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The protein balance in animals is influenced partly by the efficiency of nitrogen (N) utilization in animals. A simple strategy to increase the efficiency of N utilization is by reducing the N content in the feed converted to urea (urea production), which has been found strongly related (correlation is r2= 0.77; Harmeyer and Martens, 1980). However, this was mainly based on studies with mature, slow growing ruminants, which absorb and convert a high proportion of N to urea to prevent a negative N balance (Lapierre and Lobley, 2001). Lapierre and Lobley (2001) summarized that in growing young sheep and cattle, less strong relationships were found between N intake and urea production (r2=0.33 and r2=0.58 respectively). However, reducing the N intake levels to increase urea recycling kinetics is not always realizable due to minimal absolute N requirements in animal feed, especially for growing animals.
Next to N intake, the protein balance level is influenced by the efficiency of N recycling in animals, especially in ruminants. A short definition of urea recycling is: the absorption of urea from the digestive tract into the blood and the flow from urea in the blood into the digestive tract again so that urea nitrogen salvage could happen. Nitrogen recycling takes place between blood and the digestive tract in the form of endogenous protein-N, secreted-N (e.g. enzymes in saliva) and urea-N (Reynolds and Kristensen, 2008). In this thesis, the focus will be on urea-N recycling. The principles of urea recycling are explained in this chapter. Summarized (Figure 3.1), ammonia (NH3) and to a lesser extent amino acids, are absorbed from the digestive tract into the portal bloodstream and are converted to urea in the liver. Urea can (re)enter the digestive tract, mainly through the rumen wall, where it can be absorbed again or be (re)used for microbial protein synthesis and finally anabolic purposes. Urea recycling thus allows conversion of catabolic N into anabolic N, enabling that N remains longer in the body and increases the chance to utilize dietary N sources efficiently (Lapierre and Lobley, 2001).
Figure 3.1. Principle of urea recycling. Amino acids and NH3 are absorbed into the portal bloodstream and converted into urea in the liver (ureagenesis). Urea can reenter the rumen, where it can be absorbed (again) or be used for microbial protein synthesis.
Absorption of amino acids and ammonia
Urea is the mammalian end-product of the amino acid metabolism. In the rumen, proteins are degraded into amino acids and finally into carbon dioxide (CO2) and ammonia (NH3) by means of rumen fermentation due to the bacterial population (Shingu et al., 2007). Then, absorption of NH3 and to a lesser extent amino acids through the rumen wall and intestine walls can take place, to enter the portal circulation (figure 3.1). NH3 absorption into the blood seems to depend on the pH and the NH3 to NH4 ratio in the rumen. Furthermore, high ruminal NH3 concentrations result in a higher absorption of NH3 into the blood (Siddons et al., 1985).
In the liver, detoxification of NH3 takes place, because urea is synthesized from the nitrogen (N) compound of both NH3 and amino acids (Obitsu and Taniguchi, 2009). The synthesis of urea, called ureagenesis, takes place by means of the urea or ornithine cycle. This cycle of biochemical reactions occurs in many animals that produce urea ((NH2)2CO) from NH3, mainly in the liver and to a lesser extent in the kidneys. Mitochondrial NH3 and cytosolic aspartate are precursors for the ornithine cycle (Van den Borne et al., 2006). However, the key compound is ornithine, which acts as a carrier on which the urea molecule is 'built up'. It is suggested that higher levels of this amino acid should increase the ornithine production, because arginine is needed for ornithine production. Furthermore, ornithine, citrulline and arginine (components of the ornithine cycle) seem to stimulate urea production, with a subsequent decrease in plasma NH3 (Bender, 2008). But also vice versa, temporarily high NH3 flows seem to stimulate amino acid utilization for the urea production (Milano and Lobley, 2001). At the end of the ornithine cycle reaction sequence, urea is released by the hydrolysis of arginine, yielding ornithine to start the cycle again (Bender, 2008).
The Urea Entry Rate into the blood is dominated by urea synthesis in the liver (Lapierre and Lobley, 2001). The UER is often higher than the amounts of urea eliminated in the urine (UUE: Urinary Urea Excretion). This is because urea produced in the liver is next to excretion in the urine, also is reabsorbed in the distal renal tubules, where it maintains an osmotic gradient for the reabsorption of water (Bender, 2008). Furthermore, urea from the blood (PUN: Plasma Urea Nitrogen) can re-enter the digestive tract via saliva, secretions or directly across the rumen wall in the form of endogenous proteins or urea (Lapierre and Lobley, 2001; Shingu et al., 2007; Obitsu and Taniguchi, 2009). Thus, not all urea is secreted directly into the urine after entering the bloodstream.
