# Presentation: 677287

## Impact of selection for decreased somatic cell scoreon productive life andculling for mastitis	Abstr. #13

## Concern

- How does long-term genetic selection for lower SCS in milk affect frequency of mastitis infections?
- PTA SCS available since 1994

- & Objective

- Determine how 2 generations of selection for SCS affects culling for mastitis and productive life (PL)

## Data

- 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

## 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)

## Cow numbers

|  | MGS quintile |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
|  | 1 | 2 | 3 | 4 | 5 |
| 1 | 144,258 | 99,267 | 84,013 | 105,042 | 123,500 |
| 2 | 127,642 | 92,279 | 72,028 | 92,364 | 106,587 |
| 3 | 107,877 | 74,823 | 62,374 | 78,155 | 91,018 |
| 4 | 127,449 | 90,647 | 72,259 | 93,357 | 107,295 |
| 5 | 156,326 | 113,183 | 90,180 | 114,129 | 130,350 |

**Sire quintile**

- Low PTA SCS

- High PTA SCS

## 1st-parity SCS

|  | MGS quintile |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
|  | 1 | 2 | 3 | 4 | 5 |
| 1 | 2.64 | 2.69 | 2.74 | 2.76 | 2.82 |
| 2 | 2.76 | 2.84 | 2.87 | 2.92 | 2.95 |
| 3 | 2.85 | 2.90 | 2.95 | 3.00 | 3.05 |
| 4 | 2.92 | 2.99 | 3.04 | 3.09 | 3.15 |
| 5 | 3.07 | 3.15 | 3.20 | 3.26 | 3.31 |

**Sire quintile**

- Low PTA SCS

- High PTA SCS

## Percentage of cows culled for mastitis

|  | MGS quintile |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
|  | 1 | 2 | 3 | 4 | 5 |
| 1 | 8.6 | 8.9 | 9.1 | 9.6 | 9.9 |
| 2 | 9.2 | 9.5 | 9.8 | 9.9 | 10.6 |
| 3 | 9.6 | 10.3 | 10.1 | 10.8 | 11.0 |
| 4 | 10.5 | 11.0 | 11.3 | 11.6 | 12.1 |
| 5 | 11.6 | 11.9 | 12.2 | 12.9 | 13.3 |

**Sire quintile**

- Low PTA SCS

- High PTA SCS

## Cow PL (mo)

|  | MGS quintile |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
|  | 1 | 2 | 3 | 4 | 5 |
| 1 | 28.2 | 28.3 | 27.9 | 28.0 | 27.4 |
| 2 | 28.0 | 28.0 | 27.5 | 27.4 | 27.1 |
| 3 | 27.4 | 27.4 | 27.0 | 26.9 | 26.6 |
| 4 | 27.2 | 27.3 | 26.8 | 26.6 | 26.4 |
| 5 | 26.4 | 26.3 | 25.9 | 25.8 | 25.2 |

**Sire quintile**

- Low PTA SCS

- High PTA SCS

## Model

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

## LS differences for cow PL (mo)

|  | MGS quintile |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
|  | 1 | 2 | 3 | 4 | 5 |
| 1 | 3.16 | 3.09 | 2.88 | 2.93 | 2.41 |
| 2 | 2.76 | 2.75 | 2.35 | 2.27 | 1.82 |
| 3 | 2.30 | 2.10 | 1.93 | 1.80 | 1.37 |
| 4 | 2.18 | 2.11 | 1.83 | 1.57 | 1.20 |
| 5 | 1.19 | 1.00 | 0.78 | 0.63 | 0.00 |

**Sire quintile**

- Low PTA SCS

- High PTA SCS

## LS differences for mastitis culling (%)

|  | MGS quintile |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
|  | 1 | 2 | 3 | 4 | 5 |
| 1 | −4.4 | −4.1 | −4.0 | −3.7 | −3.4 |
| 2 | −3.9 | −3.6 | −3.3 | −3.4 | −2.7 |
| 3 | −3.4 | −2.8 | −2.9 | −2.5 | −2.3 |
| 4 | −2.7 | −2.2 | −1.8 | −1.7 | −1.3 |
| 5 | −1.6 | −1.2 | −0.8 | −0.5 | 0.0 |

**Sire quintile**

- Low PTA SCS

- High PTA SCS

## 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

## 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

## 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