Source: Cornell University
An Introduction to Linear Indexes
Linear indexes combine linear trait information on several related traits into a numerical value. They are commonly used as selection tools in breeding programs to identify those animals who are predicted to transmit a desirable combination of the traits in the composite index (1).
The values are for each trait in a composite index are weighted according to their economic value and added together to arrive at the index value.
Selection indexes are designed for more rapid genetic improvement (3).
Breed Specific Indexes
Different breeds have different, unique characteristics, therefore, trait value and selection differs between breeds.
For Holsteins and Jerseys, each breed has selection indexes that are helpful tools for breeders who want to select for a combination of traits that they feel are most valuable for their breeding program.
These indexes reflect similar traits, however, the weighting they are given is breed specific in order to reach unique breeding goals for each. Since they focus on specific categories of traits they are beneficial for producers looking to improve in those particular areas.
What if a producer is simply looking to enhance profitability and overall quality?
What about other dairy breeds?
NET MERIT
Net merit is a measure of the lifetime profitability of an individual. This is a genetic-economic index to predict expected lifetime profitability of dairy cattle, regardless of breed (4).
The profit function approach used in deriving NM$ lets breeders select for many traits by combining the incomes and expenses for each trait into an accurate measure of overall profit. Official NM$ is calculated using Holstein values instead of having a slightly different NM$ formula for each breed (5). Rather than reflecting breed-specific values, Net Merit reflects value according to different milk markets; cheese merit, fluid merit, and grazing merit. Producers should use the lifetime net merit index (NM$, CM$, FM$, or GM$) that corresponds to the market pricing that they expect a few years in the future when buying breeding stock and five years in the future when buying semen (5).
So, how does Net Merit compare with TPI and JPI?
Net merit looks at very similar traits as the other two indexes; protein, fat, milk, productive life, somatic cell score, udder, feet/legs, body size, daughter pregnancy rate, heifer conception rate, cow conception rate, and calving ability (5).
Rather than putting a weighting on each trait, the PTAs for each trait are multiplied by the corresponding economic value and then summed. Therefore, Net Merit is most reflective of solely economic conditions than either TPI or JPI, since they are biased toward breed improvement and genetic progress goals.
References:
- Holstein Association USA. Understanding Genetics and the Sire Summaries. [Online]. Available from: http://www.holsteinfoundation.org/pdf_doc/workbooks/Gen_Sire_WKBK.pdf. [Accessed May 10, 2015].
- American Jersey Cattle Association. 2015 Jersey Performance Index (JPI). [Online]. Available from: https://www.usjersey.com/Portals/0/AJCA/2_Docs/Genetic-Resources/JPI-Backgrounder.pdf. [Accessed May 11, 2015].
- Lush, Jay L. Selection Indexes for Dairy Cattle. [Online]. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1439-0388.1961.tb01312.x/abstract. [Accessed May 10, 2015].
- Agricultural Research Service. Net Merit as a Measure of Lifetime Profit: 2014 Revision. [Online]. Available from: http://www.ars.usda.gov/research/publications/publications.htm?seq_no_115=310013. [Accessed May 10, 2015].
- USDA Animal Improvement Program. Lifetime Net Merit 2014. [Online]. Available from: http://aipl.arsusda.gov/reference/nmcalc-2014.htm#Net%20merit%20calculation. [Accessed May 10, 2015].
