# Presentation: 132552

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

## Holstein AI Bulls Sampled in 1995

## Holstein AI Bulls Sampled in 1995

## Holstein AI Bulls Sampled in 1995

## Top 10 Holstein elite cowsand active AI bulls

|  | May 2003 EBV (kg) |  |
| --- | --- | --- |
| Trait | Bulls | Cows |
| Milk | 2412 | 3072 |
| Fat | 88 | 122 |
| Protein | 72 | 104 |

## 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**

## 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)**

## 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**

## Bimodal herd First-lactation milk yield deviations

**Deviation**

**Density**

## 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 **

## Method

**During iteration**
**Male ancestor mean =**
**.5 (EBV****sire**** + EBV****MGS****)**
**Mendelian sampling =**
**EBV – male ancestor mean**

## Method

**Variance of Mendelian sampling**
**Lower for recent cows with fewer records or first record in progress**
**Expected variance obtained from sum of data collection ratings**
**Population variance gets weight = 20 cows when combining with herd variance**

## Method

**Ratio = population variance / herd variance**
**.5 < ratio < 2**
**Further limits proportional to herd mean**
**High yield: ****upper limit < 2**
**Low yield: ****lower limit > .5**
**Ratio frozen after 40 iterations**

## Analysis

**Holstein calvings analyzed with and without standardization of variance of Mendelian sampling to assess predictive ability**
**Separate evaluations for pre-1997 calvings compared with corresponding evaluations for February 2003**

## Results

|  | Correlation1 |  |  |  | Regression coefficient |  |
| --- | --- | --- | --- | --- | --- | --- |
|  | Standardized? |  |  | Standardized? |  |  |
| Trait | No | Yes |  | No |  | Yes |
| Milk | .77 | .78 |  | .74 |  | .79 |
| Fat | .72 | .73 |  | .69 |  | .76 |
| Protein | .81 | .82 |  | .72 |  | .78 |
| 1Between pre-1997 and February, 2003 |  |  |  |  |  |  |

## Results

|  | Bias |  |  |  | Mean EBV (kg)1 |  |
| --- | --- | --- | --- | --- | --- | --- |
|  | Standardized? |  |  | Standardized? |  |  |
| Trait | No | Yes |  | No |  | Yes |
| Milk | – 75.8 | – 58.5 |  | 2716 |  | 2566 |
| Fat | – 3.3 | – 2.5 |  | 110 |  | 95 |
| Protein | – 2.5 | – 1.9 |  | 89 |  | 78 |
| 1Top 100 cows |  |  |  |  |  |  |

## Effect of adjustment

**Correlations higher**
**Regression of EBV on parent mean was closer to 1**
**Bias lower**
**Mean EBV of top 100 cows reduced**
**Different cows (20%) in top 50,000 for PTA milk**
**Largest drop in EBV was 1324 kg**

## Future developments

**Submit national evaluations standardized for variance of Mendelian sampling to Interbull for September 2003 test evaluation**
**Possibly enhance standardization method by imposing floor and ceiling on yield deviations during iterations**

## Conclusions

**Cow evaluations overestimate eventual evaluations of sons**
**Standardization for variance of Mendelian sampling improves prediction of progeny evaluations**