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TIMELY INFORMATION
ANIMAL SCIENCE SERIES
SERIES
Protein Supplementation: Natural vs. Non-Protein Nitrogen Sources
This Timely Information Sheet provides information on protein
supplementation in beef cattle operations.
Protein is an important nutrient in beef cattle diets. Young, rapidly growing animals and highperforming 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 publication
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. Nonprotein 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.
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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.
Final thoughts?
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.
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Table 1. Nutrient composition of plant-derived protein supplements.
Dry matter,
%
Crude protein,
%
Total
digestible
nutrients, %
Calcium,
%
Phosphorus,
%
Corn gluten
feed
90
21
80
0.11
0.84
Corn gluten
meal, 41%
90
46
85
0.13
0.55
Cottonseed
meal, 44%
90
44
78
0.22
1.23
90
28
87
0.10
0.84
90
41
72
0.74
1.14
90
49
84
0.28
0.71
92
40
65
0.44
0.97
23
26
85
0.30
0.58
90
25
95
0.20
0.64
90
42
91
0.27
0.30
Feed Type
Dried
distillers
grains with
solubles
Rapeseed
meal
(extracted)
Soybean
meal, 49%
Sunflower
meal,
extracted
Wet Brewer’s
Grains
Whole
cottonseed
Whole
soybeans
†Source: NRC Beef Cattle (2000); Preston (2015) Feed Composition Tables
‡These are general estimate of feed nutritional value. A feed analysis should be conducted to obtain an accurate assessment for the given feeding
situation.
By: Kim Mullenix, Ph.D., Extension Beef Cattle Systems Specialist, and Soren Rodning, DVM, Extension
Veterinarian, Department of Animal Sciences, Auburn University, Auburn, AL. November 2016.
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