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LETTERS TO THE EDITOR |
* 123 Polk Hall Department of Animal Science, North Carolina State University, Raleigh 27695;
and
Department of Animal Science, University of Illinois, Urbana 61801
The utilization of dietary nitrogen has been the subject of much research. For example, NRC (1998)
provides a wealth of information on the subject that is based on an extensive review of literature. For a 45-kg pig fed a corn-soybean meal diet, the NRC predicts an efficiency of N utilization of 35%. This low efficiency is the result of approximately 7% of the dietary CP not being digestible and the equivalent of 8% of the dietary CP being lost in endogenous excretions. The synthesis of endogenous material results in the obligatory catabolism of the equivalent of 10% of the dietary N. These losses account for 25% of the dietary N intake. The remaining CP (75% of dietary) is available for lean tissue accretion; however, as a result of mismatches between requirements and dietary supply, approximately 30% is degraded and used for energy production. The remainder is actually used for lean tissue growth, but approximately 10% is not accreted due to inefficiencies in lean tissue growth, and summation of these losses results in N utilization in the neighborhood of 35% (van Kempen and van Heugten, 2000). Feed wastage and poor animal health can deteriorate this efficiency even further. Chung and Baker (1992) demonstrated that with diets formulated to be nearly 100% digestible (which would minimize indigestible and endogenous losses and optimally match the requirement of the animal, thereby minimizing the 30% mismatch), efficiencies of 60% were achievable in nursery pigs, lending credibility to the above calculation.
It is therefore very surprising that many researchers using commercial diets report efficiencies of N accretion in the 60 to 70% range. An example of a recent publication reporting such results is that of Shriver et al. (2003). Pigs used in these experiments were approximately 45 kg at the time of collection, and the control pigs were fed a fairly typical corn-soybean meal diet. Nitrogen retention was 60.8% of intake, or 35.4 g/d (Table 4 in Shriver et al., 2003). According to the NRC (1998), N accretion of 35.4 g/d is equivalent to a protein accretion of 221 g/d, which is equivalent to a lean tissue gain of 961 g/d. According to the actual gain observed by Shriver et al. (2003), this lean gain suggests that 91% of total gain was lean gain, a value which most would agree is unrealistic.
Possible explanations for the very high N retention include incomplete collection of excreted N or unaccounted feed waste. The very high dry matter digestibility (92% for the controls) observed by Shriver et al. (2003) suggests that this may have played a role. Another explanation, however, is N loss from the urine. It is well known that N in the form of ammonia can volatilize quickly from urine contaminated with bacterial urease unless the urine is stored in a closed container, cooled, or acidified.
Shriver et al. (2003) did not report what measures were taken to prevent N losses. Typically, urine is acidified in the collection container, which is very effective at reducing N losses from the captured urine. Another method is to cool the collection container with ice, a method often used when urine has to be collected in its native state. Our experience with the latter method is that apparent N retentions of 50 to 60% are obtained, still much too high for the diets fed (Moeser and van Kempen, 2002). A problem that we observed in those trials was that a bacterial film developed on the urine collection trays positioned below the pigs. This film may well be responsible for capturing a substantial portion of urinary N and converting it, or at least some of it, into volatile ammonia. In vitro studies in our lab have shown that urine contaminated with a small amount of fecal material and stored in a thin layer at room temperature will release 13% of urea-N at 8 h, 26% at 12 h, and 68% at 24 h.
Urinary N losses can have a profound effect on the interpretation of data in that ammonia losses from urine are influenced by many factors. For example, Shriver et al. (2003) used a test diet high in synthetic amino acids. This diet resulted in a drop in urine pH of 0.4 pH units. Ammonia emission is very sensitive to pH, as outlined by the authors. Thus, if ammonia losses were a problem in this trial, and if they were altered by urine pH, apparent N retention would be affected simply as a result of the change in urine pH.
Fiber addition to the diet may result in another problem. Fiber can result in a shift of N excretion from urine to feces because microbes in the large intestine, thriving on fiber, accrete N that comes from urea in the blood stream, originally destined for excretion in the urine. This was one of the hypotheses of Shriver et al. (2003), which was confirmed in their study. Lowering urinary N excretion, however, can be expected to affect ammonia losses from urine, in turn affecting apparent N retention.
In summary, researchers should take great care to minimize ammonia losses during metabolism trials. Acidification of the entire urine collection system, including the collection tray, is the preferred method, except where urine has to be collected as is. In those cases, frequent collections and cleaning of the entire urine collection system are paramount. After analyzing samples, researchers should critically inspect their efficiency figures and determine whether N retention is in line with observed growth of the animals. All of us who conduct N balance trials would also be well served by reading the excellent review article by Wallace (1959), which details the many pitfalls of N balance trials.
1 Correspondence—phone: 919-515-4016; fax: 919-515-7780; E-mail: t_vankempen{at}ncsu.edu).
Literature Cited
Chung, T. K., and D. H. Baker. 1992. Ideal amino acid pattern for 10-kilogram pigs. J. Anim. Sci. 70:3102–3111.[Abstract]
Shriver, J. A., S. D. Carter, A. L. Sutton, B. T. Richert, B. W. Senne, and L. A. Pettey. 2003. Effects of adding fiber sources to reduced-crude protein, amino acid-supplemented diets on nitrogen excretion, growth performance, and carcass traits of fnishing pigs. J. Anim. Sci. 81:492–502.
Moeser, A. J., and T. A. T. G. van Kempen. 2002. Dietary fiber level and xylanase affect nutrient digestibility and excreta characteristics in grower pigs. J. Sci. Food Agric. 82:1606–1613.
NRC. 1998. Nutrient Requirements of Swine. 10th ed. Natl. Acad. Press, Washington, DC.
van Kempen, T., and E. van Heugten. 2000. Reducing Pig Waste and Odor Through Nutritional Means. Lesson 10 in USDA/EPA National Manure Stewardship Curriculum. Washington, DC.
Wallace, W. M. 1959. Nitrogen content of the body and its relation to retention and loss of nitrogen. Fed. Proc. 18:1125–1130.[Medline]
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