Protein is an important nutrient in beef cattle diets. Young, rapidly growing animals and high-performing lactating animals often require protein supplementation. Because of the high cost of protein supplementation, optimizing the use of protein in the ration is needed. This article addresses commonly asked questions regarding protein supplements.
What is a protein supplement?
The National Research Council publication entitled Nutrient Requirements of Beef Cattle (2000) defines a protein supplement as a feedstuff with greater than or equal to 20% crude protein (CP). The CP concentration of a feed is determined by multiplying its nitrogen (N) concentration by 6.25. Most proteins contain 16% N (1 ÷ 0.16 = 6.25), leading to the conversion factor of 6.25. Example: 2% N in sample x 6.25 = 12.5% CP.
What are natural protein sources?
Natural protein is true protein found in naturally occurring sources such as plants. True protein means that the protein is derived from amino acids, or the building blocks of protein. The most common commercial sources of plant protein are derived from cottonseed and soybean. Other oilseed meals such as linseed, sunflower, sesame, and rapeseed represent less widely used sources. Corn gluten feed and dried distillers grains represent co-product feeds often utilized as protein supplements (see Table 1 for nutrient composition).
What is non-protein nitrogen?
All proteins contain nitrogen, but not all nitrogen is contained in proteins. Nitrogen-based compounds other than natural protein are commonly referred to as non-protein nitrogen (NPN) sources. Beef cattle have the ability to use natural protein or some NPN to meet their protein requirements. Non-protein nitrogen sources do not contain amino acids, and thus are not considered to be natural (i.e., true) protein. However, rumen microbes have the ability to convert NPN to protein in the presence of an adequate energy source. Common NPN sources include urea and biuret.
How is NPN used by the ruminant animal?
Rumen microbes convert NPN to protein through the production of ammonia. The ammonia that is released from urea is utilized or excreted in one of two ways. The first is in the production of microbial protein, which can then be absorbed and utilized for protein by the animal. Excess ammonia that cannot be used effectively in the production of microbial protein is absorbed as ammonia and goes to the liver, where it is detoxified and excreted in the urine. However, when too much ammonia escapes the rumen because the microbes are not able to utilize enough of it for protein, the capacity of the liver for excretion can be overwhelmed and toxicity can occur.
In order for ruminants to effectively convert an NPN source into usable protein, there must be an adequate quantity of a readily available energy source provided in the diet at the same time. However, most forage-based diets do not contain an especially great amount of rapidly degradable energy, thereby limiting the amount of NPN that can be used effectively and safely from the diet.
Special Considerations When Using NPN
Caution should be used when feeding urea to prevent toxicity issues. Urea is converted to ammonia in the rumen, which is either used by bacteria to produce protein or enters the bloodstream directly. Rumen bacteria require an adequate energy source in the diet to be able to effectively use urea. If too much urea is consumed at once, or energy is limited in the system, ammonia may enter the bloodstream at levels too high to be processed by the liver. This may quickly lead to ammonia toxicity or death.
Follow label directions for supplements containing urea very carefully. Do not give hungry cattle access to NPN-based supplements to prevent overconsumption and potential toxicity issues. Allow cattle access to ample quantities of forage prior to feeding these supplements. Rumen microbes need time to acclimate to urea. Allowing cattle to consume too much urea too quickly, without allowing adequate time for rumen microbes to acclimate, can also quickly lead to ammonia toxicity or death.
Does NPN increase forage intake?
Urea is essentially 100% degradable intake protein (DIP), or protein that is available to rumen microbes for growth and digestive processes. Supplementing forages with DIP sources has been shown to increase forage digestion and intake of low-quality forage (less than 8% CP and 52% TDN). By increasing forage intake, the available energy in the diet also increases. However, there is a limit to the amount of DIP from NPN in the diet that can be used to replace natural, plant-based protein. It is recommended that no more than 15% of the total CP in the diet should be from an NPN source for this reason. Anything above this percentage is not utilized effectively by the animal.
Thus, NPN is not appropriate for feeding with low-quality forages commonly found in the Southeast because 1) energy is limited for microbial digestion and 2) increasing intake of low-quality forage may lead to nutrient deficiencies and/or potentially digestive tract compaction in cattle.
The feed analysis tag lists a value for % CP and % NPN. What does this mean?
Many commercial supplements will report the total CP concentration of the supplement and the percentage coming from NPN. For example, a 30% protein supplement may contain 15% CP (i.e., half the total) from natural sources and 15% CP (the other half) from NPN.
The decision to provide supplemental nutrients should be based off of the nutrient requirements of the class of livestock being fed. Refer to ANR-0060 Nutrient Requirements of Beef Cattle for more information.