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* Prairie Swine Centre Inc., Saskatoon, SK, Canada S7H 5N9 and
and
Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, SK, Canada S7N 5A8
3 Correspondence:
P.O. Box 21057, 2105 8th St. E. (phone: 306-373-9922; fax: 306-955-2510; E-mail:
ruurd{at}sask.usask.ca).
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Key Words: Excretion • Fibers • Nitrogen • Pigs • Protein • Urea
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Reducing dietary protein while balancing for AA reduces urinary and total N excretion (Dourmad et al., 1993; Canh et al., 1998a). Fiber may shift N excretion from urea in urine to bacterial protein in feces (Morgan and Whittemore, 1988). Oathulls are used commonly in sow diets as a fiber source, and are digested to a small extent in the large intestine (Moore et al., 1986).
Excess dietary N is converted into urea by the liver and excreted in urine by the kidney. Plasma urea concentration is affected by dietary protein quality and quantity (Eggum, 1970) and has therefore been used to predict protein quality (Orok and Bowland, 1975). Plasma urea can also predict AA requirements (Coma et al., 1995). Increased plasma urea coincides with increased urinary urea excretion (Brown and Cline, 1974), and excreted urinary N is mostly urea (Patience and Chaplin, 1997). Plasma urea may thus be related to urinary N excretion, and thereby enable the prediction of urinary N excretion (Herrmann and Schneider, 1981).
The objectives of this study were: 1) to determine if dietary protein reduction or oathull fiber inclusion would reduce urinary N excretion in grower pigs, 2) to determine if plasma urea could predict urinary N excretion among diets differing in protein and fiber content with an expected range in N excretion patterns, and 3) to determine the postprandial time point to sample blood for the best prediction.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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). Oathulls, an ingredient high in fiber, especially insoluble fiber in the form of cellulose, were included as a fiber source in the high-fiber diets. Experimental diets were formulated based on apparent ileal digestible AA and DE to 3.25 Mcal DE/kg, 2.2 g of digestible lysine/Mcal DE for low- and medium-protein diets, and 2.4 g/Mcal DE for high-protein diets (Table 2) using feed formulation software (Version 7, Brill Corporation, Norcross, GA). Diets were supplemented with synthetic AA to balance to an ideal AA ratio and fortified to exceed vitamins and minerals requirements (NRC, 1998).
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During the 5-d collection, representative feed samples were collected. Feces were collected, pooled, and stored at -20°C. Urine was collected twice daily, weighed, and a 5% aliquot was stored at -20°C. Twenty milliliters of 12 N HCl was added to the collection container at the start of each collection to prevent volatilization of urinary N. After the collection, feces and urine samples were thawed, homogenized, and subsampled, and feces were then freeze-dried.
Catheterization and Blood Sampling
On d 10 or 11, pigs were catheterized in the vena cava using 60 cm of clear vinyl tubing (Dural Plastics and Engineering, Auburn, NSW, Australia; 1.5 o.d., 1.0 mm i.d.), according to procedures described by Kingsbury and Rawlings (1993) with modifications. Catheters were inserted under diazepam (0.5 mg/kg BW; Valium, Hoffmann-LaRoche Ltd., Etobicoke, ON, Canada) and ketamine (5 mg/kg BW; M.T.C. Pharmaceuticals, Cambridge, ON, Canada) anesthesia, which were administered through the ear vein. Catheters were maintained functional by flushing with heparinized saline (10 U/mL) once daily. Pigs recovered quickly from the catheterization procedure and were fed the regular allowance before and after the procedure. Out of 36 attempts, 25 catheters were installed successfully; of those, 24 remained functional throughout the entire collection. On d 16 and 19, five blood samples were collected from pigs with catheters in 2-h intervals starting immediately before the morning feeding for a total of 10 blood samples per pig. For pigs without catheters, blood samples were collected before the morning feeding and 4 h postfeeding via jugular venipuncture on d 19. Blood was centrifuged, and plasma was frozen at -20°C until analyses.
