Animal nutrition · the basics
Beyond Guaranteed Analysis
A label tells you useful things about composition. It takes more evidence to know what an animal can digest, absorb, and use.
For poultry, horses, cattle, sheep, and goats. Other species need separate guidance; examples are labeled by species.
In this guide
A guarantee is useful. It is not a complete feeding test.
Read “minimum” and “maximum” exactly as printed. A minimum crude-protein guarantee is a lower bound, not an exact sample result. Typical livestock labels include protein and fat minima and a fiber maximum, but guarantees vary with product and purpose. Use the intended species, directions, and cautions too. [1]
What proximate values actually describe
Classical proximate analysis divides feed into moisture, crude protein, ether extract, crude fiber, ash, and nitrogen-free extract. These are method-defined fractions. They do not identify every molecule or reproduce digestion inside an animal. [2]
Crude protein is estimated from nitrogen. Non-protein nitrogen can contribute, and the calculation does not identify amino-acid balance. A CP percentage is therefore not a digestible-amino-acid result. [3]
Ether extract includes solvent-soluble material, not a test of fat quality. Crude fiber misses some plant fiber. Ash combines inorganic residue rather than identifying individual minerals. Nitrogen-free extract is calculated by subtraction and carries errors from the other measurements; it is not a direct starch assay. [2]
Content, digestion, absorption, and use
Digestibility studies add evidence beyond composition. Apparent total-tract digestion measures intake minus fecal output, which includes the animal’s own cells and secretions. Species, age, health, processing, and measurement site matter. In monogastric animals, ileal amino-acid measurements better describe supply before hindgut microbial changes than fecal disappearance does. [4]
For poultry, phytate-bound phosphorus demonstrates the distinction: phosphorus can be present in feed without being fully available. Digestive enzymes and mineral interactions affect access. “Bioavailable” asks whether a nutrient can be absorbed and used for the relevant body function, not merely whether it exists in the bag. [5]
The same protein content can hide a difference
- Invented Feed A
- Analyzed CP: 18 g per 100 g as-fed
Assumed total-tract apparent CP disappearance: 90% - 18 × 0.90 = 16.2 g
- Invented Feed B
- Analyzed CP: 18 g per 100 g as-fed
Assumed total-tract apparent CP disappearance: 60% - 18 × 0.60 = 10.8 g
These calculated amounts describe the assumed apparent CP disappearance only. They are not usable protein, digestible amino acids, or bioavailability. They do not rank real feeds or predict any species’ performance. The starting values are invented analyzed contents, not bag-label minima.
Same starting content
Read the analyzed-content line in each feed example.
Different assumptions
Read the assumed-disappearance line in each feed example.
Different calculated amounts
Read each equation using its own assumption.
Apparent disappearance is not the same as absorbed dietary protein. Endogenous material affects the result, and microbial changes complicate interpretation in fermenting species. The example shows why an analyzed content alone cannot supply a digestion figure. [4]
Ask for the next piece of evidence
Compare analyses on the same moisture basis: as-fed includes water, while dry matter excludes it. Use a representative forage or feed sample. Chemical composition helps plan and troubleshoot a ration; intake and animal response are also part of evaluating feeding value. [6]
Ask about the assay method, the intended species and stage, amino-acid or mineral information where relevant, digestibility evidence, and the basis of predicted energy values. Keep the guarantees as useful screening information, then evaluate them within the whole feeding plan.
For the label’s practical instructions, read How to Read Your Feed Tag. For protein quality, see the limiting-amino-acid barrel.
Sources
Original Tucker Milling guide, informed by these primary sources. The cited institutions do not endorse Tucker Milling. Follow the current product label.
-
Feed Inspector’s Manual: labeling ↩
Model guidance; state rules vary. Accessed October 8, 2026.
-
Accessed October 8, 2026.
-
Procedures Used by OSU Soil, Water and Forage Analytical Laboratory ↩
Accessed October 8, 2026.
-
Measurement of Feed and Nutrient Utilization ↩
Accessed October 8, 2026.
-
Interpreting Forage and Feed Analysis Reports ↩
Accessed October 8, 2026.