<p class=MsoNormal align=center style='text-align:center'>Net Merit: August 2003 Revision

Net Merit: August 2003 revision

by Tony Seykora and Paul VanRaden

With the August 2003 USDA genetic evaluations, the Net Merit (NM$) index was revised to incorporate cow fertility and calving ease traits, and to update economic values for the other traits. NM$ is defined as the expected lifetime profit as compared with the breed base cows born in 1995. NM$ began in 1994 with production and health traits and was last revised in August 2000 when the type traits (udder, feet-and-legs, and body size composites) were added.

The correlation between the previous NM$ (NM$ 2000) and the new NM$ (NM$ 2003) is .98. This means that the revised index will cause only small changes in the rankings for most bulls. Extreme bulls (either good or bad) for daughter pregnancy rate (DPR), service sire calving ease (SCE), or daughter calving ease (DCE) are more likely to have their rankings change appreciably.

NM$ calculation . . .

The economic values used in NM$ 2000 and NM$ 2003 are shown in Table 1. The major change was adding DPR, SCE, and DCE to the index. The relative weights placed on each of the traits are also given in Table 1. Under NM$ 2000, 62 percent of the emphasis was on production traits – milk, fat, and protein. For NM$ 2003, only 55 percent of the emphasis is placed on production traits (fat and protein) with correspondingly more emphasis placed on selected fitness traits. Emphasis on productive life decreased slightly from 14 to 11 percent of the index because more emphasis is now assigned to the individual traits such as DPR that contribute to productive life. Because non-Holstein breeds do not have calving ease information, the relative weights on other traits for those breeds increase. For the non-Holstein breeds the relative weights are: fat, 23%; protein, 35%; productive life, 11%; SCS, -10%; udder composite, 7%; feet-and-legs composite, 4%; body size composite, -3%; and DPR, 7%.

 

Table 1. Comparison of NM$ 2000 and NM$ 2003

Trait

Value

 

Relative weight

NM$ 2000

NM$ 2003

 

NM$ 2000

NM$ 2003

 

-------- $/PTA unit --------

 

------------ % ------------

Milk

.18

0

 

5

0

Fat

2.14

2.54

 

21

22

Protein

4.76

4.81

 

36

33

Productive life

28

26

 

14

11

SCS

-154

-166

 

-9

-9

Udder composite

29

33

 

7

7

Feet & legs composite

15

15

 

4

4

Body size composite

-14

-12

 

-4

-3

DPR

…

17

 

…

7

SCE*

…

-5

 

…

-2

DCE*

…

-5

 

…

-2

*Available only for Holsteins

 

Table 2. NM$ calculation for sample Holstein bull

Trait

PTA

Value

PTA ΄ value

 

 

------ $/PTA unit ------

------ $ ------

Milk (lbs)

1843

0

0

Fat (lbs)

76

2.54

193

Protein (lbs)

49

4.81

237

Productive life (mo)

2.4

26

62

SCS

(2.5 – 3.1)

–166

100

Udder composite

1.3

33

42

Feet & legs composite

1.9

15

29

Body size composite

1.0

–12

–12

DPR (%)

–1.5

17

–26

SCE

(7 – 8)

–5

5

DCE

(5 – 8)

–5

15

 

 

NM$ = $646

Calculation of NM$ using a sample Holstein bull is illustrated in Table 2. The example bull’s NM$ is $646. His daughters, on average, would be expected to have $646 more lifetime profit than the daughters of a bull with a NM$ of $0. To calculate NM$, the PTAs for each trait are multiplied by the corresponding economic value and then summed. An average calving ease of 8 percent is first subtracted from the PTAs of both SCE and DCE, and the following breed averages are subtracted from the PTA for somatic cell score (SCS):

 

Ayrshire

2.92

Brown Swiss

3.08

Guernsey

3.21

Holstein

3.10

Jersey

3.31

Milking Shorthorn

3.02

Economic weights . . .

DPR:Evaluations for DPR became available for the first time in February 2003 and allow direct selection for increased fertility. Poor reproduction has been cited as the number one cause for culling cows. Additional costs associated with poor fertility include increased semen required; increased labor and supplies required for heat detection, inseminations, and pregnancy checks; and yield losses because of lactation intervals that are less than optimal. An average loss of about $1.50 per day open was calculated, which converts to $17 per PTA unit of DPR on a lifetime basis and receives a relative weighting of 7 percent in NM$.

