| Group | Intercept | Slope | R2 | P Level |
| All cows | 0.133 | 0.933 | 0.683 | <0.001 |
| First lactation | 0.167 | 0.858 | 0.635 | <0.001 |
| Mature cows | 0.131 | 0.908 | 0.688 | <0.001 |