Demonstration of constant nitrogen and energy amounts in pig urine under acidic conditions at room temperature and determination of the minimum amount of hydrochloric acid required for nitrogen preservation in pig urine

Article information

Anim Biosci. 2023;36(3):492-497
Publication date (electronic) : 2022 November 13
doi : https://doi.org/10.5713/ab.22.0243
1Department of Animal Science and Technology, Konkuk University, Seoul 05029, Korea
*Corresponding Author: Beob Gyun Kim, Tel: +82-2-2049-6255, Fax: +82-2-455-1044, E-mail: bgkim@konkuk.ac.kr
Received 2022 June 23; Revised 2022 August 10; Accepted 2022 September 11.

Abstract

Objective

The objectives were to demonstrate that the nitrogen and energy in pig urine supplemented with hydrochloric acid (HCl) are not volatilized and to determine the minimum amount of HCl required for nitrogen preservation from pig urine.

Methods

In Exp. 1, urine samples of 3.0 L each with 5 different nitrogen concentrations were divided into 2 groups: 1.5 L of urine added with i) 100 mL of distilled water or ii) 100 mL of 6 N HCl. The urine in open plastic containers was placed on a laboratory table at room temperature for 10 d. The weight, nitrogen concentration, and gross energy concentration of the urine samples were determined every 2 d. In Exp. 2, three urine samples with different nitrogen concentrations were added with different amounts of 6 N HCl to obtain varying pH values. All urine samples were placed on a laboratory table for 5 d followed by nitrogen analysis.

Results

Nitrogen amounts in urine supplemented with distilled water decreased linearly with time, whereas those supplemented with 6 N HCl remained constant. Based on the linear broken-line analysis, nitrogen was not volatilized at a pH below 5.12 (standard error = 0.71 and p<0.01). In Exp. 3, an equation for determining the amount of 6 N HCl to preserve nitrogen in pig urine was developed: additional 6 N HCl (mL) to 100 mL of urine = 3.83×nitrogen in urine (g/100 mL)+0.71 with R2 = 0.96 and p<0.01. If 62.7 g/d of nitrogen is excreted, at least 240 mL of 6 N HCl should be added to the urine collection container.

Conclusion

Nitrogen in pig urine is not volatilized at a pH below 5.12 at room temperature and the amount of 6 N HCl required for nitrogen preservation may be up to 240 mL per day for a 110-kg pig depending on urinary nitrogen excretion.

INTRODUCTION

Metabolizable energy (ME) in feeds has been widely employed in swine diet formulations [1] as energy utilization is better reflected in the ME system compared with the gross energy (GE) or digestible energy (DE) system. Feed ME values are determined by subtracting urinary and gaseous energy from ingested DE. In this calculation, gaseous energy is often neglected due to the small quantity in pigs. Thus, an accurate measurement of urinary energy is essential for determining ME values in feeds [25].

Energy in the pig urine consists mainly of urea which can be hydrolyzed to ammonia and evaporated into the air [6]. As the prevention of ammonia volatilization from urine is essential for an accurate ME determination, the addition of acids in the urine collection containers is a general practice to keep the urine acidic [7,8]. However, the amounts of acids used for nitrogen preservation vary among experiments [4,911]. To our knowledge, little information is available on the amount of hydrochloric acid (HCl) required for nitrogen preservation in pig urine. Therefore, the objectives of the present experiments were to demonstrate that nitrogen and energy in pig urine are not volatilized under acidic conditions and to determine the amounts of HCl required for nitrogen preservation.

MATERIALS AND METHODS

Animal care

The experimental protocol was approved by the Institutional Animal Care and Use Committee at Konkuk University, Republic of Korea (KU17049 and KU19058).

