Understanding the Guaranteed Analysis
Every animal feed product, from pet food to livestock feed, is required by regulatory bodies like the AAFCO to display a 'Guaranteed Analysis' on its label. This section provides minimum and maximum percentages for key nutrients. For protein, you will find a figure for 'Crude Protein' (CP), which represents the minimum guaranteed percentage of protein in the product.
What "Crude Protein" Really Means
The term 'crude' doesn't relate to the quality of the protein, but rather the method used to measure it. It is a chemical estimation of the total protein content based on the amount of nitrogen present in the feed. This is because protein is approximately 16% nitrogen, so labs multiply the nitrogen content by a constant factor of 6.25 to estimate the total crude protein.
Key components of crude protein include:
- True Protein: The actual protein composed of amino acid chains that can be directly utilized by the animal.
- Non-Protein Nitrogen (NPN): Nitrogen from other sources, like urea or ammonia, which is not true protein but is still measured in the chemical analysis. For ruminants, NPN can be used by rumen microbes to synthesize microbial protein. For non-ruminants, however, NPN is not readily utilized and offers little nutritional benefit. This is a key reason why 'crude protein' is a less precise measure than it might seem.
The Scientific Measurement: Kjeldahl and Dumas
Historically, the Kjeldahl method was the standard for determining nitrogen content. It involves digesting a feed sample with sulfuric acid, which converts organic nitrogen into ammonium sulfate. The amount of nitrogen is then quantified and converted to crude protein. Today, the faster and more automated Dumas method is also widely used, burning the sample at high temperatures to release nitrogen gas for measurement. Both methods produce the 'crude protein' value you see on the label.
As-Fed vs. Dry Matter Basis: A Critical Comparison
One of the biggest challenges when comparing feed labels is the variable moisture content between products, particularly between dry and wet foods. The percentages listed in the Guaranteed Analysis are on an 'as-fed' basis, meaning they include the moisture. To make a true, 'apples-to-apples' comparison, you must convert the protein percentage to a 'dry matter basis' by removing the moisture component.
Calculating Protein on a Dry Matter Basis
To convert a protein percentage from an as-fed to a dry matter basis, follow these steps:
- Find the moisture content: Look for the maximum moisture percentage listed in the Guaranteed Analysis. If not listed, it's typically around 10% for dry food and 75% for wet food.
- Calculate the dry matter percentage: Subtract the moisture percentage from 100%.
- Calculate the protein on a dry matter basis: Divide the crude protein percentage (as-fed) by the dry matter percentage, then multiply by 100.
For example:
- Dry Kibble: 25% Crude Protein, 10% Moisture. Dry Matter = 90%. Dry Matter Protein = (25 / 90) * 100 = 27.8%.
- Canned Food: 8% Crude Protein, 75% Moisture. Dry Matter = 25%. Dry Matter Protein = (8 / 25) * 100 = 32%.
Comparison Table: Wet vs. Dry Feed Protein
This table illustrates why converting to a dry matter basis is crucial for accurate comparisons.
| Feature | Dry Kibble | Canned/Wet Food |
|---|---|---|
| Guaranteed Analysis (As-Fed) | Appears higher (e.g., 25%) | Appears lower (e.g., 8%) |
| Moisture Content | Low (approx. 10%) | High (approx. 75%) |
| Protein (Dry Matter Basis) | Lower than As-Fed (e.g., 27.8%) | Higher than As-Fed (e.g., 32%) |
| Calculation | Divisor is approx. 0.9 | Divisor is approx. 0.25 |
Beyond the Numbers: Assessing Protein Quality
While crude protein gives you a quantity, it doesn't indicate the quality or digestibility of the protein. A feed's amino acid profile is a better indicator of protein quality.
The Importance of the Ingredient List
The ingredient list, ordered by weight, can provide clues about the protein quality. Ingredients high on the list indicate a higher proportion in the formula. Look for named animal-based protein sources like 'chicken meal' or 'deboned lamb,' as these typically provide a higher-quality, more digestible protein than non-specific terms like 'meat and bone meal' or 'animal protein products'. Similarly, high-quality plant-based protein sources like 'soybean meal' can be excellent, particularly for livestock. Some advanced feed labels will list guaranteed minimum percentages for key amino acids, such as lysine, which is especially important for growth and repair.
Conclusion: Making an Informed Choice
Reading a feed label for protein goes beyond simply looking at the 'Crude Protein' percentage. By understanding that this is a minimum value measured by total nitrogen, you can interpret the information more accurately. Learning to convert the 'as-fed' protein to a 'dry matter basis' is essential for comparing different feed types. Finally, by scrutinizing the ingredient list for high-quality protein sources and checking for specific amino acid guarantees, you can make a truly informed decision about the nutritional value of the feed you provide for your animals. For more detail, you can consult resources like this informative feed tag guide from Cornell University.
