Energy on the report is a model output

Routine NEL and TDN values are usually calculated from chemistry and assumptions, not collected as direct substances from the sample.

4 min read · 2026-09-17

Energy predictions depend on equations, feed classification, digestibility assumptions, and sometimes intake level. Two laboratories can begin with similar chemistry and print different NEL values because their models differ. The energy line is a transparent estimate, not a vial of calories poured out of the sample.

NASEM and extension report guides treat TDN and NEL as calculated from other results. When energy changes unexpectedly, compare the underlying nutrients and the equation before blaming the crop. Keep the same laboratory and model for trend work whenever possible. Treat reported energy as a model output — not an independent laboratory fact.

Check in the barn

What to check: Did DM, NDF, NDFD, fat, or ash move in a way that would drag the energy equation? Did the laboratory or the equation change between reports? Are you comparing NEL/lb with NEL/kg? Ask for the inputs before you rewrite the diet on the energy line alone.

Sister notes on this universe cover nfc-is-not-starch, never-drop-the-digestibility-clock, and a-number-without-a-unit-is-not-a-result. Those are the usual reasons two energy numbers refuse to match. Educational only — not a diagnosis, prescription, or ration formula.

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A number without a unit is not a result

Percent, ppm, mg/kg, Mcal/lb, and Mcal/kg can turn the same digits into very different decisions.

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Home plate: Rumen · Reading carbohydrates & energy · TDN

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Sources

Nutrient Requirements of Dairy Cattle, 8th revised edition — NASEM (2021)
Understanding and Using a Feed Analysis Report — University of Nebraska–Lincoln Extension
Reading a Forage Analysis Report — University of Kentucky Extension

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