Exp. 1. Nitrogen and energy contents in pig urine under acidic condition

Urine samples were collected from 5 barrows (Landrace× Yorkshire) with a mean body weight (BW) of 68.1±4.0 kg for 24 h with no acid in the urine collection containers and were filtered using cotton cloth (0.5 mm pore size) to remove impurities. The samples were stored in a sealed container at −20°C. Nitrogen concentrations in the urine samples were 0.29%, 0.58%, 0.63%, 0.66%, and 0.68%. Each urine sample (approximately 3.0 L) was divided into 2 groups of 1.5 L which were supplemented with either 100 mL of 6 N HCl to obtain a pH below 2 or 100 mL of distilled water. Each 200 mL urine sample added with HCl, or distilled water was placed in a plastic container. All plastic containers with the urine samples were placed on a laboratory table for 10 d at room temperature of 18°C to 23°C. The weight, nitrogen concentration, and GE concentration of urine were determined every 2 d. Urinary nitrogen concentrations were determined using an automatic Kjeldahl analyzer (method 990.03) as described in AOAC [12] and urinary GE concentrations were determined using the procedure described by Kim et al [13].

Experimental data were analyzed using the MIXED procedure (SAS Inst. Inc., Cary, NC, USA). The statistical model included day, supplementation of 6 N HCl, and interaction between day and supplementation of 6 N HCl as fixed variables, and the day was the repeated term in this model. The values of least squares mean were calculated. Orthogonal polynomial contrasts were used to test the linear and quadratic effects of day and the interaction between day and supplementation of 6 N HCl. Each plastic container was an experimental unit. Statistical significance and tendency were declared at alpha less than 0.05 and 0.10, respectively.

Exp. 2. A maximum pH for nitrogen preservation in pig urine

Urine samples were collected from 10 barrows ([Landrace× Yorkshire]×Duroc) with a mean BW of 41.2±2.1 kg with no acid in the urine collection containers and were filtered using cotton cloth (0.5 mm pore size) to remove impurities. The samples were stored in a sealed container at −20°C. Three urine samples were selected to contain variable nitrogen concentrations of 0.12, 0.53, and 0.94 g/100 mL. To determine of the maximum pH for nitrogen preservation, six 100-mL aliquots from each urine sample were added with 6 N HCl to achieve pH values of 0.6, 1.1, 2.2, 4.7, 7.1, and 9.3 in 18 plastic containers. The plastic containers with the urine of various pH values were placed on a laboratory table at room temperature for 5 d. Nitrogen concentrations were analyzed (method 990.03; AOAC, 2019) at the beginning and after 5 d to determine the nitrogen losses from the pig urine.

A break point of a pH value for nitrogen preservation was estimated by a one-slope broken-line model using the NLIN procedure of SAS (SAS Inst. Inc., USA). A plastic container was an experimental unit and statistical significance was declared at an alpha less than 0.05.

Exp. 3. A minimum amount of HCl required for nitrogen preservation in pig urine

Five urine samples were selected from 10 samples of Exp. 2 to obtain variable nitrogen concentrations of 0.12, 0.26, 0.53, 0.61, and 0.94 g/100 mL. The pH changes of each 100 mL of urine samples were measured every addition of 0.2 mL of 6 N HCl using a pH meter (SevenEasy pH Meter S20; Mettler Toledo, Columbus, OH, USA).

The NLIN procedure (SAS Inst. Inc., USA) was employed to develop exponential equations for estimating urine pH by the volume of added HCl in each urine sample with various nitrogen concentrations. An equation for determining the minimum amount of 6 N HCl for nitrogen preservation in urine was generated by the REG procedure of SAS (SAS Institute, 2012) with 6 N HCl concentrations in urine as a dependent variable and nitrogen concentrations in urine as an independent variable. A plastic container was an experimental unit and statistical significance was declared at alpha less than 0.05. Additionally, the amounts of 6 N HCl required for nitrogen preservation were calculated based on actual daily nitrogen excretion data from 9 published experiments and 8 unpublished experiments conducted in our laboratory.

