have predicted transmitting ability (PTA) SCS evaluations
based on 35 or more daughters. A weighted function (WPTA) of
sire and maternal grandsire PTA for SCS was used: (sire PTA +
0.5 maternal grandsire PTA)/1.5. The 3 dependent variables were
PL, frequency of cows culled for mastitis, and first-lactation
SCS. The model included effects of herd, birth year, and WPTA
(WPTA was categorized into groups: <2.70, 2.70 to 2.79, ...,
3.20 to 3.29, 
3.30). For
analysis of first-lactation SCS, calving year and calving month were
substituted for birth year. Differences among WPTA groups were highly
significant: as WPTA increased, PL decreased, whereas percentage
culled for mastitis and first-lactation SCS increased. The range in
PL from lowest to highest WPTA was 5.07 mo for Holsteins and 4.73 mo
for Jerseys. Corresponding differences for percentage culled for
mastitis were 7.0 and 5.6% and for SCS were 0.95 and 1.04 (for
Holsteins and Jerseys, respectively). Although phenotypic studies
suggest that cows with extremely low SCS were less resistant to
mastitis, our results showed consistent improvements in PL,
percentage culled for mastitis, and SCS of daughters when bulls were
chosen for low PTA SCS.
Key Words: selection • mastitis • culling
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