What is Crude Protein?
Crude protein (CP) is a measure of a feed's total nitrogen content. This is based on the fact that protein is the primary nitrogen-containing compound in most food sources, containing roughly 16% nitrogen. Scientists measure the nitrogen level and multiply it by a conversion factor, typically 6.25, to arrive at the CP value. The most common method used is the Kjeldahl procedure. However, the Kjeldahl method doesn't differentiate between true protein (amino acids) and non-protein nitrogen (NPN), such as urea. This limitation means that CP can overstate the amount of protein truly available to an animal, especially in feeds with significant NPN.
What is Digestible Protein?
In contrast, digestible protein (DP) is the amount of protein an animal can actually break down, absorb, and use. This offers a more accurate assessment of a feed's nutritional quality compared to CP. DP isn't a single value and can be assessed in different ways:
- Apparent Digestibility: This is a basic calculation of protein intake minus the protein found in feces. It gives a general idea but is less precise as it doesn't account for protein from the animal's own body (endogenous protein) that ends up in the feces.
- True Digestibility: This method provides a more accurate measure by correcting for endogenous nitrogen losses. Determining true digestibility is more complex and may involve special techniques or feeding trials with protein-free diets.
The protein source, its amino acid makeup, and how the feed is processed (like heat treatment) can all affect digestibility.
The Digestive Process and Protein Availability
Different animals process protein differently. Ruminants (like cattle) use microbes in the rumen to break down some protein (rumen degradable protein), which the microbes then use to produce microbial protein – a key amino acid source for the animal. Some protein bypasses the rumen and is digested in the small intestine (rumen undegradable protein). Monogastric animals (like pigs and chickens) digest protein mostly through enzymes in their stomach and small intestine.
Key Differences Between Crude and Digestible Protein
Understanding the difference is critical for creating effective diets, particularly in commercial livestock farming. For instance, animals with high production needs may require more digestible protein than microbial protein alone can provide, necessitating supplemental bypass proteins.
| Feature | Crude Protein (CP) | Digestible Protein (DP) |
|---|---|---|
| Definition | Total nitrogen content in feed. | Protein absorbed and utilized by the body. |
| Measurement Basis | Total nitrogen content, calculated by methods like Kjeldahl. | Digestion trials comparing protein intake to fecal output. |
| Accuracy | Less accurate; can overestimate usable protein due to NPN. | More accurate and relevant to nutritional value. |
| Inclusion of NPN | Includes NPN as if it were true protein. | Does not include NPN in the absorbed fraction. |
| Influencing Factors | Primarily a chemical value of the feed. | Affected by animal type, age, health, and feed processing. |
| Resulting Value | Equal to or higher than DP. | Equal to or lower than CP. |
Conclusion
The distinction between crude protein and digestible protein is fundamental for accurate feed evaluation. CP gives a basic estimate based on nitrogen, while DP provides a much more meaningful measure of the protein an animal can actually use. Grasping this difference allows for more precise diet formulations that meet specific needs, leading to better animal health, growth, and performance. Focusing on DP is essential for scientifically sound and cost-effective feeding strategies.
The Importance of Advanced Protein Evaluation
Utilizing protein effectively goes beyond simple digestion, considering factors like essential amino acid balance and the presence of anti-nutritional compounds. The industry is advancing with methods like the Protein Digestibility Corrected Amino Acid Score (PDCAAS), which evaluates protein quality based on both amino acid profile and digestibility. Techniques such as ileal digestibility measurements offer an even more precise assessment of protein availability by measuring absorption at the end of the small intestine. These advanced evaluations emphasize that high CP doesn't equate to high nutritional quality; the protein must be digestible and provide the necessary amino acids to be truly valuable. Further research on protein digestibility is available from sources such as ScienceDirect.