Chemical Analyses
Feed and freeze-dried fecal samples were ground through a 1-mm screen in a Retsch mill (Brinkman Instruments, Rexdale, ON, Canada). Chemical analyses were conducted in duplicate. Feed, fecal, and urinary samples were analyzed for N by combustion (method 968.06; AOAC, 1990) using a Leco protein/nitrogen determinator (model FP-528, Leco Corp., St. Joseph, MI). Dry matter content of feed and feces was determined by drying at 135°C in an airflow-type oven for 2 h (method 930.15; AOAC, 1990). Chromic oxide content was analyzed in feed and feces (Fenton and Fenton, 1979) with a Pharmacia LKB-Ultrospec III spectrophotometer (model 80-2097-62; Cambridge, England) at 440 nm after ashing at 450°C overnight. Gross energy in feed and feces was measured in an adiabatic bomb calorimeter (model 1281, Parr Instrument Co., Moline, IL).
Feed was analyzed for crude fiber (method 978.10; AOAC, 1990) and ether extract content (method 920.39; AOAC, 1990). The ADF and NDF contents were determined using an Ankom200 fiber analyzer (Ankom Technology Co., Fairport, MI). Feed samples were analyzed for AA (method 994.12; AOAC, 1995). Methionine was determined as methionine sulfone and cystine as cysteic acid after oxidation with performic acid. Tryptophan was determined after alkaline hydrolysis with lithium hydroxide by reversed-phase HPLC. Plasma urea was analyzed using the Abbott Spectrum urea nitrogen test (Series II, Abbot Laboratories, Dallas, TX). Apparent total-tract digestibility of N and energy, N retention, and DE were calculated using chromic oxide concentration in diets and feces, using the indicator method.
Statistical Analyses
The individual pig was considered the experimental unit. Variables were analyzed using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). The statistical model included effects for dietary treatment (protein, fiber, and protein x fiber interaction) and group. Means comparisons were performed using the probability of difference. Linear and quadratic effects of dietary protein were examined with orthogonal contrasts. The degree of association between postprandial plasma urea and urinary N excretion was determined using Pearson’s correlation coefficients. Regression analysis was used to predict daily urinary N excretion as a function of plasma urea. Repeated-measures analysis was used to evaluate the effect of time after feeding on plasma urea. Values are reported as least squares means.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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). Total lysine content increased with increasing dietary protein and was higher for high-fiber compared to low-fiber diets.
Nitrogen Balance
The study was designed to result in different levels of protein intake; thus, N intake differed among dietary protein treatments (Table 3). Fecal, urinary, and total N excretion (g/d) decreased linearly with decreasing dietary protein (Table 3; P < 0.001). For low-protein compared to high-protein diets, fecal, urinary, and total N excretion was reduced by 23, 48, and 40%, respectively. The ratio of urinary to fecal N decreased linearly with decreasing dietary protein (P < 0.001), resulting in a 32% reduced ratio for low-protein compared to high-protein diets. Nitrogen retention was reduced linearly with decreasing dietary protein (P < 0.001), resulting in a 17% reduction for low-protein compared to high-protein diets.
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; P < 0.001). Fecal N excretion was not affected by dietary protein (P > 0.10). Urinary N was reduced 9% units for the low-protein compared to high-protein diet. Total N excretion was decreased 8% units and N retention was increased 8% units for the low-fiber compared to high-fiber diet.
Nitrogen intake was 3.5% higher for high-fiber compared to low-fiber diets (Table 3; P < 0.001). In grams per day, fecal N excretion was 9% higher (P < 0.10) and N retention was 7% higher (P < 0.10) for high-fiber compared to low-fiber diets, which may be due to differences in N intake. To determine possible fiber effects, N variables are better expressed as % of N intake. With the correction, fiber did not affect fecal, urinary, or total N excretion or N retention (P > 0.10).
Energy and Nitrogen Digestibility and Animal Performance
Although diets were formulated to an equal DE content (3.25 Mcal/kg), differences among diets were measured (Table 4). Digestible energy content was 4.3% lower for the low-protein, high-fiber diet and 6.8% higher for the low-protein, low-fiber diet compared to the calculated DE. Medium- and high-protein diets for both fiber levels were close to calculated values. A quadratic response (P < 0.05) of DE to dietary protein was observed. Digestible energy content was affected by fiber (P < 0.001) with a protein x fiber interaction (P < 0.001). Overall, DE content was 4.6% higher for low-fiber compared to high-fiber diets (P < 0.001). Specifically, for low-fiber compared to high-fiber, DE content was 12% higher for low-protein (P < 0.001), and similar for medium- and high-protein diets (P > 0.10).