RESULTS AND DISCUSSION

Exp. 1. Nitrogen and energy contents in pig urine under acidic condition

The amount of nitrogen in the urine showed a linear interaction (p<0.001) between acid supplementation and time (Table 1). The amount of nitrogen in the urine supplemented with distilled water decreased linearly with time, whereas that supplemented with 6 N HCl remained constant regardless of the time. The amount of GE in the urine had a tendency for linear interaction (p = 0.053) between acid supplementation and time. The amount of GE in the urine supplemented with distilled water tended to decrease linearly with time, whereas that supplemented with 6 N HCl remained constant regardless of the time. These results indicate that urea was hydrolyzed to ammonia molecules and volatilized under alkaline conditions, but not under acidic conditions. In the energy metabolism experiments, urine is collected daily basis with acids in the collection containers and subsamples are stored in the freezer [3,4,9,14]. The present results also indicate that urine samples can be stored at room temperature for at least 10 d without nitrogen or energy volatilization if the urine pH is kept below 2. Under acidic conditions, a large quantity of hydrogen ions traps ammonia as ammonium ions [15,16] that do not dissociate into hydrogen ions and ammonia [17].

Effects of hydrochloric acid supplementation to pig urine on nitrogen and gross energy (GE) contents at room temperature for 10 days, Exp. 11),2)

As urea is a major energy source in urine [18], the constant urine energy under acidic conditions regardless of the time is reasonable. The amounts of energy at d 0 were expected to be the same between the 2 groups. However, the amount of energy in the distilled water-added urine was 30.6% less (12.2 vs 17.6 kcal; Table 1) than that in the acid-added urine on d 0. This unexpected result is most likely due to the volatilization of ammonia from distilled water-added urine during the lyophilization procedure before GE determination. Thus, the addition of acids to pig urine is critical to prevent ammonia volatilization from urine during collection and sample drying processes for an accurate determination of urinary GE.

Exp. 2. A maximum pH for nitrogen preservation in pig urine

Based on the one-slope broken-line analysis (Figure 1), nitrogen in pig urine was not volatilized at pH below 5.12 (R2 = 0.98, standard error = 0.71, and p<0.01). The present results demonstrate that the use of acid in the urine collection container to keep pH below 5 would be sufficient for nitrogen preservation. This result agrees with the previous suggestions that the pH of the urine should be kept below 5 to avoid nitrogen volatilization [19,20]. Although urine pH values are rarely measured in energy metabolism or nitrogen balance experiments, the pH of collected urine may have exceeded 5 due to insufficient addition of acid to the urine containing a large quantity of urea. Thus, the addition of a sufficient amount of acid to make the pH less than 5.12 is critical for nitrogen preservation in pig urine.

Figure 1

A broken-line analysis of nitrogen losses for 5 d at 6 urine pH values acquired by different inclusion rates of 6 N hydrochloric acid into the 100 mL of each urine sample (Exp. 2). Each data point represents the least squares mean of 3 observations. The amount of nitrogen was 0.53 g on average in 100 mL urine at the beginning. A one-slope broken-line model of nitrogen losses for 5 d indicates that a maximum pH of 5.12 is needed to prevent nitrogen volatilization from the pig urine. The break point was estimated based on following equation: Y = 0.04×(X–5.12)–0.01 where X is more than 5.12 (standard error = 0.712 and p<0.01) in Exp. 2.

Exp. 3. A minimum amount of HCl required for nitrogen preservation in pig urine

Exponential models were developed for each of the 5 urine samples to estimate urine pH changes by adding 6 N HCl (Figure 2). Using these models, the amount of 6 N HCl required for 100 mL of urine to achieve the pH 5.12 was calculated for each of the 5 urine samples with various nitrogen concentrations. Based on these data, an equation was generated using nitrogen concentration in urine (g/100 mL) as an independent variable to determine a minimum amount of 6 N HCl required for nitrogen preservation in pig urine (Figure 3).

