NPN: Any implications for ruminant nutrition?

NPN: Any implications for ruminant nutrition?

Slow-release NPN is an important source of soluble protein, which ensures optimal levels of ammonia in the rumen.

*C.A. Sgoifo Rossi, G. Baldi and R. Compiani are with the department of health, animal science and food safety at the University of Milan, Italy. S. Vandoni and M. Agovino are with Alltech.

IT is possible that the improper use of urea — a non-protein nitrogen (NPN) source — in the past has led to it being considered useless or even a dangerous product.

Today, thanks to the development of innovative technologies and improved knowledge of animal nutrition, the fundamental role that this dietary protein source plays in ruminants has been clarified.

Urea is an amide, mainly synthesized in the liver in order to detoxify physiologically produced ammonia that is largely the result of the catabolism of exogenous or endogenous amino acids. This biochemical mechanism allows for the safe transport via blood and excretion — mainly in the urine — of excess ammonia, which is highly toxic.

Indeed, urea is a neutral molecule, highly soluble and nontoxic (Lehninger, 1983). Urea plays an essential role in metabolism through preventing excessive dehydration by controlling urine concentration, contributing to the regulation of blood pressure and maintaining an optimum concentration level of sodium in the blood stream (Boulpaep and Boron, 2003).

The hepatic production of urea in a beef animal that eats 210 g of nitrogen daily — equal to 1,312.5 g per day of crude protein (CP) — is 137.3 g per day (Eisemann et al., 1996).


Ruminant diets

All ruminants eliminate urea through saliva, a fundamental biological mechanism, because it allows the recovery of part of the nitrogen product and introduces it back in the rumen. Ruminal microorganisms are, in fact, equipped with urease, an enzyme that hydrolyzes urea into carbon dioxide and two molecules of ammonia, which are then fixed to carbon skeletons — products of carbohydrate fermentation — to synthesize amino acids.

This process is very important for the ruminant as it ensures the vitality of rumen microflora and provides the animal with a source of high-biological value protein in the form of microbial protein. Approximately 15-20% of dietary nitrogen comes from NPN sources — such as free amino acids, nucleic acids, purine and pyrimidine bases, ammonia, ammonium salts, urea, nitrates and nitrites — and also urea from saliva.


Microbial protein

Diets provided to beef cattle to produce high-quality meat are characterized by high concentrations of non-structural carbohydrates (NSC), and as a direct consequence, in the rumen, the active proliferation of bacteria that are able to degrade such NSC substrates occurs. These bacteria are able to deaminate amino acids from feed proteins to ammonia and then synthetize new amino acids required for their development and multiplication.

Some other bacteria, like cellulolytic bacteria, do not have this ability and need free ruminal ammonia for microbial synthesis (Russell et al., 1992). This difference in the ability to ferment non-fibrous carbohydrates and structural carbohydrates between bacterial populations avoids competition between them with respect to plant-sourced amino acids as a source of ammonia (Griswold et al., 2003).

Ammonia plays a key role in ruminal metabolism, as confirmed by many in vitro studies. Reynal and Broderick (2005) showed a gradual increase in the microbial growth values of ammonia at the rumen level equal to 12.3 mg/100 mL.

This was also confirmed by Boucher et al. (2007), who found optimal microbial synthesis with values between 11 and 13 mg/100 mL.

Adding NPN into the diets of ruminants with high or low levels of rumen degradable protein (RDP) leads to an increase in the in vitro digestibility of dry matter (DM), organic matter, neutral detergent fiber, acid detergent fiber and NSC. This improvement comes from optimizing the growth of the ruminal microbial population and its better utilization of dietary nitrogen for protein synthesis (Griswold et al., 2003).

An increase in microbial protein synthesis is not only related to improved efficiency of carbohydrate degradation but also ensures an increased intake of amino acids. Indeed, microbial protein is characterized by a significantly higher biological value compared to any vegetable protein source, with an amino acid profile very close to that of meat (Table 1).

Considering that the main energy sources in beef cattle diets are characterized by a medium (corn meal) or a high (high-moisture corn or corn silage) rate of ruminal starch degradation, the addition of NPN sources helps rumen bacteria produce amino acids using carbon skeletons coming from rapid starch fermentation.

The increase of protein synthesis, together with the buffering effect of urea and other NPN sources, was found to improve the growth performance of animals fed a higher level of RDP and with corn meal and corn flakes as the main energy sources (Dicostanzo et al., 2007).

Studies conducted in the U.S. showed that the addition of a high level of urea (greater than 1.00% of total diet DM) in diets with quite a low amount of protein (13%) and high degradable starch sources (corn flakes) improved daily gain, feed efficiency and carcass dressing percentage (Gleghorn et al., 2004).

In European livestock production, the use of urea is much lower (less than 0.7% of the diet's total DM) than in other parts of the world. Only recently has the sector begun to understand the important role played by the proper intake of soluble nitrogen, particularly NPN, in the optimization of growth and performance and in improving the welfare of the animal through reducing the risk of digestive disorders.

The inclusion of NPN sources in the diets of beef cattle receiving the same protein level leads to an increase in microbial protein production, as shown in projections from the Cornell Penn Miner (CPM) software.

In Table 2, there are two possible diet reformulations, including urea with different levels of corn silage and corn meal. From a nutritional point of view, the best diet is diet 2. It provided the maximum concentration of starch that, through its fermentation, will produce volatile fatty acids with which to synthesize amino acids from ammonia.

In contrast, diet 1 is based on a reduced-cost diet achieved by increasing the amount of corn silage. Even this solution can provide an increase in microbial protein; however, it is less than diet 2.