Entry into digestive tract
Urea, which flows from the blood through the ruminal wall into the rumen and enters the digestive tract, is hydrolyzed by bacterial urease to carbon dioxide (CO2) and ammonia (NH3) (figure 3.1). Lapierre and Lobley (2001) summarized that the GER can increase the digestible N with 43-85% in growing steers, 50-60% in dairy cows and 86-130% in growing sheep.
Factors affecting the urea entry into digestive tract
The amount of urea entering the digestive tract (GER: Gut Entry Rate) is, until certain levels (sheep: 6 mM = 84 mg/L; cattle: 4 mM = 56 mg/L (Harmeyer and Martens, 1980; cattle: 80 mg/L (Kennedy and Milligan, 1978)) affected by the urea concentration in blood plasma (PUN) (Harmeyer and Martens, 1980). Above these concentrations, boundary layer effects of NH3 inhibit the GER of urea (Lapierre and Lobey, 2001). Thus it seems that the GER could be influenced by the concentration gradient of urea between blood plasma and urea (RUN: Ruminal Urea Nitrogen) or NH3 (RAN: Ruminal NH3 Nitrogen) in the digestive tract.
Furthermore, a high RAN result in a lower activity of ureolytic bacteria and urease (Bunting et al., 1989a; Marini et al., 2004), which inhibit the GER. Therefore, the GER can be influenced by diverse bacteria-influencing compounds in feed (Harmeyer and Martens, 1980). Furthermore, the activity of ureolytic bacteria and urease influence the concentration gradient between PUN and RUN (Kennedy and Milligan, 1978; Bunting et al., 1989a). Thus high NH3 concentrations result in decreased GER, due to interrelated effects between the concentration gradient and ureolytic bacterial activity.
However, too low RAN values (below 3.5 mM) can limit microbial growth, e.g. because cellulolytic bacteria in the rumen require ammonia-N to ferment fiber (Bryant, 1973). On the other hand, at limiting ruminal NH3 concentrations, bacteria (at/near rumenwall) are probably able to 'catch' NH3 out of blood (Bunting et al., 1989b). This suggests that providing low N intakes result in higher recycled urea-N utilization (Marini and Van Amburgh, 2003).
It has been suggested that transport of urea in the colon and epithelia in sheep occurs through carriers or facilitative transport (Ritzhaupt et al., 1997). Those carriers and urea transporters (UT) permit bi-directional flows (Ritzhaupt et al., 1997), and thus can it be possible that the total GER is an underestimation when urea is reabsorbed but not metabolized (Lapierre and Lobley, 2001).
Location of urea entry into digestive tract
Urea can enter all parts of the digestive tract, including via saliva and pancreatic juice. However, the entering rates depend on the site of entering and the type of feed ingested.
As summarized by Lapierre and Lobley (2001), in sheep, the part of the total GER transferred to the rumen varies (27-60; 27-54%: Kennedy and Milligan, 1978 and Siddons et al., 1985 respectively) depending on type of diet. This proportion seems to increase when animals get high levels of rumen-degradable energy in feed (Lapierre and Lobley, 2001; Theuer et al., 2002). The majority of conversions of urea into anabolic compounds by the microbial population occur in the fore-stomach, mainly the rumen.
Also saliva contributes to the GER into rumen, depending on the type of diet ingested. E.g. this proportion varies extensively from 15 (Kennedy and Milligan, 1978) to almost 100% (Norton et al., 1978) in sheep. It has been found in growing beef steers that forage diets, e.g. alfalfa hay, result in higher amounts of saliva entering the gut (36% of GER) (Taniguchi et al., 1995) compared to high concentrate diets (17% of GER) (Guerino et al., 1991). Thus the fore-stomachs are important for the anabolic salvage of N, however, this depends on the type of feed ingested.
Also the small intestine contributes to the anabolic salvage of N. It has been found in sheep that 37 and 48% of the total GER of urea entered the small intestine when feeding a diet of grass in silage or grass which is dried, respectively (Siddons et al., 1985). However, the quantities of anabolic N formed may by small, e.g. because NH3 production seems to exceed urea entry in the small intestine, although this depends on the feed type ingested (Lapierre and Lobley, 2001).
