Net Merit – Updated in August 2006
The 2006 revision of Net Merit (NM$) includes an improved definition of productive life and new genetic evaluations for service sire and daughter stillbirth. It is important that dairy producers routinely review their genetic selection criteria and know where service sires rank compared with other active AI bulls.
Extension Dairy Educator
Northwest Lower Michigan
Net Merit (NM$) was revised in August, 2006 to ensure its usefulness as a genetic selection tool that includes economically important production and fitness traits. The enhancements include a modification to the definition of productive life, addition of two traits influencing calving ability, and updates to the economic weights for traits in NM$.
Researchers at USDA’s Animal Improvement Programs Laboratory (AIPL) strive to provide genetic evaluations that meet the needs of today’s dairy producers. During the past decade, USDA-AIPL has developed methods to use DHI data to generate genetic evaluations for several fitness traits that impact a cow’s profitability (e.g., somatic cell score, productive life, daughter pregnancy rate). Two new traits, Service Sire Stillbirth and Daughter Stillbirth, were added to NM$ for Holsteins with the August, 2006 genetic update.
The revised calculation of productive life gives credit to extended lactations and months in milk after 84 months of age. The economic weights for traits in the NM$ calculation now more accurately reflect the traits’ current economic impact in commercial dairy herds.
PTAs for Productive Life
Predicted transmitting abilities for productive life (PTA PL) are based on months in lactation through a specified age for a cow. Since PTA PL was introduced in 1994, cows have been credited with up to 10 months per lactation through 7 years of age (84 months). Days in milk data beyond 305 days and months in lactation past 7 years of age were not included in productive life calculations.
The revised productive life trait now incorporates days in milk data for the entire lactation and months past 84 months of age to the total herd life of the cow. Each month of lactation receives a different amount of credit based on the shape of the lactation curve. The largest weightings will be given to those months at the peak of lactation and extended months in milk will receive diminishing credit. More emphasis is placed on beginning a new lactation than continuing an extended lactation. First lactations will receive less credit than later lactations in proportion to the difference in average production. Lactation-curve credits ensure that cows with multiple lactations get more credit than cows with a single long lactation.
The range of PTA PL is increased by about 40%. Based on August, 2006 data for Active AI bulls, PTA PL ranges are from -4.7 to +7.5 months for Holsteins and from -1.0 to +4.2 months for Jerseys. Active AI Holstein bulls had an average PTA PL of +0.8 months while the average PTA PL for Jersey bulls is +1.7 months.
Stillborn calves represent a significant economic loss to dairy producers. USDA-AIPL reported stillbirth incidences of 12% for calves of first lactation cows and 5% for calves of second or later lactation cows. For the genetic evaluations, stillbirth is defined as calves born dead or that die within 48 hours of birth, with a vast majority of the stillborn calves being born dead.
Two stillbirth evaluations are calculated for Holstein sires with adequate data. Service sire stillbirth (SSB) measures the tendency of calves from a particular sire to be stillborn. As an example, a sire with SSB of 8% will produce 4% fewer stillborn calves than a sire with a 12% SSB. Daughter stillbirth (DSB) estimates the tendency of daughters of a particular sire to produce stillborn calves.
Heritability of stillbirths is relatively low – approximately 3%. Consequently, the most effective way to incorporate stillbirth evaluations in a sire selection program is to include them in a selection index, such as NM$.
For Holsteins only, NM$ now includes stillbirth genetic evaluations. The stillbirth evaluations (SSB and DSB) plus daughter calving ease (DCE) and service sire calving ease (SCE) have been combined into a Calving Ability composite (CA$). CA$ is the lifetime dollar benefit of less calving difficulty and fewer stillbirths. In CA$ calculations for Holsteins, relative emphasis on calving ease is 40% and 60% on stillbirth.
CA$ is calculated for Brown Swiss using only service sire and daughter calving ease evaluations. CA$ is not available for the other dairy breeds because calving ease and stillbirth genetic evaluations are not calculated due to insufficient data.
Updated Economic Weights
Economic weights for the traits in the NM$ calculation were updated in August, 2006. NM$ estimates lifetime profit based on incomes and expenses relevant for today’s dairy producers. Table 1 lists the relative emphasis of each trait in NM$ for Holsteins and Brown Swiss. Calving Ability composite (CA$) is not calculated for the other dairy breeds. For those breeds, the relative emphasis on each trait excluding CA$ can be estimated by multiplying the values in Table 1 by 1.06.
A significant outcome of updated economic weights is that overall emphasis on non-production traits is now 54%, compared with 45% for NM$ 2003. The production traits (fat and protein) have a combined emphasis of 46%. Since 2003 fat and protein are the only production traits included in NM$ to reflect that a majority of milk in the U.S. is now marketed based on multiple component pricing.
The weight for somatic cell score is negative because lower values are more profitable. The negative weight for body size reflects that large Holstein cows tend to generate lower lifetime profit than their moderate-sized contemporaries in commercial dairy herds.
Daughter pregnancy rate now receives a relative weight of 9% compared with 7% for NM$ 2003. Higher replacement heifer prices contributed to the increased emphasis on productive life (17% compared with 11% in the 2003 version of NM$).
Dairy producers should routinely review their genetic selection criteria and make adjustments as needed. Table 2 lists the NM$ values for various percentile ranking levels for each breed. Knowing where service sires rank relative to other active AI bulls is helpful in determining if the sires meet your goals. To maximize genetic improvement, it is recommended that the service sires in your herd average at or above the 80th percentile.
Net Merit $ is a useful tool in predicting the total performance of a bull’s daughters over their lifetime. Revisions to NM$ and the traits included in the index continue to ensure that dairy producers have an up-to-date tool for making sire selection decisions.
Cassell, B. 2006. Net Merit gets new look this August. Hoard’s Dairyman. July 2006.
Cassell, B. 2006. They’re tracking dead calves. Hoard’s Dairyman. May 25, 2006.
Seykora, T., P. VanRaden, and J. Cole. 2006. Net Merit receives face-lift. Hoard’s Dairyman. August 25, 2006.
Various authors. 2006. Changes to evaluation systems – August 2006. Animal Improvement Programs Laboratory, United States Dept. of Agriculture. <http://www.aipl.arsusda.gov/reference/changes/eval0608.html>.
VanRaden, P.M. 2006. Net merit as a measure of lifetime profit: Animal Improvement Programs Laboratory, United States Dept. of Agriculture. <http://www.aipl.arsusda.gov/reference/nmcalc.htm>.