Use the Net Merit index to make dairy sire selection decisions
Net Merit is a genetic index that simplifies the process of selecting service sires based on their genetic merit for a combination of economically important traits.
February 26, 2011 - Author: Kathy Lee, Michigan State University Extension
Genetic evaluations for dairy cattle are calculated for a number of economically important traits. Predicted transmitting abilities (PTAs) are available for:
- production traits (milk, fat, protein),
- type classification traits (final score and linear type traits), and
- health and fitness traits (somatic cell score, productive life, daughter pregnancy rate, calving ease, and stillbirth).
To simplify the process of selecting service sires based on their genetic merit for a combination of economically important traits, selection indexes have been developed. Several selection indexes are provided through USDA and the dairy breed associations. Dairy farmers can choose to use a specific selection index based on how closely the relative weights for the traits represent the importance of the traits for their herd management.
Net Merit (NM$), which is computed by USDA, estimates lifetime profit based on incomes and expenses relevant for today’s dairy producers. The relative economic weights for fat and protein used by USDA to compute NM$ closely reflect the multiple component pricing for milk that applies to Michigan dairy producers. The emphasis on the health and fitness traits is based on the relative value of reducing those particular problems in a herd.
The traits included in NM$ and their relative weights for Holsteins and Brown Swiss are:
|Productive life (m)||22%|
|Somatic cell score||-10%|
|Body size composite||-6%|
|Daughter pregnancy rate||11%|
|Calving ability $||5%|
Calving Ability $ (CA$) is an index of four traits for Holsteins: service-sire calving ease, daughter calving ease, service sire stillbirth, and daughter stillbirth. CA$ for Brown Swiss includes only service-sire calving ease and daughter calving ease. CA$ is not calculated for the other dairy breeds. For those breeds, the relative emphasis on each trait in NM$ can be estimated by multiplying the relative weights listed above by 1.05.
The weight for somatic cell score is negative because lower values are more profitable. The negative weight for body size reflects that large cows tend to generate lower lifetime profit than their moderate-sized contemporaries in commercial dairy herds.
The relative emphasis of production traits compared to health/fitness and type traits in NM$ has shifted significantly since its inception. In fact, currently the combined emphasis on health/fitness and type traits (65%) exceeds the overall emphasis on production traits (35%) When NM$ was introduced in 1994, the emphasis on production traits was 74% compared to 26% emphasis on the health and fitness traits.
Net Merit $ is a useful tool in predicting the total performance of a bull’s daughters over their lifetime. Dairy producers can use selection indexes, such as NM$, to more efficiently consider a combination of economically important traits when choosing the sires of their future herd replacements.
For more information, visit the USDA Animal Improvement Programs Laboratory.