Likely most microbial protein synthesized from urea that enters the hindgut is excreted. Lapierre and Lobley (2001) suggest that the hindgut only serves catabolic purposes of urea, (Lapierre and Lobley, 2001).
Fate of urea entered the digestive tract
Urea utilized for anabolic purposes
Urea entered the digestive tract by means of saliva or flowing through the gut wall is hydrolyzed to NH3 which could be used for microbial protein synthesis for anabolic purposes (UUA: Urea Utilization for Anabolic purposes) (Sarraseca et al., 1998; Lapierre and Lobley, 2001; Shingu et al., 2007). This is a mechanism for the salvage of urea-N into bacterial protein which can be digested and yields amino acids to the animal when they are absorbed in the intestine. Thus, urea nitrogen incorporated in microbial protein gets 'a second chance' for absorption and deposition/anabolic purposes. Therefore, urea recycling can be regarded as a mechanism with positive effects at the protein balance of ruminants (Lapierre and Lobley, 2001). The UUA depends partly on the gut entry location, because for example urea entered the rumen has more chance to be utilized for microbial protein synthesis compared to urea entered the hindgut. Lapierre and Lobley (2001) summarized that the amount of urea utilized for anabolic purposes proportional to the GER varied from 52 to 60% for dairy cows and 45 to 51% in sheep (Lobley et al., 2000).
Within the rumen, also N is recycled. Proteolytic bacteria and protozoa degrade rumen bacteria, what results in higher NH3 levels and absorption, but a lower intestinal microbial protein flow. This indicates that changing ruminal microbial populations can affect the urea recycling kinetics and the anabolic N flow largely (Lapierre and Lobley, 2001).
Urea return to ornithine cycle
Next to utilization for anabolic purposes, NH3 could be returned to the ornithine cycle (ROC: Return to Ornithine Cycle) in the liver (Sarraseca et al., 1998; Lapierre and Lobley, 2001; Shingu et al., 2007). Much of the NH3 is in the liver used for glutamate and glutamine synthesis, and then a variety of other nitrogenous compounds (Bender, 2008). Furthermore, multiple recycling results in an efficient reuse of N because urea returns to the digestive tract more than once, especially in sheep, dairy cows and growing steers, summarized by Lapierre and Lobley (2001). The ROC and especially the recycling via the rumen results in a higher chance that a catabolic N product is used in microbial protein synthesis and is deposited in an anabolic N source. Indeed, it was found that the existence of urea recycling possibly results in improvements of 22 to 49% of GER used for anabolic purposes in both cattle and sheep (Lapierre and Lobley, 2001).
The absorption of NH3 from the rumen accounts for about 46.5% of the available N in the rumen, in cattle and sheep (Lapierre and Lobley, 2001). Furthermore, it has been found that the ROC proportional to the GER was approximately 42% (32-52%) in sheep (Sarreseca et al., 1998), and 33.5% (26-41%) in cattle (Archibeque et al., 2001). Lapierre and Lobley (2001) summarized that the amount of urea recycled to the ornithine cycle (ROC) proportional to the GER varied from 34 to 38% for dairy cows and 42 to 51% in sheep (Lobley et al., 2000). From this amount of ROC, relative much NH3 is obtained from recycled urea compared to digested N, because Siddons et al. (1985) found that NH3 in the rumen proportional to the urea GER was 33% in sheep fed dried grass (low N content), while this percentage was lower in case of grass silage (high N content). This declares why some urea kinetic fluxes can exceed (apparent) digestible N intakes. This likely results in a high amounts of absorbed amino acids relative to the digestible N intakes, varying from 27 to 279% for cattle and sheep, respectively. Those amounts decrease (24-58%) when e.g. the GER into the rumen has been taken account of. Of course, amino acid absorption cannot be larger than the (apparent) digestible N, except when N sources due to catabolism or urea recycling are considered (Lapierre and Lobley, 2001).
Urea excretion into feces
Next to UUA and ROC, urea can also be excreted into the feces. This has been found most likely to happen when urea inters the hindgut (Lapierre and Lobley, 2001). Lapierre and Lobley (2001) summarized that the amount of urea excreted into the feces proportional to the GER varied from 6 to 10% for dairy cows and 3 to 7% in sheep (Lobley et al., 2000).
