Somatic cell count (SCC)

Definition

Somatic cell count (SCC) is the number of cells per milliliter of milk, mostly white blood cells that move into the udder in response to infection. It is the standard measure of subclinical mastitis and of milk quality. Results are reported in cells/mL, often written in thousands (e.g. “180” = 180,000 cells/mL).

There are two different SCC measures, and they answer different questions.

Bulk tank SCC

Bulk tank SCC (BTSCC) is measured on a sample of each milk pickup. It reflects the whole shipped milk supply, and it is what regulators and milk buyers grade on. A herd’s BTSCC for a period is the volume-weighted average of its pickups, because a large load contributes more cells to the supply than a small one. A single pickup result is never “the herd’s SCC”.

Individual cow SCC and linear score

Individual cow SCC comes from monthly DHI test-day samples (or in-line sensors). Because cow SCC is highly skewed, it is usually converted to a linear score (LS), a log scale on which each 1-point rise doubles SCC. LS 4 corresponds to 200,000 cells/mL. Cow-level data is used to find infected cows and to measure infection dynamics; it is not a substitute for bulk tank SCC.

Regulatory limits

US (PMO, Grade “A” raw cow milk for pasteurization): individual producer limit 750,000 cells/mL (goat milk 1,500,000). Enforcement is based on repeated samples taken on separate days, not one result: when 2 of the last 4 counts exceed the standard the regulatory agency sends a written notice, and when 3 of the last 5 exceed it the permit is suspended (PMO 2025 Revision, Sections 6 and 7, as of November 2026). States adopt each PMO revision on their own schedule and can set a stricter state limit.
EU: raw cow milk ≤ 400,000 cells/mL, as a rolling geometric average over three months with at least one sample per month (Regulation (EC) 853/2004, Annex III, Section IX). US milk used in dairy products certified for export to the EU has to meet this at the farm level under the USDA AMS EU health certification program.

Premium thresholds

Most milk buyers pay a quality premium below set BTSCC tiers, and some deduct above them. Tiers are buyer-specific; common breakpoints are 100,000, 150,000, 200,000 and 250,000 cells/mL, often combined with a bacteria count (SPC) requirement. See the milk check for how these appear on settlement.

New infection rate and chronic rate

Test-day cow SCC is used to classify each cow by comparing her current test with her previous test, using a cutoff of 200,000 cells/mL (LS 4):

Previous test	This test	Class
≤ 200,000	≤ 200,000	Healthy
≤ 200,000	> 200,000	New infection
> 200,000	≤ 200,000	Cured
> 200,000	> 200,000	Chronic

Fresh cows have no previous test in the current lactation. They are reported separately as the fresh-cow rate (first test over 200,000), or compared with their last test of the previous lactation to measure dry-period cures and dry-period new infections.

Formula

Volume-weighted bulk tank SCC over a period (each pickup i with milk weight w and SCC s):

BTSCC (cells/mL) = Σ (s_i × w_i) / Σ w_i

\mathrm{BTSCC} = \frac{\sum_i s_i\,w_i}{\sum_i w_i}

Linear score (SCC in cells/mL):

LS = log2(SCC / 100,000) + 3

\mathrm{LS} = \log_2\!\left(\frac{\mathrm{SCC}}{100{,}000}\right) + 3

This is the USDA somatic cell score definition (USDA AIPL).

LS	SCC (cells/mL)
2	50,000
3	100,000
4	200,000
5	400,000
6	800,000

Test-day rates (cows tested on both tests, cutoff 200,000):

New infection rate (%) = cows ≤200k last test and >200k this test
                         / cows ≤200k last test × 100
Chronic rate (%)       = cows >200k on both tests / cows tested on both × 100
Prevalence (%)         = cows >200k this test / cows tested × 100
Fresh-cow rate (%)     = fresh cows >200k at first test / fresh cows tested × 100

\mathrm{NIR} = \frac{N_{\le 200 \rightarrow > 200}}{N_{\le 200,\ \text{prev}}} \times 100

How to pull it in DC305

DC305 holds test-day cow SCC from DHI or meter data. It does not hold bulk tank SCC; that comes from pickup data (see Bulk tank pickup data).

Common items are SCC (test-day SCC, usually in thousands) and LGSCC (linear score).

Related items in common setups: PRVLG (previous-test LS), PSCC (previous-test SCC), LOG1 (first-test LS), SCC1 (first-test SCC), DRYLG (LS at dry-off) and MAXLG (highest LS).

LIST ID PEN DIM SCC LGSCC FOR LACT>0 LGSCC>4      cows over 200k
SUM LGSCC BY PEN FOR LACT>0                       average LS by pen
COUNT FOR LACT>0 LGSCC>4                          prevalence numerator
PCT SCC>0 SCC<300 BY LCTGP FOR LACT>0             % of tested cows under 300k, by lactation group
SUM SCC=200 PSCC=200 FOR SCC>0 PSCC>0             2×2 table: this test vs previous test at 200k
SUM LOG1=4 DRYLG=4 FOR LACT>1 LOG1>0 DRYLG>0      2×2 table: LS at dry-off vs first test (dry period)
ECON\S                                            bulk tank SCC report

The 2×2 SUM tables give new infections (low → high), chronic (high → high) and cures (high → low) between two tests.

DC305 also has udder-health summary reports (new infection, chronic, cured by test date).

Mastitis treatment events and user-defined SCC items vary by herd; check the herd’s private dc305-vocab mapping before pulling them.

Typical range

Herd context: US Holsteins, freestall or dry-lot, monthly DHI testing.

Measure	Typical	Good
Bulk tank SCC	150,000–250,000	< 150,000
% cows > 200,000 (prevalence)	15–25%	< 15%
Monthly new infection rate	8–12%	< 5–8%
Chronic rate	5–15%	< 5%
Fresh-cow rate (first test > 200k)	10–20%	< 10%
Average LS	2.5–3.5	< 2.5

US national average SCC has fallen over the past two decades and in recent years has been under 200,000 cells/mL.

Seasonality: SCC usually peaks in summer (heat, humidity, environmental bacteria) and is lowest in winter.

Common mistakes

Averaging pickup SCCs. A simple average of single-pickup results over-weights small loads. Use the volume-weighted formula, and never report one pickup as the herd’s SCC.
Averaging cow SCC instead of LS. Raw cow SCC is skewed by a few very high cows. Average LS, or report % of cows over 200,000.
Treating test-day cow SCC as bulk tank SCC. Cow-weighted DHI averages ignore milk volume and include cows whose milk is withheld; they will not match the tank.
Ignoring withheld milk. Treated and fresh cows’ milk is not in the tank, so BTSCC can look good while cow-level infection is high.
Mixing units. Some systems report SCC in thousands, others in cells/mL. Check before applying a threshold.
Computing new infection rate across a gap. If a cow missed a test, her “previous test” is not the immediately preceding one; exclude or flag her.
Judging regulatory status on one sample. PMO and EU limits use repeated samples or a rolling geometric mean.