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Nonprotein Nitrogen Compounds

Nonprotein nitrogen compounds (NPN) are a group of high-protein feedstuffs. NPN feeds are feeds which contain nitrogen in a form other than peptides or proteins. NPN compounds are classified as organic or inorganic NPN compounds. Organic compounds contain carbon and include such compounds as urea, amides, amines, amino acids, and peptides.

Figure 7.7 illustrates the chemical structure of urea.

Inorganic NPN compounds do not contain carbon and include inorganic salts such as ammonium chloride, ammonium phosphate, and ammonium sulfate.

Although a number of feedstuffs contain NPN compounds, in the feed trade NPN compounds refer to added NPN compounds. NPN compounds are fed exclusively to mature ruminants. NPN compounds are not fed to poultry, swine, or horses. The one exception is amino acids are fed to all species.

Mature ruminants are the only class and species of livestock animal that can efficiently use the NPN compounds. The following is a brief description of the metabolism of NPN compounds. In the rumen, NPN compounds are metabolized to ammonia by the rumen microorganisms. Then, further metabolism combines ammonia with carbon chains to form amino acids. Finally, the rumen microorganisms metabolize and incorporate the amino acids into rumen microbial protein. As the rumen contents turnover, rumen microorganisms, including the microbial protein, pass onto the omasum, are digested in the abomasum and small intestine, are absorbed in the small intestine, and are subsequently metabolized in the tissues of the ruminant.

The efficiency of metabolism of NPN compounds to microbial crude protein is determined by factors such as type and number of microorganisms in the rumen and the presence of other required substrates such as readily available carbohydrates, minerals, and vitamins. The primary two dietary substrates for microbial crude protein synthesis are a source of nitrogen, i.e. ammonia, and a source of energy, i.e. readily available carbohydrates. To maximize efficiency of microbial crude protein synthesis, the amount and availability of substrates must be synchronized. Ruminants fed a source of readily available carbohydrates, such as grains, perform best on NPN compounds. Animals fed roughages with low amounts of readily available carbohydrates will perform better on true proteins compared to NPN compounds.

The primary NPN compound fed is urea.

Figure 7.8 illustrates feed urea.

The crude protein content of feed grade urea is 281%. Recall, crude protein is determined by multiplying nitrogen content by 6.25. The nitrogen content of urea is 45%; therefore, 45 times 6.25 equals 281%. Additional NPN compounds fed at a lesser extent are biuret and ammonium phosphate. The majority of the discussion of NPN compounds will focus on urea. The information may also be applicable to other NPN compounds.

The following section will list basic management strategies to optimize the use of urea. A number of these points also apply to additional NPN compounds.

1. Feed only to ruminants.
2. Supply adequate carbohydrates. Carbohydrates provide both energy for the microorganisms and a carbon skeleton for the amino acids.
3. Provide a balanced diet.
4. Mix feed well.
5. Do not feed excessive levels of urea. Urea should be limited to one-third of the total protein in the ration and to 2% in complete feeds.
6. Feed additional protein sources to young, growing animals and high-producing mature animals.
7. Feed small amounts frequently, rather than large amounts infrequently.
8. Provide a transition period for the animal to adapt to urea.
9. Provide adequate water.

Urea may be delivered to the ruminant via a number of methods including being mixed in concentrate feeds, added to the liquid supplement, added to the salt block, combined with silage, added to dry roughages, or as a slow-release product. Slow-release urea products are designed to maximize the crude protein synthesis and reduce the opportunity for urea toxicity. Method of delivery will be influenced by protein requirements of the animal, availability of urea, opportunity for toxicity, cost, availability of energy, and other protein sources in the ration.