Figure 2

Urine pH changes by the addition of 6 N hydrochloric acid (HCl) to urine (Exp. 3). Exponential models were developed for each urinary nitrogen concentration: Y = −9.28+9.75×(1+e−0.81×X), with p<0.001 for nitrogen 0.12 g/100 mL; Y = −10.16+10.41×(1+e−0.38×X), with p<0.001 for nitrogen 0.26 g/100 mL; Y = −10.26+10.57×(1+e−0.30×X), with p<0.001 for nitrogen 0.53 g/100 mL; Y = −10.80+10.89×(1+e−0.24×X), with p<0.001 for nitrogen 0.60 g/100 mL; Y = −10.98+10.93×(1+e−0.18×X), with p<0.001 for nitrogen 0.94 g/100 mL. The required concentrations of 6 N HCl to achieve a urine pH less than 5.12 were 0.92, 2.01, 2.61, 3.24, and 4.20 mL/100 mL for urinary nitrogen concentrations of 0.12, 0.26, 0.53, 0.60, and 0.94 g/100 mL, respectively.

Figure 3

Minimum amounts of 6 N hydrochloric acid (HCl) required for nitrogen preservation for 100 mL pig urine based on urine nitrogen concentration (g/100 mL; Exp. 3). The plotted data were based on the x-axis values for pH = 5.12 and urinary nitrogen concentrations presented in Figure 2.

Nitrogen concentrations in pig urine are affected by water intake and dietary fiber concentrations due to changes of water absorption to the circulation system of pigs and urinary water excretion [2,7,21]. However, these 2 factors do not affect absolute amounts of urinary nitrogen excretion. For the calculation of an amount of HCl required for nitrogen preservation in urine, the amount of excreted urinary nitrogen should be considered. High dietary protein contents [2224] and an imbalance of dietary amino acids [25,26] cause an elevation of urinary nitrogen excretion in pigs.

Based on actual data for daily nitrogen excretion, the amount of 6 N HCl required for nitrogen preservation was calculated to be 48 to 240 mL per day for the largest daily urinary nitrogen excretion for each BW range (Table 2). In this calculation, the maximum quantity of daily nitrogen excretion (g/d) for each BW range was multiplied by the required amount of 6 N HCl for nitrogen preservation per gram of urinary nitrogen (3.83 mL/g) which is the slope in Figure 3. In energy metabolism or nitrogen balance experiments, the amount of urinary nitrogen excretion is largely variable and difficult to predict accurately. Therefore, a sufficient amount of acids should be used for nitrogen preservation based on the calculations provided in this work.

The ranges of daily urinary nitrogen excretion (g/d) according to body weight (BW) and required amounts of 6 N hydrochloric acid (HCl) for urinary nitrogen preservation, Exp. 31)

CONCLUSION

The amount of nitrogen and gross energy in pig urine remains constant at room temperature if the urine is highly acidic. The urine pH needs to be below 5.12 to inhibit nitrogen volatilization from pig urine. Although urinary nitrogen excretion is largely variable depending on many factors including experimental diets and growth stage of pigs, and the amount of 6 N HCl required for nitrogen preservation may be up to 240 mL per day for a 110-kg pig depending on urinary nitrogen excretion.

Notes

CONFLICT OF INTEREST

We certify that there is no conflict of interest with any financial organization regarding the material discussed in the manuscript.

FUNDING

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP; Ministry of Science, ICT & Future Planning) (No. 2021R1A2C2009921).