Meat quality

The first studies to evaluate the effects of dietary urea on meat quality were performed in the 1960s. Loosli and McDonald, in a review from 1968, reported no difference in terms of meat acceptability when urea that provided up to one-third of the total nitrogen was added into the diet (Brueggemann et al., 1962).

More recent studies conducted on buffalo meat showed no differences in terms of meat quality in animals fed 0.0, 0.5, 1.0, 1.5 or 2.0% urea on a DM basis (Burque et al., 2008).

The same results were reported in a study by Roeber et al. (2005) in which no difference in sensory perception was found for the parameters of tenderness, juiciness and flavor when animals were fed 1.16, 0.83, 0.38 or 0.00% urea in the total DM of the diet. Similarly, no statistical differences were found for dressing percentage, fat cover, longissimus dorsi muscle area and U.S. Department of Agriculture quality grade when urea was included in the diet at different concentrations ranging from 0.0 to 3.0% of total DM (Galyean et al., 2012; Ceconi et al., 2012).

Further studies showed no differences in these parameters or in terms of drip loss and shelf-life of sliced (McClelland et al., 2012) and ground beef meat (Johnston et al., 2012).


New technologies

The main factor influencing the production of microbial protein is the simultaneous availability of fermentable carbohydrates and ammonia (Huntington and Archibeque, 1999).

From this point of view, urea is characterized by very quick rumen degradability, equal to 200% per hour, which is even quicker than high-fermentable energy feeds such as sugar sources that have an average ruminal degradability equal to 40-60% per hour (Van Amburgh et al., 2012).

Immediately after urea consumption, a peak in ammonia concentration in the rumen is not followed by a similar peak in the availability of carbon chains derived by the degradation of fermentable carbohydrates. These conditions reduce potential rumen microbial synthesis. In order to achieve optimal synthesis, technologies have recently been developed to guarantee a steady urea release in the rumen, avoiding the previously described differences in ruminal fermentation kinetics.

These technologies are characterized by ruminal degradation kinetics that are even slower than soybean meal (Figure; Palmer et al., 2008) and can promote better nitrogen efficiency, which is essential for bacterial growth.

The replacement of urea with a slow-release NPN additive in beef diets promotes uremia and blood ammonia reduction (Huntington et al., 2006; Garcia-Gonzales et al. 2007; Taylor-Edwards et al., 2009a; Holder et al., 2013), in addition to a reduction in the amount of urea excreted from the liver of up to 33% in the 10 hours after intake (Taylor-Edwards et al., 2009b).

The improved efficiency of nitrogen utilization potentially reduces the excretion of nitrogen in the environment (Sinclair et al., 2012). In relation to this potential reduction of emissions, great importance was also placed on the effective use of these nitrogen sources in terms of environmental impact, which is currently quantified by determining the carbon footprint as the equivalent of carbon dioxide emitted into the atmosphere.

Recent studies on dairy cows in Europe showed a 17% reduction in grams of carbon dioxide equivalent produced per kilogram of DM intake when soybean meal was replaced with a slow-release NPN source, thus reducing the greenhouse effect (Gehman, 2012).



In conclusion, not all diets for ruminants are able to guarantee the appropriate level of ruminal ammonia to maximize bacterial growth, and this is particularly relevant in the case of cellulolytic bacteria. Rumen microbes are able to synthesize proteins starting from ammonia from dietary and salivary NPN sources.

A slow-release NPN is an important source of soluble protein, which ensures optimal levels of ammonia in the rumen. NPN does not affect the chemical characteristics and quality of the meat. In general, an adequate supply of slow-release NPN has the ability to reduce the environmental impact of cattle and ruminants.



Boucher, S.E., R.S. Ordway, N.L. Witheouse, F.P. Lundy, P.J. Kononoff and C.G. Schwab. 2007. Effect of incremental urea supplementation of a conventional corn silage-based diet on rumen ammonia concentration and synthesis of microbial protein. J. Dairy Sci. 90:5619-5633.

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1. Essential amino acids (% of protein) of muscle protein, ruminal bacteria protein and the proportion of RDP of some important protein sources used in beef diets




Soybean meal,

Sunflower meal,

Distillers dried

Corn gluten




44% CP***

26% CP***

grains w/solubles***








































































*INRAN, 2013.

**Clark et al. (1992).

***CPM Dairy 3.0.


2. Examples of diet reformulation including urea for beef cattle (CPM Dairy 3.0)


No urea

Diet 1 with urea

Diet 2 with urea

Corn silage, kg




Wheat straw, kg




Corn meal, kg




Beet pulp dry, kg




Wheat bran, kg




Soybean meal 44% CP, kg




Sunflower meal 26% CP, kg




NPN, kg



Mineral and vitamin mix, kg




Chemical analysis




DM, kg/day




CP, % DM




Rumen undegradable protein, % CP








Soluble protein, % CP




Neutral detergent fiber (NDF), % DM




Physically effective NDF, % DM




Non-forage carbohydrates, % DM




Starch, % DM




Ether extract, % DM




Calcium, % DM




Phosphorus, % DM




Effect on bacteria growth




Production of bacterial DM, g/day


2,224.92 (+29.25)

2,236.14 (+40.47)

Production of bacteria protein, g/day


1,390.58 (+18.28)

1,397.56 (+25.26)

Cost of diet, euros/head/day*




*Based on prices at Granaria di Milano, Italy, on July 4, 2013.

NPN: Any implications for ruminant nutrition?


Volume:85 Issue:53

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