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Concern

Impact of selection for decreased somatic cell score on productive life and culling for mastitis Abstr. #13. Concern. & Objective. How does long-term genetic selection for lower SCS in milk affect frequency of mastitis infections? PTA SCS available since 1994.

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Concern

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  1. Impact of selection for decreased somatic cell scoreon productive life andculling for mastitisAbstr. #13

  2. Concern & Objective • How does long-term genetic selection for lower SCS in milk affect frequency of mastitis infections? • PTA SCS available since 1994 • Determine how 2 generations of selection for SCS affects culling for mastitis and productive life (PL)

  3. Data • Holstein AI bulls with August 2007 PTA SCS based on 35 daughters • Cows with birth dates from 1993 through 1999 and calving dates from 1995 through 2005 • Cows excluded if they changed herds, had missing lactation records within first 5 parities, or were in herds with <5 cows • Final data set of 2,556,402 Holstein cows in 27,167 herds

  4. Data (cont.) • Holstein bulls assigned to quintiles based on PTA SCS • 25 cow groups based on sire and maternal grandsire (MGS) quintiles Examples • Cows in group11 had sire and MGS in the lowest quintile (bulls with lowest PTA SCS) • Cows in group55 had sire and MGS in the highest quintile (bulls with highest PTA SCS)

  5. Cow numbers Low PTA SCS Sire quintile High PTA SCS

  6. 1st-parity SCS Low PTA SCS Sire quintile High PTA SCS

  7. Percentage of cows culled for mastitis Low PTA SCS Sire quintile High PTA SCS

  8. Cow PL (mo) Low PTA SCS Sire quintile High PTA SCS

  9. Model • Least-square (LS) differences between cow groups examined for cow PL • Analysis on a within-herd basis with cow birth year in model

  10. LS differences for cow PL (mo) Low PTA SCS Sire quintile High PTA SCS

  11. LS differences for mastitis culling (%) Low PTA SCS Sire quintile High PTA SCS

  12. LS results • Advantage for cows with sire or MGS with lower PTA SCS nearly linear across groups within MGS or sire quintile • Difference in cow PL between highest and lowest PTA SCS quintiles • 2.0 to 2.4 mo for sire quintiles • 0.8 to 1.2 mo for MGS quintiles

  13. Economics • Each month of additional PTA PL valued at $29 in current USDA lifetime net merit index • 200-cow herd in group11 worth nearly $6,000 more annually than corresponding herd in group55 • Additional income or expense associated directly with SCS not considered

  14. Conclusion • Selection for decreased SCS across generations expected to result in cows that produce longer and have a smaller likelihood of leaving the herd because of mastitis • 3.2 mo longer PL • 4.4% less likely to be culled for mastitis

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