References

1. Kong C, Adeola O. Evaluation of amino acid and energy utilization in feedstuff for swine and poultry diets. Asian-Australas J Anim Sci 2014;27:917–25. https://doi.org/10.5713/ajas.2014.r.02 .
2. van Kempen TATG, Baker DH, van Heugten E. Nitrogen losses in metabolism trials. J Anim Sci 2003;81:2649–50. https://doi.org/10.2527/2003.81102649x .
3. Maison T, Liu Y, Stein HH. Digestibility of energy and detergent fiber and digestible and metabolizable energy values in canola meal, 00-rapeseed meal, and 00-rapeseed expellers fed to growing pigs. J Anim Sci 2015;93:652–60. https://doi.org/10.2527/jas.2014-7792 .
4. Kong C, Kim KH, Ji SY, Kim BG. Energy concentration and phosphorus digestibility in meat meal, fish meal, and soybean meal fed to pigs. Anim Biosci 2021;34:1822–8. https://doi.org/10.5713/ab.21.0102 .
5. Sung JY, Kim BG. Prediction equations for digestible and metabolizable energy concentrations in feed ingredients and diets for pigs based on chemical composition. Anim Biosci 2021;34:306–11. https://doi.org/10.5713/ajas.20.0293 .
6. Nelson DW. Gaseous losses of nitrogen other than through denitrification. In : Stevenson FJ, ed. Nitrogen in agricultural soils Madison, WI, USA: ASA; 1982. p. 327–63.
7. Canh TT, Verstegen MW, Aarnink AJ, Schrama JW. Influence of dietary factors on nitrogen partitioning and composition of urine and feces of fattening pigs. J Anim Sci 1997;75:700–6. https://doi.org/10.2527/1997.753700x .
8. Knowlton KF, McGilliard ML, Zhao Z, Hall KG, Mims W, Hanigan MD. Effective nitrogen preservation during urine collection from Holstein heifers fed diets with high or low protein content. J Dairy Sci 2010;93:323–9. https://doi.org/10.3168/jds.2009-2600 .
9. Kerr BJ, Jha R, Urriola PE, Shurson GC. Nutrient composition, digestible and metabolizable energy content, and prediction of energy for animal protein byproducts in finishing pig diets. J Anim Sci 2017;95:2614–26. https://doi.org/10.2527/jas.2016.1165 .
10. Li PL, Chen YF, Lyu ZQ, et al. Concentration of metabolizable energy and digestibility of amino acids in Chinese produced dehulled double-low rapeseed expellers and non-dehulled double-low rapeseed co-products fed to growing-finishing pigs. Anim Feed Sci Technol 2017;234:10–9. https://doi.org/10.1016/j.anifeedsci.2017.09.001 .
11. Zhang Z, Liu Z, Zhang S, Lai C, Ma D, Huang C. Effect of inclusion level of corn germ meal on the digestible and metabolizable energy and evaluation of ileal AA digestibility of corn germ meal fed to growing pigs. J Anim Sci 2019;97:768–78. https://doi.org/10.1093/jas/sky469 .
12. Horwitz W, Latimer GW. Official methods of analysis of AOAC International 21st edth ed. Gaithersburg, MD, USA: AOAC International; 2019.
13. Kim BG, Petersen GI, Hinson RB, Allee GL, Stein HH. Amino acid digestibility and energy concentration in a novel source of high-protein distillers dried grains and their effects on growth performance of pigs. J Anim Sci 2009;87:4013–21. https://doi.org/10.2527/jas.2009-2060 .
14. Park CS, Aderibigbe AS, Ragland D, Adeola O. Digestible and metabolizable energy concentrations and amino acid digestibility of dried yeast and soybean meal for growing pigs. J Anim Sci 2021. 99skaa385. https://doi.org/10.1093/jas/skaa385 .
15. Christianson CB, Carmona G, Klein MO, Howard RG. Impact on ammonia volatilization losses of mixing KCl of high pH with urea. Fertil Res 1994;40:89–92. https://doi.org/10.1007/BF00750092 .
16. Ghosh BC, Bhat R. Environmental hazards of nitrogen loading in wetland rice fields. Environ Pollut 1998;102:123–6. https://doi.org/10.1016/S0269-7491(98)80024-9 .
17. Mathews CK, van Holde KE, Appling DR, Anthony-Cahill SJ. Biochemistry 4th edth ed. Upper Saddle River, NJ, USA: Pearson education, Inc; 2013.
18. Farrell DJ. Metabolizable energy in feeding systems for pigs and poultry. Proc Aust Soc Anim Prod 1978;12:62–7.
19. Fuller MF, Cadenhead A. The preservation of faeces and urine to prevent losses of energy and nitrogen during metabolism experiments. In : Blaxter KL, Kielanowski J, Thorbek G, eds. Energy metabolism of farm animals Newcastle-upon-Tyne, England: Oriel Press; 1967. p. 455–60.
20. Adeola O. Digestion and balance techniques in pigs. In : Lewis AJ, Southern LL, eds. Swine nutrition Washington, DC, USA: CRC Press; 2001. p. 903–16.
21. Low AG. Role of dietary fibre in pig diets. In : Haresign W, Cole DJA, eds. Recent advances in animal nutrition London, UK: Butterworths; 1985. p. 87.
22. Canh TT, Aarnink AJA, Schutte JB, Sutton A, Langhout DJ, Verstegen MWA. Dietary protein affects nitrogen excretion and ammonia emission from slurry of growing–finishing pigs. Livest Prod Sci 1998;56:181–91. https://doi.org/10.1016/S0301-6226(98)00156-0 .
23. Valaja J, Siljander-Rasi H. Effect of dietary crude protein and energy content on nitrogen utilisation, water intake and urinary output in growing pigs. Agric Food Sci 1998;7:381–90. https://doi.org/10.23986/afsci.72869 .
24. Portejoie S, Dourmad JY, Martinez J, Lebreton Y. Effect of lowering dietary crude protein on nitrogen excretion, manure composition and ammonia emission from fattening pigs. Livest Prod Sci 2004;91:45–55. https://doi.org/10.1016/j.livprodsci.2004.06.013 .
25. Figueroa J, Lewis A, Miller PS. Nitrogen balance and growth trials with pigs fed low-crude protein, amino acid-supplemented diets. Nebraska Swine Reports 2000;100:25–8.
26. Grandhi RR. Effect of dietary ideal amino acid ratios, and supplemental carbohydrase in hulless-bar ley-based diets on pig performance and nitrogen excretion in manure. Can J Anim Sci 2001;81:125–32. https://doi.org/10.4141/A00-064 .