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; P < 0.05) with decreasing dietary protein, resulting in 1.1% units higher energy digestibility for low-protein compared to high-protein diets. Energy digestibility was affected by fiber (P < 0.001), resulting in 4.3% units lower energy digestibility for high-fiber compared to low-fiber diets, with a trend for a protein x fiber interaction (P < 0.10). Energy digestibility was similar among high-fiber diets, but 2.0% units higher for low-protein compared to medium- and high-protein within the low-fiber diets. Nitrogen digestibility ranged from 81.2 to 83.9% among treatments, but was not affected by dietary protein or fiber (P > 0.10). Dietary treatments did not affect ADG or feed efficiency (P > 0.10).
Plasma Urea
On d 19 with 42 pigs and on d 16 with 24 pigs, reduced dietary protein decreased plasma urea linearly before feeding and 4 h after feeding (Table 4; P < 0.001). On d 19, plasma urea was reduced 40 and 47% for low-protein compared to high-protein diets before feeding and 4 h after feeding, respectively. Similarly, plasma urea on d 16 was reduced 41 and 53% for low-protein compared to high-protein diets before feeding and 4 h after feeding, respectively. Fiber did not affect plasma urea on d 16 and 19 (P > 0.10).
Postprandial plasma urea concentration was affected by dietary protein, time postprandial, and a protein x time interaction (Figure 1; P < 0.001). Plasma urea did not differ between d 16 and 19 (P > 0.10). Plasma urea increased postprandially for high- and medium-protein diets (P < 0.05) and peaked 4 h after feeding at 30% above preprandial concentrations before declining toward preprandial concentrations by 8 h after feeding. Plasma urea did not increase postprandially for the low-protein diets (P > 0.10).
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; R2 = 0.39). Preprandial plasma urea ranged from 3.4 to 7.4 mmol/L for high-protein, from 2.3 to 4.8 for medium-protein, and from 1.9 to 4.7 for low-protein diets.
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). Plasma urea at 4 h postfeeding ranged from 3.9 to 8.2 mmol/L for high-protein, from 4.0 to 5.9 for medium-protein, and from 2.4 to 4.3 for low-protein diets. Daily urinary N excretion (g/d) was predicted by 3.03 + 2.14 x plasma urea concentration (mmol/L) at 4 h postfeeding on d 19 (Figure 3; R2 = 0.66), with a 95% confidence interval from 0.45 to 5.61 for the intercept and from 1.65 to 2.63 for the slope. The standard error of prediction was 2.38 g/d and the residual standard deviation (RSD) was 2.35 g/d. The 95% confidence interval ranged from 13.0 to 14.5 g/d urinary N excretion and the 95% prediction interval ranged from 8.9 to 18.6 g/d, both for the average plasma urea concentration of 5.0 mmol/L.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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The reduction of dietary protein decreased fecal, urinary, and total N excretion in the present study, similar to the findings of Dourmad et al. (1993). In contrast, a reduction of dietary protein decreased urinary and total N excretion, but not fecal N excretion in other studies (Gatel and Grosjean, 1992; Canh et al., 1998a) because N digestibility decreased with reduced dietary protein and compensated for N intake differences. In the present study, N digestibility did not differ among dietary protein levels, indicating that differences in fecal N excretion were due to changes in N intake. For each percentage of reduction in dietary protein, urinary N excretion was reduced 8% (1.4 g/d), fecal N excretion was reduced 4% (0.3 g/d), and total N excretion was reduced 7% (1.7 g/d), indicating that urinary and total N excretion were reduced relatively more than fecal N excretion. Thus, the concept was supported that low-protein diets supplemented with AA will reduce N excretion, and especially urinary N excretion. Moreover, the dietary treatments resulted in a large range in urinary N excretion among pigs (6.8 to 22.0 g/d), so that the relation of daily urinary N excretion to plasma urea could be tested properly.
