In ruminant animals a considerable amount of degradation and synthesis of protein occurs in the rumen and the material that becomes available for digestion can differ considerably from what was originally present in the food. Consequently, different approaches for evaluating protein sources are necessary for ruminant and non-ruminant animals.
Ruminants have protein requirements at two levels, with one being the nitrogen (N) requirements of rumen microbes and the other is the protein needs of the host. An important feature of ruminant animals is having rumen microbes that are able to synthesise protein using nitrogen in the diet as well as having the ability to synthesise protein from non-protein nitrogen (NPN) sources.
The protein percentage of any feed is calculated by determining the amount of nitrogen (N) in that feed and multiplying it by 6.25 (protein generally contains 16% nitrogen hence 100/16 = 6.25).
- True Protein– is the nitrogen (N) associated with ‘natural protein’ (from protein meals, pasture, grains etc.).
- g. Soybean Meal at 7.2% N x 6.25 = 45% protein.
- Non-Protein Nitrogen (NPN)– is a term used in animal nutrition to refer to ingredients such as urea and ammonia sulphate, which are not proteins but can be converted into proteins by rumen microorganisms.
- g. Urea with 46% N x 6.25 = 287% protein.
- g. Ammonium Sulphate with 20.2% N x 6.25 = 126% protein.
- Rumen Degradable Protein (RDP)– is part of the feed protein which is degraded in the rumen and it can either be rapidly degraded (urea) or slowly degraded (cottonseed meal and copra meal).
- Microbial Crude Protein (MCP)– the microorganisms in the rumen use ammonia to build their own bodies (synthesis of microbial crude protein). The microbes are then flushed from the rumen and the animal digests the MCP in the abomasum. The resultant amino acids are then absorbed in the small intestine.
- Undegradable Dietary Protein (UDP)– is protein which escapes or ‘bypasses’ the rumen fermentation and gets digested in the small intestine to supply amino acids directly to the animal. Bypass protein is extremely important for high producing ruminants (lactating and rapidly growing young stock) as their protein requirements are above what can be provided by MCP alone.
The table below shows the protein composition of some common feed ingredients on an as fed basis. (Note: protein percentage of ‘natural’ protein will vary depending on crop quality and the season.)
| Product | Crude Protein % | Bypass protein % |
| Cottonseed Meal | 37 | 43 |
| Copra Meal | 21 | 56 |
| Soybean Meal | 45 | 35 |
| Whole Cottonseed | 21 | 20 |
| Barley | 12 | 27 |
| Sorghum | 10 | 57 |
| Molasses | 4.3 | Nil |
| Urea | 287 | Nil |
| Ammonium Sulphate | 126 | Nil |
Let’s look at some common information found on the labels of ruminant feeds a little closer to help understand the product that is being purchased.
Label Example
| Ingredient | Label 1 |
| Crude Protein % | 34 |
| Equivalent Crude Protein % | 21 |
| Urea % | 6 |
| Ammonium Sulphate % | 3 |
| Salt % | 5 |
| Calcium (g/kg) | 36 |
| Phosphorus (g/kg) | 15 |
| Sulphur (g/kg) | 20 |
What do these items listed on the label mean?
- Crude Protein (CP)– is used to indicate the total amount of protein in a feed and it includes true protein and NPN. CP is sometimes listed on labels as total protein. In the above example the CP is 34% and the ECP is 21% so this tells us that 21% of the protein is derived from NPN sources and 13% from true protein.
- Crude Protein – Equivalent Crude Protein = True Protein
- 34 – 21 = 13
- Equivalent Crude Protein (ECP)– measures the proportion of crude protein that originated from sources of NPN such as urea. In the example above the urea and ammonium sulphate are added together to get the ECP value.
- Urea at 6 x 287 / 100 = 17.22
- Ammonium Sulphate at 3 x 126 / 100 = 3.78
- 17.22 + 3.78 = 21% ECP
- Urea– the percentage of urea found in the supplement.
- Ammonium Sulphate– the percentage of ammonium sulphate found in the supplement.
- Salt– The minerals sodium (Na) and chloride (Cl) that form salt are crucial electrolyte minerals required by the animal for a number of metabolic functions. Salt is also used in supplements to regulate intake. The above example contains 5% salt and if intake needs to be reduced more salt can be added to help decrease palatability.
- Calcium (Ca)– is often added to supplements and is a structural component of bone. It is important for nerve and muscle function, blood clotting, hormone and enzyme action. Ca is inter-related to phosphorus status.
- Phosphorus (P) – is a structural component of bone and is very important for growth, feed intake, fertility and milk production.Adequate phosphorus supplementation is crucial for phosphorus deficient country and care must be taken to keep the calcium to phosphorus ratio to a minimum of 2 parts Ca to 1 part P.
- Sulphur (S)– The rumen microorganisms use S in combination with N in the production of a number of amino acids.Protein supplements should contain a sulphur to nitrogen ratio of around 1 part S to 10 parts N.
- General units of measure – labels can list different units of measure for components, here are some conversions to enable effective comparisons:
- E.g. 36g/kg to % = 36g/1000g x 100 = 3.6%
- E.g. mg/kg = ppm and are often used interchangeably – 20mg/kg = 20ppm
Summary
Successful feeding of protein supplements can increase the digestion and performance of stock grazing low protein and high fibre diets. Supplements that contain both NPN such as urea and protein meal like copra or cottonseed are preferable as the urea provides a quick boost to rumen microbes, while the protein from the meal is slowly degraded in the rumen and also supplies the animal with bypass protein. This combination enhances the rate of digestion, stimulates appetite and allows the animal to effectively graze and digest dry pastures for longer.
For supplementation to be successful it is important to start feeding animals before they start to lose condition and an adequate supply of dry feed (and water) must be available to stock at all times.
By Shannon Godwin (BAppSC GDTL)