Topics:
Animal Nutrition, Animal Refresher
Assumptions of Crude Protein Calculation
Which of the following assumption(s) is/are true for the calculation of crude protein?
- All feed protein contains 16% Nitrogen
- All Nitrogen of food comes from true protein
- Both a & b — Correct Answer
- All Nitrogen comes from crude protein
Explanation:
Correct Answer: Both a & b
Crude Protein calculation by the Kjeldahl method requires two simultaneous assumptions: (a) All food protein contains 16% Nitrogen — hence the conversion factor 100/16 = 6.25, and (b) All Nitrogen in food comes from true protein — ignoring NPN sources. Both assumptions must be accepted for the CP formula: CP = N% × 6.25.
Weende Proximate Analysis — CP Estimation
| Step | Detail |
|---|---|
| Method | Kjeldahl method (N estimation) |
| Assumption 1 | All protein contains 16% N → factor = 100/16 = 6.25 |
| Assumption 2 | All N in feed is from true protein (ignores NPN) |
| Formula | CP% = N% × 6.25 |
| Limitation | Overestimates protein when NPN (urea, amides) is present |
Why Both Assumptions Are Needed
- Without Assumption 1, we cannot convert % N to % protein (need a multiplier).
- Without Assumption 2, the method would require separating NPN from true protein — which Kjeldahl does not do.
- This is why Crude Protein = True Protein + Non-Protein Nitrogen × 6.25.
Why Other Options Are Wrong
- Only assumption (a) → incomplete; assumption (b) is equally essential.
- Only assumption (b) → incomplete; assumption (a) is required for the 6.25 factor.
- All N comes from crude protein → circular definition; NPN IS part of crude protein — but that is a result, not an assumption.