In the present study, N retention was reduced for the low- and medium-protein compared to the high-protein diets, similar to results found by Kerr and Easter (1995) and Lenis et al. (1999). In contrast, reduced dietary protein only reduced N retention numerically in other studies (Dourmad et al., 1993; Canh et al., 1998a). A reduced N retention may be attributed to AA imbalances (Kerr and Easter, 1995), different digestible AA levels, or different efficiencies of AA utilization among diets. Deviations of actual ileal digestible AA content from calculated values may have affected the dietary AA balance; however, total AA analysis indicated that dietary AA content was balanced according to an ideal protein ratio. In the present study, pigs had restricted access to feed; thus, maximal lean gain was not achieved because lysine intake was limited. Based on total AA analysis and calculated digestible lysine levels, the 1.1 g/d higher intake of digestible lysine for the high-protein diet caused 2 g/d of the 4 g/d differences in N retention between the high-protein and low-protein diets, indicating that different levels of digestible lysine were partly responsible for the linear reduction in N retention with reduced dietary protein. Pigs fed the high-fiber diets, which had a slightly higher lysine content than the low-fiber diets, responded with an increased N retention, suggesting that lysine was indeed the limiting nutrient for N retention. The remainder of the reduced N retention of low-protein diets may be explained by the theory that efficiency of utilization of synthetic AA for protein deposition is lower when pigs are fed infrequently (Batterham et al., 1984; Partridge et al., 1985), although this theory was disputed recently for pigs fed at least two meals per day (Le Bellego et al., 2001), and may thus not be valid for the conditions of the present study.
Reduction of dietary protein did not affect ADG and feed efficiency, as found by Dourmad et al. (1993) and Canh et al. (1998a). Tuitoek et al. (1997) reduced dietary protein from 16.6 to 13% without negative effects on ADG, ADFI, feed efficiency, and carcass characteristics. Although ADG and feed efficiency were not affected by dietary protein in the present study, N retention was reduced for low- and medium-protein compared to high-protein diet. The discrepancy between ADG and N retention can be attributed in part to a higher fat deposition for pigs fed low- and medium-protein diets because of energy sparing due to low protein content (Noblet et al., 1987). In addition, ADG and feed efficiency coincided numerically with patterns in N retention among protein diets. Considering that ADG and feed efficiency were measured over a short period, differences may have been detected if performance was measured for a longer period or if more pigs were included in the study.
Oathulls are obtained as byproduct during dehulling of oats to produce oat groats, and represent 25% of the intact grain. Oathulls have a high concentration of fiber consisting primarily of cellulose and lignin (Bach Knudsen, 1997). Inclusion of oathulls in the diets did not affect N retention, digestibility, or excretion patterns (% of N intake), or plasma urea in the present study. Within protein level, low- and high-fiber diets were formulated to an equal digestible nutrient profile. Thus, whereas the inclusion of oathulls was accomplished by substituting it for other ingredients, the substitution was primarily accomplished by alterations in energy-contributing fractions (i.e., corn starch and canola oil), rather than in protein-contributing fractions. Therefore, an altered digestible AA profile was mostly avoided and the substitution itself was not a contributor to the observed N responses. The higher fecal N excretion and N retention (g/d) for high- compared to low-fiber diets were caused by a higher N intake, which was probably due to a higher-than-average protein content in the specific batch of oathulls. Fermentable fiber will shift N excretion from urea in urine to bacterial protein in feces (Canh et al., 1997; 1998b,c). The lack of a shift in the present study therefore suggests that fiber sources high in insoluble fiber, such as oathulls, may not be fermented in the hindgut of grower pigs.
Oathulls may not have affected N excretion in urine for three reasons. First, 5% of oathulls were included in the high-fiber diet, which may be an inclusion level too low to affect N excretion patterns. Second, because of the high cellulose content of oathulls (Bach Knudsen, 1997), an adaptation period longer than 2 wk may be required to stimulate hindgut fermentation (Gargallo and Zimmerman, 1981; Longland et al., 1993). Finisher pigs may also respond better than grower pigs to ingredients high in cellulose (Kennelly and Aherne, 1980a). Third, oathulls have a high degree of lignification (Bach Knudsen, 1997), which may result in a resistance to bacterial fermentation in the hindgut and, therefore, a low energy digestibility in pigs (Stanogias and Pearce, 1985; Moore et al., 1986).
