Standardization of lactation records for variance of Mendelian sampling to reduce bias in evaluations of bull dams

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Standardization of lactation records for variance
of Mendelian sampling to reduce bias in
evaluations of bull dams
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Holstein AI Bulls Sampled in 1995
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Holstein AI Bulls Sampled in 1995
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Holstein AI Bulls Sampled in 1995
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Top 10 Holstein elite cowsand active AI bulls
May 2003 EBV (kg) May 2003 EBV (kg)
Trait Bulls Cows
Milk 2412 3072
Fat 88 122
Protein 72 104
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Possible reasons for bias

• Preferential treatment
• Build up over generations as biased daughters
  contribute to their dams
• Incomplete standardization of variances
• Inadequate adjustment of outlier production
  records
• Heritability increase from 25 to 30 in 1997

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Efforts to account for bias

• Heterogeneous variance adjustment based on
  phenotypic variance (since 1991)
• Floor and ceiling at 4 SD on production records
  (since 1997)

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Efforts to understand bias

• Estimation of bias in embryo-transfer cows
• Investigation of distribution of records within
  herd (search for bimodal distributions)
• Merchandisable cows
• Contemporaries with low yields

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Bimodal herd First-lactation milk yield
deviations
Density
Deviation
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Plan

• Calculate variance of Mendelian sampling using
  male ancestor mean to avoid possible bias from
  dam evaluations
• Calculate variance within herd and 5 yr of first
  calving groups
• Adjust records by ratio of population SD to herd
  SD

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Method

• During iteration
• Male ancestor mean
• .5 (EBVsire EBVMGS)
• Mendelian sampling
• EBV