As alluded to, urea toxicity is an important issue to address when feeding nonprotein nitrogen compounds. Technically, urea toxicity is ammonia toxicity. Typically, urea toxicity occurs following over consumption of urea in a short period of time. The initial step in urea toxicity is the amount of ammonia available in the rumen significantly exceeds the amount of ammonia able to be metabolized by the rumen microorganisms. Following this occurrence, excess ammonia will be absorbed across the rumen wall and transported to the liver. The liver has the ability to detoxify the ammonia; converting the ammonia to urea. Urea can then be safely recycled in the ruminant’s body or excreted via the urine. In the case of urea toxicity, the amount of ammonia to the liver exceeds the liver’s capacity to convert the ammonia to urea and the excess ammonia enters the bloodstream. Excess ammonia in the bloodstream is toxic to the animal. For cattle, toxic concentrations of blood ammonia are in the range of 1 mg/100 mL.

Signs of urea toxicity can be apparent as soon as 30 minutes post-consumption. Signs of urea toxicity include nervousness, staggering, and kicking at the flank, which will usually be followed by the animal being down. Signs may also include labored breathing, incoordination, tetany, slobbering, and bloating. In the early stages of urea toxicity, the animal may be treated via acidification of the rumen. Reducing the pH will reduce the absorption rate of ammonia and reduce the opportunity to develop ammonia toxicity in the body. In the later stages, treatment is not effective. The two primary outcomes of urea toxicity are the animal either dies or survives with few residual effects. The amount required for a fatal dose is dependent upon if the animal is adapted to the urea, consumption of other feedstuffs prior to the over-consumption of urea, type of diet the animal is adapted to, etc. In general, a dose of 40-50 grams per 100 kilograms of body weight in a 30 minute period may prove lethal. Prevention of urea toxicity is the most effective way to decrease economic loss due to urea toxicity. Prevention focuses on feeding management of NPN compounds.

Methods to improve the utilization of urea have been identified. The first method is the use of NPN compounds as silage additives. Addition of NPN compounds to forage at the time of ensiling increases crude protein content of the forage in a relatively cost-effective manner. The majority of the crude protein will be present as ammonia. More information on the use of NPN compounds as silage additives will be provided in a later module. The second method is the manufacture of extruded grain-urea mixtures. Brand names of extruded grain-urea mixes include Starea and GoldenPro. The mixes provide a source of energy and a source of slow-release NPN compound. Production data indicate satisfactory performance. Various other products are briefly described on page 106 of the text. On an additional note, research data indicate factors other than the rate of release primarily influence utilization of ammonia from urea.

Another common NPN compound fed is biuret.

Figure 7.9 illustrates the chemical structure of biuret.

In comparison to urea, biuret is more expensive and less available. Biuret is also less soluble and therefore less likely to cause toxicity compared to urea. Compared to urea, biuret may produce a more favorable response when fed with low-quality forages. Biuret is more palatable than urea. Biuret cannot be fed to lactating dairy cattle.

Ammoniated feeds are another group of NPN compound feeds. Both organic and inorganic salts of ammonia have been used to effectively increase the nitrogen content of feeds fed to ruminants. Examples of salts used include ammonium acetate, bicarbonate, butyrate, carbonate, chloride, lactate, polyphosphate, propionate, and sulfate. Ammoniated feedstuffs include molasses, beet and citrus pulp, cottonseed meal, rice products, and forages such as silage and hay. More information on treating forages with ammonia will be presented in a later module. To detoxify the feeds, aflatoxin-contaminated feeds have also been treated with ammonia. Two ammoniated liquid feeds described in the text are ammonium lignin sulfonate and fermented ammoniated condensed whey. Ammonium lignin sulfonate may be used in finishing feeds or in a molasses mixture with satisfactory production results. The crude protein content of ammonium lignin sulfonate is 15-25%. Fermented ammoniated condensed whey is 69% crude protein on a dry-matter basis. Feeding to ruminants has produced favorable production results.

Other nonprotein nitrogen compounds include individual amino acid-based products such as methionine hydroxyl analog, protected methionine, and protected lysine. The NPN compounds are a source of the respective amino acids for monogastrics and high producing ruminants such as lactating dairy cattle.

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