DPR value is different for different herds and management situations. The value of getting cows bred on schedule is higher for herds with seasonal calving than for those that calve year round. As of August 2003, 8 of the 12 largest Holstein populations in the world include cow fertility in their national indexes with relative weightings of 1 percent in Germany; 7 percent in the United States and the Netherlands; 8 percent in Australia 9 percent in Denmark 10 percent in New Zealand and Sweden and 13 percent in France

Calving ease: A difficult birth reduces production, delays reproduction, increases incidence of metabolic problems in early lactation, increases likelihood of stillbirths, and may kill the cow. In addition, increased labor and veterinary costs are associated with difficult calvings. Calving difficulty is influenced both by the sire of the calf (SCE) and the sire of the mother (DCE). SCE evaluations have been available since 1978, and DCE evaluations first became available in August 2002. SCE and DCE are each valued at -$5 per PTA unit in NM$, which gives a relative weighting of about 2 percent for each trait and a total relative weighting of 4 percent for calving ease. As of August 2003, only 5 of the 12 countries with large Holstein populations include calving ease or incidences of stillbirths in their national indexes. Relative weighting on calving ease is 3 percent for Germany, 4 percent for the United States, 6 percent for Denmark, 10 percent for The Sweden.

Dairy producers have used SCE evaluations since 1978 to avoid Holstein bulls that are poor for calving ease when breeding heifers, and that practice still makes sense. The value of -$5 per PTA unit of SCE used in NM$ is a weighted average of losses for cows and heifers. The actual calculated loss for heifers is -$9 per PTA unit of SCE, and the value for cows is only -$3 per PTA unit of SCE.

Productive life: The economic value of productive life depends mainly on the price difference in costs of replacements versus the salvage value obtained when selling cull cows. Even though the value for productive life was reduced in the new NM$, increased genetic progress is expected for productive life because the total emphasis on fitness traits is greater.

SCS: Selection for lower SCS leads to higher milk prices in markets where quality premiums are paid. Selecting for lower SCS also reduces clinical mastitis. Costs associated with clinical mastitis include increased labor, antibiotics, and discarded milk as well as a greater chance of accidental antibiotic residue in the bulk tank.

Yield traits: Milk prices vary over time and by use of the milk. A base milk price of $12.70 per hundred-weight was assumed after hauling and promotional charges were deducted. To account for various milk uses, fluid merit (FM$) and cheese merit (CM$) indexes are calculated in addition to NM$. Component prices per pound are shown in Table 3. Subtracting feed costs from the component price and than multiplying by an average of 3 lactations times .89 (ratio of actual production to mature equivalent yield) gives the value per PTA pounds that are used to calculate the lifetime merit indexes.

 

Table 3. Economic values for production traits

Index

Component value

 

Index value

Milk

Fat

Protein

Milk

Fat

Protein

 

----------------------------------- $/lb ------------------------------------

 

------------------------------ $/PTA lb --------------------------

NM$

.012

1.30

2.30

 

.000

2.54

4.81

CM$

-.009

1.30

3.00

 

- .056

2.54

6.68

FM$

.051

1.30

1.00

 

.104

2.54

1.33

Feed cost

.012

.35

.50

 

. . .

. . .

. . .

The majority of producers in the U.S. are paid for milk, fat, and protein such that they should select on NM$. However, producers in fluid markets that receive less than $1.65 per pound of protein will be better off selecting for FM$. Conversely, those producers that receive more than $2.65 per pound of protein should select for CM$.

Type traits: The conformation composites are calculated differently for different breeds as illustrated in Table 4. For Holsteins the composite indexes are published by Holstein Association USA as standardized transmitting abilities with standard deviations for true transmitting abilities equal to 1. The published composite indexes are used directly to calculate NM$. For other breeds, the published PTAs for linear traits are first converted to standardized transmitting abilities by dividing by their standard deviations of true transmitting abilities. Then they are used to compute the composite indexes for NM$ calculations.

 

Table 4. Relative weights for composite traits

Composite and traits

Holstein

Brown Swiss

Jersey and other breeds

Udder

 

 

 

Fore udder

16

21

20

Rear udder height

16

6

18

Rear udder width

12

1

8

Udder cleft

10

2

3

Udder depth

30

35

26

Teat placement

16

11

7

Teat length

. . .

