Lifetime Net Merit $ (NM$) Index 2010 Reflects Increasing Emphasis
on Non-Yield Traits
By: Jack McAllister, Ph.D.
USDA introduced its first genetic-economic index for genetic evaluation of dairy cattle
in 1971. The first index, known as Predicted Difference $, included two traits – milk yield with
52% of the emphasis and fat yield with 48% of the emphasis and was expected to reflect
expected genetic merit for lifetime profit (Table 1) which the animal transmitted to their
offspring. The emphasis on each trait was determined by the relative economic value of the
trait and the relative genetic variation in that trait and the genetic relationship of that trait to
the other traits in the index. Over the years, milk protein yield was added to the index. The
Predicted Difference $ index was established as guide, nationally, to aid in the selection of
dairy cattle for their genetic value for lifetime profit.
In 1994, productive life and somatic cell score were added to the index (Table 1). The
name of the index was changed to what it is today – Net Merit $. With the addition of these
traits to the yield traits, the relative emphasis changed based upon the principles of relative
economic value and genetic relationships of traits. Milk yield received only 6% while fat yield
received 25% emphasis and protein yield 43%. This relative emphasis among the yield traits
came about as a result of the relative value and use of fluid milk, pounds of butterfat and
pounds of milk protein in the consumption of all dairy products in the US. Thus, the yield traits
received 76% of the emphasis while the non-yield traits which had been added received 24%
of the emphasis in the index.
The type traits - udder composite, feet and legs composite and body size composite
were added in 2000 and received, as a group, 15% of the relative emphasis (Table 1). This
addition of type traits resulted in another shift in the relative emphasis in the traits of the
index. Overall, the emphasis on the yield traits had been reduced from 74% to 62%.
The reproduction traits daughter pregnancy rate, service sire calving difficulty and
daughter calving difficulty were incorporated in 2003 (Table 1). In the 2003 NM$ index, milk
yield now received no emphasis, fat yield’s emphasis was increased from 21 to 22% and
protein yield’s emphasis was reduced from 36% to 33%, giving a total emphasis of 55% to the
yield traits. The remaining 45% emphasis was spread among productive life with 11%,
somatic cell score with 9% and the type traits with 14% emphasis collectively and the
reproductive traits with 11%. Thus, with the addition of type traits and then reproduction traits,
yield traits now have been reduced in relative emphasis from 74% down to 55%.
In the 2006 NM$ index, calving assistance $ replaced the service sire and daughter
calving difficulty. The emphasis on protein yield dropped from 33 to 23% and productive life
went up to 17% from 11% with the other traits receiving about the same emphasis as the
2003 index. The total emphasis for yield traits fell again from 55% to 46%.
The traits in the 2010 NM$ index are the same as for 2006 but the emphasis on the
traits has changed again. Fat and protein yield dropped to 19 and 16% relative emphasis,
respectively. This now gives the yield traits only 35% relative emphasis. The shift to an
increasing emphasis on non-yield traits came from productive life now receiving 22% of the
emphasis, up from 17%. Also, somatic cell score, type traits and daughter pregnancy rate all
increased in relative emphasis as a group by 7% while calving assistance $ dropped from 6%
to 5%. Over the years, the change in relative emphasis has come about because of the
increasing economic value of cows with long productive lives which reproduce regularly and
are relatively free of mastitis in relation to the economic value of their lactation yields of
butterfat and protein.
Because NM$ is a national index, it reflects the emphases that are appropriate for the
national supply of milk and its fat and protein components used in dairy products nationally.
Today, in addition to NM$ there are two other indexes which can be used for parts of the
country where the milk supply is used primarily for either cheese production or fluid milk
products. The index for cheese production is called Cheese Merit $ (CM$). The index for fluid
milk production is Fluid Merit $ (FM$). Here again, these indexes contain the same traits but
the relative emphasis on the different traits varies according to the relative economic value of
milk, milk fat or milk protein in the production of either cheese or fluid milk dairy products.
Educational programs of Kentucky Cooperative Extension serve all people regardless of race, color, age, sex,
religion, disability, or national origin.