| Group | Intercept | Slope | R2 | P Level |
| All cows | 0.253 | 0.818 | 0.587 | <0.001 |
| First lactation | 0.288 | 0.756 | 0.489 | <0.001 |
| Mature cows | 0.270 | 0.814 | 0.582 | <0.001 |