Table 3.
Linear mixed regression models between fat-to-protein ratio in milk (FPR) and daily energy balance (EB, MJ of ME/day) in Holstein-Friesian dairy cows during early lactation (1–20 WIM), and comparisons of regression slopes between WIM intervals within each lactation.
| Lactation | WIM 1 | n | Intercept | s.e. | Slope 2 | s.e. | p-Value | Adj. RMSE 3 | p-Value for Difference in Slope (WIM) 4 | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1–4 | 5–8 | 9–12 | 13–16 | 17–20 | |||||||||
| 1 | 1–4 | 1554 | 49.0 | 6.34 | −54.6 | 3.73 | <0.001 | 21.4 | -- | 0.012 | 0.133 | <0.001 | <0.001 |
| 5–8 | 1851 | 50.5 | 5.63 | −47.5 | 3.09 | <0.001 | 17.0 | -- | 0.271 | <0.001 | <0.001 | ||
| 9–12 | 1869 | 65.4 | 5.65 | −50.3 | 3.37 | <0.001 | 15.4 | -- | <0.001 | <0.001 | |||
| 13–16 | 1796 | 53.3 | 5.67 | −38.1 | 3.53 | <0.001 | 14.6 | -- | 0.026 | ||||
| 17–20 | 1637 | 49.2 | 5.50 | −32.3 | 3.85 | <0.001 | 14.7 | -- | |||||
| 2 | 1–4 | 1359 | 39.1 | 7.21 | −57.2 | 4.79 | <0.001 | 24.4 | -- | 0.053 | >0.500 | >0.500 | <0.001 |
| 5–8 | 1585 | 68.7 | 7.00 | −64.4 | 4.66 | <0.001 | 21.1 | -- | 0.011 | 0.049 | <0.001 | ||
| 9–12 | 1579 | 70.5 | 6.71 | −55.6 | 4.50 | <0.001 | 19.3 | -- | >0.500 | 0.001 | |||
| 13–16 | 1525 | 75.6 | 6.80 | −57.5 | 4.62 | <0.001 | 17.8 | -- | <0.001 | ||||
| 17–20 | 1367 | 59.1 | 7.79 | −43.0 | 5.62 | <0.001 | 18.6 | -- | |||||
| 3 | 1–4 | 814 | 46.4 | 10.4 | −72.8 | 7.28 | <0.001 | 28.1 | -- | >0.500 | <0.001 | 0.027 | <0.001 |
| 5–8 | 986 | 69.6 | 7.86 | −72.8 | 5.11 | <0.001 | 23.8 | -- | <0.001 | 0.010 | <0.001 | ||
| 9–12 | 957 | 113.2 | 8.55 | −92.1 | 6.04 | <0.001 | 22.3 | -- | <0.001 | <0.001 | |||
| 13–16 | 931 | 79.9 | 8.76 | −61.5 | 6.29 | <0.001 | 20.1 | -- | 0.001 | ||||
| 17–20 | 818 | 63.2 | 10.1 | −44.4 | 7.37 | <0.001 | 21.2 | -- | |||||
| ≥4 | 1–4 | 802 | 66.6 | 9.84 | −86.2 | 6.54 | <0.001 | 30.6 | -- | 0.118 | 0.028 | <0.001 | <0.001 |
| 5–8 | 970 | 78.8 | 9.57 | −77.6 | 6.13 | <0.001 | 26.2 | -- | 0.455 | 0.015 | <0.001 | ||
| 9–12 | 953 | 90.1 | 9.84 | −73.7 | 6.42 | <0.001 | 25.1 | -- | 0.106 | <0.001 | |||
| 13–16 | 932 | 85.4 | 9.09 | −65.1 | 6.33 | <0.001 | 23.2 | -- | 0.039 | ||||
| 17–20 | 830 | 78.5 | 10.6 | −53.1 | 7.40 | <0.001 | 23.5 | -- | |||||
1 Weeks in milk/stage of lactation; 2 linear rate of decrease in EB (MJ of ME/day) per 0.1 units of FPR; 3 root mean square error adjusted by the random crossed effects of study and cow; 4 pairwise comparisons of regression slopes according to T-test. Models for first lactation cows resulted in better curve fitting (lower RMSE) compared to models for multiparous cows. While regression slopes differed between many of the four-week periods within each lactation, the trends were generally inconsistent. However, irrespective of parity, regression slopes showed a significant decreased between 1–4 and 17–20 WIM (p < 0.001), and between 13–16 and 17–20 WIM; p ≤ 0.039).