Figure ... shows an overview of the urea kinetics with data summarized by Lapierre and Lobley (2001), which are averages for dairy cows, steers and sheep (Archibeque et al., 2001; Sarraseca et al., 1998). The urea synthesis is assumed to be approximately similar to the digested N. When a PUN of 100% is assumed, 33% of the urea in blood is excreted into the urine (UUE) and 67% flows (returns) into the digestive tract (GER). Approximately 50% of this latter GER flow is reconverted to microbial proteins and can be used for anabolic purposes. Another 40% of the GER is reabsorbed as NH3 into the portal blood and reconverted into urea (PUN) from which point it can be divided between UUE (33%) and GER (67%) again. The remaining 10% of the GER is excreted in the feces (Lapierre and Lobley, 2001).
Figureâ€¦Assume the UER is approximately similar to the digested N. From the PUN (100%), 33% is the UUE and 67% the GER. Approximately 50% of this GER flow is destined for UUA. Another 40% of the GER is the ROC, which appears as PUN which can again be divided in UUE (33%) and GER (67%). The remaining 10% of the GER is the UFE (Lapierre and Lobley, 2001).
Thus the utilization of N in the feed could be improved by a higher urea recycling, which results in an increased UUA. However, feed N utilization can also be improved by reduced NH3 absorption in combination with a relative higher flow available for UUA.
The utilities of urea recycling
Both ruminants and non-ruminants, including omnivores, make use of the urea recycling mechanism. However, in ruminants, much more urea is recycled compared to non-ruminants, which emphasizes the importance of urea recycling in ruminants (Lapierre and Lobley, 2001). Next to reducing feed costs (due to less dietary N requirement), there are three main reasons to obtain a good and efficient urea recycling in ruminants (Huntington and Archibeque, 1999):
Maximization of the microbial functioning in the rumen;
Optimization of the amino acid supply to the host ruminant - improvements of adaptation;
Minimization of the negative effects of nitrogen excretion into the environment.
Maximization of microbial functioning
A higher level of urea recycling results in a higher production of microbial protein. This protein source will be largely used for anabolic uses and performance which will result, on the long term, in improved production efficiency (Lapierre and Lobley, 2001). Urea recycling increases the body N flows to synthesis more anabolic N from a catabolic N source, thus it conserves the N in the body, which provides an ability to respond fast to challenges.
Optimization of amino acid supply - adaptation
As a consequence of the salvage mechanism to recover some N, urea recycling in ruminants provides an ability to respond fast to challenges and is important regarding the adaptation to different environmental (living) circumstances but mainly to nutritional conditions. Examples are periods of dietary protein deficiency or an asynchronous supply of carbohydrates and proteins (Reynolds and Kristensen, 2008). An increase in the total urea flux, caused by the ROC, is considered to serve as a labile N pool in the whole body to permit metabolic plasticity under a variety of physiological (productive), environmental and nutritional conditions (Obitsu and Taniguchi, 2009; Lapierre and Lobley, 2001).
Minimization of N excretion into the environment
Finally, a more efficient urea recycling in ruminants results in a less urea-N excretion in the urine. This is will minimize the negative effects of nitrogen excretion into the environment (Huntington and Archibeque, 1999).
Factors influencing urea recycling kinetics
A general conclusion, based on previous research is that urea recycling kinetics, like urea production, excretion and GER, is affected by several dietary factors. Those dietary factors mainly include feed and N intake levels (Sarraseca et al., 1998; Marini and Van Amburgh, 2003; Marini et al., 2004), dietary roughage to concentrate ratio (Huntington et al., 1996) and carbohydrate fermentability/digestibility (Theurer et al., 2002). Also differences between animal species and even animal breeds are found (Lapierre and Lobley, 2001; Obitsu and Taniguchi, 2009; Shingu et al., 2007). Furthermore, urea recycling kinetics can be affected by physiological characteristics and productive priorities of animals (Obitsu and Taniguch, 2009).
Feed and N intake levels
With changing feed intakes, nutrient intakes like the N intake, change at the same time and cannot be distinguished from each other. Furthermore, with both, increasing feed and N intakes, UER and PUN increase. Therefore, feed intake and N intake levels will be dealt with in one paragraph. An overview, based on several studies, of the effects of increased feed/N intake at parameters of the N balance and urea kinetics are showed in tableâ€¦
Table â€¦ Overview of the effects of increased feed/N intake at parameters of the N balance and urea kinetics.