MSU Extension Dairy

Genetic improvement is a key factor in managing a profitable dairy herd. Genetic progress in dairy herds depends on the use of genetically-superior service sires to produce high-quality replacement heifers. 

Dairy producers have a multitude of information available to make sire selection decisions. Predicted transmitting abilities (PTAs) are computed for a number of traits (production, health/fitness and type).  The most effective way to consider a group of traits when choosing service sires is to use a selection index. 

A selection index incorporates several traits into a single genetic value. Traits are combined into the index based on relative economic weights of each trait. Selection indexes are computed by various organizations including Council on Dairy Cattle Breeding (CDCB), artificial insemination (AI) companies, dairy breed associations and genomic testing businesses.

Net Merit (NM$) is one of the selection indexes used on dairy farms for making sire selection decisions. NM$ was updated in August 2018 to add six new health traits for Holsteins. Changes in the NM$ calculation are described in the Michigan State University Extension  article titled, “ Health traits added to Net Merit index”.

The genetic merit of bulls marketed through AI organizations continues to improve with each release of updated genetic evaluations. Consequently, sire selection goals set by dairy producers a year ago may be outdated today. 

The following table lists the December 2018 NM$ values for various percentile ranking levels for each breed.  Knowing where service sires rank among active AI bulls is helpful in determining if the sires meet the herd’s genetic goals.

NM$ levels of top percentiles for AI sires by breed. December 2018 CDCB Sire Summary:

To maximize genetic improvement using NM$ as your selection index, Michigan State University Extension recommends that a herd’s service sires have an average at or above the 80^th percentile. For example using the table above, the service sires in a Holstein herd would need to average $675 NM or greater to achieve the 80^th percentile goal.

To ensure that a herd’s replacement heifers will be genetically superior, sire selection criteria should be reviewed and updated periodically. It is also important to communicate and discuss these goals with semen sales representatives.