USDA Somatic Cell Score Evaluation (Feb 2005)

Country (or countries)

United States of America

Main trait group

Health - Somatic Cell Score



Trait definition(s) and unit(s) of measurement

Somatic Cell Score = log base 2 (SCC / 100,000) + 3; where SCC is somatic cells per milliliter. Lactation SCS is the average of test day SCS across the first 305 days of lactation.

Method of measuring and collecting data

Collected by Dairy Herd Improvement Associations using ICAR approved methods.

Time period for data inclusion

First calving 1984 and later plus pedigree from birth year 1950 and later.

Age groups (e.g. parities) included

 First five parities are included.

Other criteria (data edits) for inclusion of records

Valid sire ID required. Lactations from cows >40 DIM and cows removed from the herd >15 DIM are included. TD by 90 DIM required.

Criteria for extension of records (if applicable)

Records < 305 d are extended to 305 d by best prediction.

Sire categories

AI and NS included. B&W and R&W HOL evaluated together.

Environmental effects, pre-adjustments

Multiplicative adjustments for age at calving, month of calving, and for differing SD by parity.

Method (model) of genetic evaluation

ST BLUP Repeatability Animal Model

Environmental effects in the genetic evaluation model

Fixed: management group (flexible herd-year-seasons), parity x age, regression on inbreeding, Random: permanent environment, and herd by sire interaction. HOL management group definition includes registry status. Published PTA includes the expected future inbreeding (EFI) multiplied by the regression coefficient as a post-processing step.

Adjustment for heterogeneous variance in evaluation model


Use of genetic groups and relationships

Unknown parents grouped by birth year. For cows, sires and dams grouped separately; for bulls, sires and dams together. [HOL} Canadian animals grouped separately.

Blending of foreign/Interbull information in evaluation

Foreign evaluations of parents are not included.

Genetic parameters in the evaluation

See Page 3.

System validation

Means and SD for all variables are calculated and examined overall. Means for new bulls, changes for high bulls, largest changes, and key statistics for recent AI bulls are checked. Genetic trends for all breeds are validated by methods 1, 2, and 3.

Expression of genetic evaluationsIf standardised (e.g. RBV), give standardisation formula in the appendix


Definition of genetic reference base.
Next base change

A phenotypic mean of 3 is added to PTA SCS. Current base: cows born in 2000 (stepwise, 5 year).
February 2010 (when the base will be cows born in 2005).

Calculation of reliability

Daughter equivalents from parents and from progeny are summed by processing progeny from the youngest to oldest generation and then parents from oldest to youngest. Starting values for mate reliability are from the previous evaluation.

Criteria for official publication of evaluations

At least 10 daughters with usable first lactation record. Interbull evaluations are reported as official in the US if: they include data from an additional country; the US has no evaluation; or Interbull excludes US data and the Interbull evaluation has higher reliability than the US evaluation.

Number of evaluations / publications per year

Four, in February, May, August, and November.

Use in total merit index

SCS receives 9% of the total emphasis for NM.

Anticipated changes in the near future


Key reference on methodology applied

Schutz, M.M 1994. Genetic evaluation of somatic cell scores for United States dairy cattle. J. Dairy Sci. 77:2113.

Schutz, M.M. P.M. Vanraden, G.R. Wiggans, and H.D. Norman. 1995. Standardization of lactation means of somatic cell scores for calculation of genetic evaluations. J. Dairy Sci. 78:1843.

VanRaden, P.M. and Wiggans, G.R. 1991. Derivation, calculation, and use of national animal model information. J. Dairy Sci. 74:2737.

Key organization:

Animal Improvement Programs Laboratory
USDA, Agricultural Research Service
Building 005, Room 306, BARC-West
10300 Baltimore Avenue
Beltsville, Maryland 20705-2350
Tel: 301-504-8334 Fax: 301-504-8092
web site:

Updated: Jan 12 2017 .

Genetic Parameters for Udder Health

Country (or countries):

United States of America

Main trait group:







Genetic variance

Official proof standardization formula

Milk Somatic Cell Score:

Somatic Cell Score = log base 2 (SCC / 100,000) + 3; where SCC is somatic cells per milliliter.


[AYS] .40

StandEval = PTASCS + 3




[BSW] .38

StandEval = PTASCS + 3




[GUE] .43

StandEval = PTASCS + 3




[HOL] .42

StandEval = PTASCS + 3




[JER] .37

StandEval = PTASCS + 3