Article information Continued

Figure 1

A broken-line analysis of nitrogen losses for 5 d at 6 urine pH values acquired by different inclusion rates of 6 N hydrochloric acid into the 100 mL of each urine sample (Exp. 2). Each data point represents the least squares mean of 3 observations. The amount of nitrogen was 0.53 g on average in 100 mL urine at the beginning. A one-slope broken-line model of nitrogen losses for 5 d indicates that a maximum pH of 5.12 is needed to prevent nitrogen volatilization from the pig urine. The break point was estimated based on following equation: Y = 0.04×(X–5.12)–0.01 where X is more than 5.12 (standard error = 0.712 and p<0.01) in Exp. 2.

Figure 2

Urine pH changes by the addition of 6 N hydrochloric acid (HCl) to urine (Exp. 3). Exponential models were developed for each urinary nitrogen concentration: Y = −9.28+9.75×(1+e−0.81×X), with p<0.001 for nitrogen 0.12 g/100 mL; Y = −10.16+10.41×(1+e−0.38×X), with p<0.001 for nitrogen 0.26 g/100 mL; Y = −10.26+10.57×(1+e−0.30×X), with p<0.001 for nitrogen 0.53 g/100 mL; Y = −10.80+10.89×(1+e−0.24×X), with p<0.001 for nitrogen 0.60 g/100 mL; Y = −10.98+10.93×(1+e−0.18×X), with p<0.001 for nitrogen 0.94 g/100 mL. The required concentrations of 6 N HCl to achieve a urine pH less than 5.12 were 0.92, 2.01, 2.61, 3.24, and 4.20 mL/100 mL for urinary nitrogen concentrations of 0.12, 0.26, 0.53, 0.60, and 0.94 g/100 mL, respectively.

