Energy-corrected milk (ECM)

Definition

Energy-corrected milk (ECM) is milk yield adjusted to a standard energy content, so that a pound of high-component milk counts for more than a pound of low-component milk. It puts a Jersey pen and a Holstein pen, or a summer month and a winter month, on the same footing.

The common US ECM formula standardizes to roughly 3.5% fat and 3.2% protein. A cow producing milk at exactly those tests has ECM about equal to her actual milk. Higher tests give ECM above actual milk; lower tests give ECM below it.

Fat-corrected milk (FCM) is the older version: it corrects for fat only, usually to 3.5% or 4.0% fat, and ignores protein.

ECM is the usual denominator for feed efficiency, feed cost per cwt and labor cost per cwt, because it tracks the energy the cow actually exported in milk.

Several different ECM and FCM formulas are in use, and they do not give the same number. Always state which formula a figure uses before comparing it to another herd, a nutritionist’s report or a published benchmark.

Formula

This KB uses the following ECM formula as its primary definition. This exact form is the DHI equation of Orth (1992), the citation peer-reviewed papers usually give for these coefficients; it rests on the milk-energy work of Tyrrell & Reid (1965), and some papers cite Tyrrell & Reid for it directly. All quantities are in the same unit (lb or kg), per cow per day or per pen/herd per day:

ECM = 0.327 × milk + 12.95 × fat + 7.2 × protein

where fat     = milk × fat% / 100     (fat yield)
      protein = milk × protein% / 100 (protein yield)

\mathrm{ECM} = 0.327\,M + 12.95\,F + 7.2\,P

Other common formulas

3.5% fat-corrected milk (the Gaines 4% FCM rescaled to 3.5% fat: 0.4 ÷ 0.925 = 0.4324 and 15 ÷ 0.925 = 16.216):

3.5% FCM = 0.4324 × milk + 16.216 × fat

\mathrm{FCM}_{3.5} = 0.4324\,M + 16.216\,F

4% fat-corrected milk (Gaines 1928, Illinois Agricultural Experiment Station Bulletin 308):

4% FCM = 0.4 × milk + 15 × fat

\mathrm{FCM}_{4.0} = 0.4\,M + 15\,F

Close variants of the primary ECM formula circulate with slightly different coefficients (e.g. 0.323 × milk + 12.82 × fat + 7.13 × protein, which some papers attribute to NRC 2001). They give results within about 1% of each other, but a published figure should name the exact coefficients used.

European work often uses an ECM form that also includes lactose (e.g. Sjaunja et al. 1990), which gives yet another value.

Worked example

A cow gives 100 lb milk at 3.9% fat and 3.1% protein, so fat = 3.9 lb and protein = 3.1 lb.

ECM = 0.327 × 100 + 12.95 × 3.9 + 7.2 × 3.1
    = 32.7 + 50.5 + 22.3
    = 105.5 lb

How to pull it in DC305

DC305 stores test-day milk (MILK) and test-day fat and protein percent for each cow; ECM is computed from them. Depending on version and setup, an ECM or FCM item may already exist, or a herd may have a user-defined calculated item.

PCTF and PCTP are the standard items for fat % and protein % at the last test.

To build it from the test-day items:

LIST ID PEN DIM MILK PCTF PCTP FOR LACT>0 MILK>0

Export the list, compute fat lb, protein lb and ECM per cow, then sum the pounds for pens or the herd (see Common mistakes). If the herd has a calculated ECM item, check its definition in the herd’s private dc305-vocab mapping before quoting it; the formula behind a user-defined item varies by herd.

For herd-level ECM per cow, many operations use shipped milk and shipped component tests (from pickup data) divided by cows milking, rather than DC305 test-day data. Say which source a figure comes from.

Typical range

Herd context: US Holsteins, freestall or dry-lot, typical tests about 3.8–4.2% fat and 3.0–3.3% protein.

ECM ÷ actual milk: about 1.03–1.10 for Holsteins; about 1.20–1.30 for Jerseys (around 4.8–5.0% fat, 3.6–3.8% protein).
Herd ECM per cow per day: roughly 75–90 lb in average-to-good US Holstein herds; high herds exceed 95–100 lb.

Because summer tests are lower, ECM falls more than actual milk in heat stress months.

Common mistakes

Mixing formulas. ECM by one formula and 3.5% FCM by another can differ by several percent for the same milk. Don’t compare across reports without checking.
Plugging percentages into a pounds formula. The formula takes fat and protein yield (lb), not test (%). Using 3.9 for “fat” only works when milk is exactly 100 lb.
Averaging cow ECMs or tests unweighted. Compute pen or herd ECM from summed milk, fat and protein pounds, not from an unweighted average of cow ECMs or of fat %.
Mixing data sources. Test-day (DHI/meter) milk and shipped (creamery) milk don’t tie out. Use one source for the numerator and stay consistent.
Wrong cow denominator. ECM per cow differs if divided by milking cows, lactating cows or all cows including dry. State it.
Treating ECM as income. ECM measures energy, not pay price. Component pricing weights fat and protein by market value, not by energy.