# Presentation: 705093

## Genetic evaluation and best prediction of lactation persistency

## Introduction

**At the same level of production**** cows with high persistency milk more at the end than the beginning of lactation.**
**Best prediction of persistency is calculated as a function of trait-specific standard lactation curves and the linear regression of a cow’s test day deviations on days in milk. **

## Best Prediction

**Selection Index**
**Predict missing yields from measured yields**
**Condense daily into lactation yield and persistency**
**Only phenotypic covariances are needed**
**Mean and variance of herd assumed known**
**Reverse prediction**
**Daily yield predicted from lactation yield and persistency**

## Reliabilities

**Definition**
**Squared correlation of estimated with true persistency**
**Same as Rel (breeding value)**
**Single-trait or multi-trait (M, F, P, SCS)**

## PersistencyVanRaden 1998 6th WCGALP XXIII:347-350

**Definition**
**305 daily yield deviations (DIM - DIM****o****)**
**Uncorrelated with yield by choosing DIM****o**
**DIM****o ****were ****161****, ****159****, ****166****, and ****155**** for M, F, P, and SCS**
**DIM****0**** have increased over time**
**Standardized estimate**

## Statistical Properties

| Available data |   Rel (Y) | Rel (P) | Corr (Y,P) |
| --- | --- | --- | --- |
| 305-d Daily | 100% | 100% | .00 |
| 305-d Monthly | 97 | 91 | .00 |
| 305-d Bimonthly | 94 | 82 | .00 |
| 100-d RIP | 76 | 33 | -.78 |
| 15-d test | 40 | 27 | -1.00 |

## Objective

**Calculate (co)variance components and breeding values for persistency of milk (****PM****), fat (****PF****), protein (****PP****), and SCS (****PSCS****) in Holsteins**
**Estimate genetic correlations among persistency and yield traits**

## Data

**8,682,138**** lactation records from ****4,375,938**** Holstein cows calving since January 1, 1997**
**Best prediction of persistency**
**Milk (****M****), fat (****F****), protein (****P****), SCS**
**Floor and ceiling of ± 4.0**
**Phenotypic reliability ≥ 50%**
**1****st**** - 5****th**** lactations (1****st**** required)**

## Cow with Average Persistency

## Highest Cow Persistency

## Lowest Cow Persistency

## Model

**Repeatability animal model:**
**	****y****ijkl**** = hys****i**** + lac****j**** + a****k**** + pe****k**** + ****β****(do****jk****) + e****ijkl**
**y****ijkl**** = persistency of milk,fat, protein, or SCS**
**hys****i**** = fixed effect of herd-year-season of calving I**
**lac****j**** = fixed effect of lactation j**
**a****k**** = random additive genetic effect of animal k**
**pe****k**** = random permanent environmental effect of animal k**
**do****jk**** = days open for lactation j of animal k**
**e****ijkl**** = random residual error**

## (Co)variance Components

|  | σa2 | σpe2 | σe2 | h2 | rept |
| --- | --- | --- | --- | --- | --- |
| PM | 0.10 | 0.09 | 0.85 | 0.10 | 0.18 |
| PF | 0.07 | 0.08 | 0.79 | 0.07 | 0.15 |
| PP | 0.08 | 0.07 | 0.70 | 0.09 | 0.17 |
| PSCS | 0.02 | 0.03 | 0.64 | 0.03 | 0.07 |

## Correlations Among Persistency Traits

|  | PM | PF | PP | PSCS |
| --- | --- | --- | --- | --- |
| PM |  | 0.83 | 0.87 | -0.48 |
| PF | 0.72 |  | 0.82 | -0.41 |
| PP | 0.91 | 0.72 |  | -0.58 |
| PSCS | -0.19 | -0.11 | -0.14 |  |

**1****Genetic correlations above diagonal, residual correlations below diagonal.**

## Phenotypic Correlations Among Persistency and Yield

|  | M | F | P | SCS |
| --- | --- | --- | --- | --- |
| PM | 0.03 | 0.07 | 0.07 | -0.03 |
| PF | 0.05 | -0.02 | 0.08 | -0.03 |
| PP | 0.04 | 0.08 | 0.05 | -0.03 |
| PSCS | 0.05 | 0.02 | 0.04 | -0.03 |

## Genetic Correlations Among Persistency and Yield

|  | M | F | P | SCS |
| --- | --- | --- | --- | --- |
| PM | 0.05 | 0.10 | 0.03 | -0.04 |
| PF | 0.12 | 0.12 | 0.00 | 0.00 |
| PP | -0.02 | 0.08 | -0.09 | -0.11 |
| PSCS | -0.23 | -0.28 | -0.20 | 0.41 |

## Distribution of Sire PTA

## Conclusions

**Heritabilities and repeatabilities are low to moderate**
**Routine genetic evaluations for persistency are feasible**
**The shape of the lactation curve may be altered without affecting production**