-24

-18

Feet & legs

 

 

 

Rear legs (side view)

-8

-48

-30

Rear legs (rear view)

18

. . .

. . .

Foot angle

24

52

70

Feet and legs score

50

. . .

. . .

Body size

Holstein and other breeds

Jersey

Stature

50

50

Strength

25

40

Body depth

15

. . .

Rump width

10

10

The emphasis placed on udder composite in NM$ is 7 percent, and significant genetic progress is expected over the next 10 years. Better udders are associated with less mastitis, less labor for milking, and greater longevity. Emphasis on the feet-and-legs composite remains at 4 percent, but expected genetic progress per year has increased because of the positive genetic correlation with other fitness traits in the index. Body size composite receives a relative emphasis of –3 percent in NM$ because the cost of increased feed for growth and maintenance exceeds the income from selling heavier cull cows and bull calves. Many dairy producers in the past selected for larger cows, but body size has negative genetic correlations with longevity, fertility, and calving ease in the current U.S. Holstein population. Smaller size is also rewarded by the American Jersey Cattle Association in their Jersey Performance Index and by the American Guernsey Association in their Production Type Index because larger cows don’t last as long.

Expected genetic progress . . .

The expected genetic change for Holsteins is shown in Table 5 for the different traits. The expected genetic change (or change in breeding value) equals twice the expected change in PTA. Thus, multiplication of annual PTA change by 20 gives expected genetic change per decade.Encouragingly, significant genetic progress is expected for most of the traits. These predictions of genetic gain assume that NM$ is the primary selection criteria for the industry.

About 10% less progress per year will be made for the production traits under the revised index than under the former NM$. However, the expected genetic change per decade is still a very respectable +2380 pounds for milk, +96 pounds for fat, and +78 pounds for protein. The slightly lower progress for production is offset by much greater progress for fitness traits.

The expected change in PTA for productive life has increased from +.18 months per year to +.24 months per year even though the relative emphasis on productive life decreased from 14 to 11 percent in NM$ 2003. More progress is expected for productive life because increased selection for DPR and calving ease also results in cows with greater longevity and adds to the genetic trend for productive life. Actually, productive life has a desirable genetic correlation with all the other traits in the index.

 

Table 5. Expected genetic change for Holsteins

Trait

PTA change per year:

 

Breeding value change per decade:

NM$ 2003

NM$ 2000

NM$ 2003

 

Milk (lbs)

135

119

 

 

2380

Fat (lbs)

5.2

4.8

 

96

Protein (lbs)

4.5

3.9

 

78

Productive life (mo)

.18

.24

 

4.8

SCS

–.002

–.022

 

–.44

Udder composite

.002

.07

 

1.4

Feet & legs composite

.015

.05

 

1.0

Body size composite

–.001

–.03

 

–.6

DPR (%)

.00

.05

 

1.0

SCE

–.07

–.13

 

–1.3

DCE

–.04

–.08

 

–1.6

 

SCS is expected to decrease by -.44 over the next 10 years because of the reduced emphasis on production and increased emphasis on udder health traits in the revised index. In other words, the Holstein breed is expected to go from an average SCS score of 3.1 to an average of 2.66, which would be a dramatic decrease in somatic cell count and decrease in mastitis incidence.

Body size is expected to experience a slight genetic decrease of -.6 over the next 10 years. Because one unit of body size composite equals 24 pounds of mature cow weight, the decrease of -.6 corresponds to a decrease of about 14 pounds in average cow size, which is a relatively insignificant genetic change in body weight.

Calving ease is affected jointly by SCE, which accounts for the service sire trend, and DCE, which accounts for only half the expected trend in dam merit. The total progress per decade is the annual SCE gain multiplied by 10 plus the annual DCE gain multiplied by 20: -1.3 + (-1.6) = -2.9 percent change in calving difficulty. In other words, the average calving difficulty should decrease from 8.0 to 5.1 percent for Holsteins Again, this decrease would be a fairly dramatic improvement in just a 10-year period.

Pregnancy rate is expected to reverse some of the past genetic decline and increase by 1 percent, which corresponds to 4 fewer days open on average. Genetic progress for DPR is slower than for other traits because it has a relatively low heritability of only 4 percent and a genetic antagonism with milk production. However, if breeders select on NM$, the trend for DPR should now move in the right direction and result in more fertile cows in the future.