Figure 3

Minimum amounts of 6 N hydrochloric acid (HCl) required for nitrogen preservation for 100 mL pig urine based on urine nitrogen concentration (g/100 mL; Exp. 3). The plotted data were based on the x-axis values for pH = 5.12 and urinary nitrogen concentrations presented in Figure 2.

Table 1

Effects of hydrochloric acid supplementation to pig urine on nitrogen and gross energy (GE) contents at room temperature for 10 days, Exp. 11),2)

Item Hydrochloric acid (d) Distilled water (d) SEM p-value3)



0 2 4 6 8 10 0 2 4 6 8 10 HCl L Q HCl×L HCl×Q
Urine weight (g) 248 238 231 223 214 199 251 237 228 222 211 203 2 0.868 <0.001 0.970 0.927 0.014
Nitrogen concentration (%) 0.574 0.602 0.626 0.643 0.668 0.707 0.563 0.478 0.442 0.403 0.366 0.326 0.071 <0.001 0.002 0.198 <0.001 0.451
Nitrogen amount (g) 1.43 1.44 1.45 1.43 1.43 1.40 1.41 1.13 1.01 0.90 0.77 0.66 0.16 <0.001 <0.001 0.234 <0.001 0.051
Corrected nitrogen4) (%) 0.574 0.571 0.575 0.574 0.572 0.571 0.563 0.452 0.404 0.359 0.311 0.265 0.064 <0.001 <0.001 0.124 <0.001 0.130
GE concentration (kcal/kg) 70.8 77.2 74.1 77.7 83.7 92.4 48.3 45.7 42.6 44.2 46.5 48.9 7.3 <0.001 <0.001 0.197 0.001 0.800
GE amount (kcal) 17.6 18.4 17.1 17.3 18.0 18.4 12.2 10.8 9.7 9.8 9.8 9.9 1.6 <0.001 0.199 0.399 0.053 0.363
Corrected GE5) (kcal/kg) 70.8 73.2 68.1 69.3 71.7 74.6 48.3 43.2 39.0 39.4 39.6 39.7 6.6 <0.001 0.259 0.276 0.042 0.564
1)

Each least squares mean represents 5 observations.

2)

The room temperature for 10 d ranged from 18.3°C to 25.9°C.

3)

HCl, supplementation of 6 N HCl in urine; L, linear effects of day; Q, quadratic effects of day; HCl×L, the interaction between supplementation of 6 N HCl in urine and linear effects of day; HCl×Q, the interaction between supplementation of 6 N HCl in urine and quadratic effects of day.

4)

Corrected nitrogen concentration in urine at d 2, 4, 6, 8, and 10 (%) = weight of urine at a specific day (g)×nitrogen concentration in urine at a specific day (%)÷weight of urine at d 0 (g).

5)

Corrected GE concentration in urine at d 2, 4, 6, 8, and 10 (%) = weight of urine at a specific day (kg)×GE concentration in urine at a specific day (kcal/kg)÷weight of urine at d 0 (kg).

Table 2

The ranges of daily urinary nitrogen excretion (g/d) according to body weight (BW) and required amounts of 6 N hydrochloric acid (HCl) for urinary nitrogen preservation, Exp. 31)

BW (kg) Number of observations Maximum nitrogen (g/d) Minimum nitrogen (g/d) Mean (g/d) Required 6 N HCl2) (mL/d)
0 to 20 88 12.6 2.1 4.6 48
20 to 40 101 29.6 2.8 15.1 113
40 to 60 270 49.6 5.3 20.0 190
60 to 80 225 59.1 7.0 23.2 227
80 to 110 121 62.7 7.1 27.9 240
1)

Data were from 9 published experiments and 8 unpublished experiments that measured daily urinary nitrogen excretion.

2)

The amount of 6 N HCl required for urinary nitrogen preservation was calculated by multiplying the maximum quantity of daily nitrogen excretion for each BW range with the required amount of 6 N HCl for nitrogen preservation per gram of urinary nitrogen (3.83 mL/g) which is the slope in Figure 3.