MSU Extension Dairy

The net merit index (NM$) incorporates economically important traits into a single value and can be used as a primary genetic selection tool.  Using NM$ simplifies the process of sorting through bulls when making sire selection decisions and maximizes genetic progress for lifetime profit.

Traits included in NM$ are weighted based on their level of economic importance.  NM$ is designed for dairy herds that sell their milk in a multiple component pricing market. 

Two significant updates were made to NM$ in August 2018 by the Council on Dairy Cattle Breeding (CDCB). The six new health traits that indicate disease resistance were added for Holsteins only.  CDCB introduced PTAs for these individual traits in April 2018. 

As described in a previous Michigan State University Extension article, heritabilities of the health traits are low (displaced abomasum, 1.1 percent; hypocalcemia or milk fever, 0.6 percent; ketosis, 1.2 percent; mastitis, 3.1 percent; metritis, 1.4 percent; and retained placenta, 1.0 percent). The expected rate of genetic change will be slow for these lowly heritable traits. Yet their inclusion in selection indexes is warranted based on their significant economic value.

The health traits are incorporated into NM$ for Holsteins as a sub-index (HTH$). HTH$ will not be published separately; however, PTAs for the individual health traits are available through AI companies and other organizations. The following table lists relative values of the traits in the sub-index.

Relative value of health traits in HTH$ index

The other major change in NM$ is the revised economic values of the traits in NM$. The new values reflect recent milk prices, hauling costs and the SCS adjustment in the multiple component milk pricing system.

Relative weighting of traits and sub-indexes in NM$ - August 2018

The relative emphasis on production traits increased slightly from 43 percent to 45 percent. The relative weighting for somatic cell score changed from -7 percent to -4 percent. Direct selection for mastitis resistance reduces the relative emphasis on indirectly selecting against mastitis based on somatic cell score.

The Council on Dairy Cattle Breeding calculates three additional merit indexes – cheese merit (CM$), fluid merit (FM$) and grazing merit (GM$).  These indexes also were revised by incorporating HTH$ and updating the relative economic values for each trait and subindex.

Selection indexes are valuable tools for choosing genetically superior sires and dams.  With the availability of genetic evaluations for numerous traits, indexes have been developed to incorporate economically important traits into a single value making selection easier and inclusive of traits based upon their economic importance.