Lifetime Net Merit $ (NM$) Index2010 Reflects Increasing Emphasis
on Non-Yield Traits
Kentucky
Dairy Notes
September 2010
For More Information
Please Contact:
Jack McAllister.
University of Kentucky
(859) 257-7540
amcallis@uky.edu
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%.
Kentucky
Dairy Notes
September 2010
For More Information
Please Contact:
Jack McAllister.
University of Kentucky
(859) 257-7540
amcallis@uky.edu
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$). Here again, these indexes contain the same
traits but the relative emphasis on the different traits varies according to the
relative economic value of milk, milk fat or milk protein in the production of
either cheese or fluid milk dairy products.
Table 1. History of main changes in USDA genetic-economic indexes for dairy cattle and percentages
of relative emphasis of traits in the indexes.
USDA genetic-economic index (and year introduced)
Traits included
PD$
MFP$
CY$
NM$
NM$
(1971) (1976) (1984) (1994) (2000)
NM$
NM$
NM$
(2003) (2006) (2010)
Milk
52
27
−2
6
5
0
0
0
Fat
48
46
45
25
21
22
23
19
Protein
…
27
53
43
36
33
23
16
Productive Life (PL)
…
…
…
20
14
11
17
22
Somatic Cell Score (SCS)
…
…
…
−6
−9
−9
−9
−10
Udder composite
…
…
…
…
7
7
6
7
Feet/legs composite
…
…
…
…
4
4
3
4
Body size composite
…
…
…
…
−4
−3
−4
−6
Daughter Pregnancy Rate (DPR)
…
…
…
…
…
7
9
11
Service sire calving difficulty
…
…
…
…
…
−2
…
…
Daughter calving difficulty
…
…
…
…
…
−2
…
…
…
…
…
…
…
…
6
5
Calving Ability
(CA) $
Adapted from Cole et al., 2010. Net merit as a measure of lifetime profit: a revision.
http://aipl.arsusda.gov/reference/nmcalc.htm
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