Ten trait groups are included in the current total merit index (Table 1), where milk production, mastitis, fertility and udder receive the highest relative weight.
Figure 1 shows how the relative weight on traits in the total merit index used for selection of NRF sires have changed over time. In the 1960’s, 70% of the weight was placed on milk. During the 1970’s fertility and health was included, and the relative weight on these traits have gradually increased while the weight on milk has decreased.
28% of the weight is placed on milk, 21 % on mastitis, and 18% on fertility (Table 1), and simultaneous genetic improvement for health, fertility and milk yield is well documented. This is obtained because the traits are given sufficient weight in the total merit index, progeny testing is based on large daughter groups to obtain reliable breeding values for traits with low heritability, and because the total merit index is used to select elite sires.
Table 1: Relative weights on traits included in the total merit
index used for selection of Norwegian Red sires.
| Traits |
Relative weight |
| Milk production |
28 |
| Mastitis |
21 |
| Fertility |
18 |
| Udder conformation |
15 |
| Leg conformation |
6 |
| Growth rate |
6 |
| Temperament |
2 |
| Disease other than mastitis |
2 |
| Milkability |
1 |
| Calving difficulty |
0,5 |
| Stillbirth |
0,5 |
Figure 1: Weight on groups of traits (ppt)