In the present study, inclusion of oathulls in the diets reduced energy digestibility and measured DE content, but did not affect N digestibility, similar to the findings of Kennelly and Aherne (1980b), Stanogias and Pearce (1985), and Moore et al. (1986). A protein x fiber interaction affected measured DE content and tended to affect energy digestibility since energy digestibility was 2% units higher for low-protein compared to medium- and high-protein within the low-fiber diets, but similar among dietary protein within the high-fiber diets. The higher content of cornstarch and barley and the lower content of wheat in the low-protein compared to the medium- and high-protein, low-fiber diets may together have resulted in a higher-than-expected energy digestibility for the low-protein, low-fiber diet, suggesting that the changes in carbohydrate fractions may have caused the interaction.
Urea is the main nitrogenous end product from AA catabolism in pigs, and is synthesized in the liver and excreted by the kidney in urine. Plasma urea concentrations depend both on quality and quantity of dietary protein (Eggum, 1970), and excess dietary protein or imbalances in dietary AA will increase plasma urea. In the present study, dietary protein, time postfeeding, and a protein x time interaction affected plasma urea concentrations. Plasma urea decreased linearly with a reduction of dietary protein either before feeding or 4 h after feeding, similar to the findings of Lopez et al. (1994) and Lenis et al. (1999). Overall, plasma urea increased postprandially and peaked at 4 h after feeding (Eggum, 1970; Herrmann and Schneider 1981; Malmlof et al., 1989). Similarly, pigs fed twice daily had a peak in plasma urea at 3.6 h postfeeding, and this peak was 32% higher than the prefeeding concentration (Cai et al., 1994). A time effect was detected for high- and medium-protein diets, but not for the low-protein diet, causing the protein x time interaction effect on plasma urea in the present study. A low plasma urea concentration reflects a high quality of dietary protein (Eggum, 1970); thus, the low plasma urea for the low-protein diets indicated a better balance of ingested AA. In addition, the lack of increase in plasma urea postprandially for the low-protein diet indicated that AA degradation was not altered postprandially, suggesting that the amount of excess AA not used for protein synthesis was negligible. Plasma urea was not affected by dietary oathulls, in agreement with urinary N excretion and other N balance data that were also not affected by dietary oathulls, a further indication that the oathull fiber was not fermented in the hindgut.
Accurate measurement of protein deposition and DE intake on swine farms throughout the grower-finisher phase is important to establish accurate nutrient requirements for grower-finisher pigs (NRC 1998) to enable precision diet formulation to meet AA requirements and thereby reduce N excretion. To measure protein deposition rate, one approach is to use real-time ultrasound, and fat-free lean content of pigs can be predicted over time (R2 = 0.69 to 0.76, RSD = 2.7 to 3.0; i.e., with a mean of 47.7 kg equals 6.3% of the mean; Cisneros et al., 1996). Another approach to measuring protein deposition may be to estimate N retention (g/d) by measuring voluntary feed intake and N content in feed, measuring fecal N excretion using the indicator method by using intrinsic acid-insoluble ash as the marker (McCarthy et al., 1974), and by estimating urinary N excretion using plasma urea as a predictor (Herrmann and Schneider, 1983). Using the latter approach, an indicator for daily urinary N excretion should be used since daily urine output is difficult to collect on swine farms. Blood is more easily collected; thus, plasma urea may be a suitable candidate for an indicator.
Plasma urea concentration was related to daily urinary N excretion, confirming Brown and Cline (1974), who suggested a relationship between plasma urea and daily urea excretion in urine. Urea is the major nitrogenous compound in urine (Patience and Chaplin, 1997); thus, the relationship could be extrapolated to total N excretion in urine. Postprandial sampling time affected plasma urea and prediction of daily urinary N excretion by plasma urea; thus, choice of sampling time postprandial is crucial, and the best prediction of daily urinary N excretion was obtained using plasma urea at 4 h postfeeding in grower pigs fed a meal twice per day. The R2 for the regression (R2 = 0.66) may suggest that daily urinary N excretion and thus protein deposition can be predicted from plasma urea. However, the 95% prediction interval for a specific plasma urea indicates that too much variation exists to predict daily urinary N excretion accurately, similar to Herrmann and Schneider (1983). The 9.7 g/d interval in N excretion will result in a 61 g/d range in predicted protein deposition.
In the present study, and in Zervas and Zijlstra (2002), the best prediction of daily urinary N excretion was with plasma urea at 4 h postfeeding. Data from both studies were pooled (n = 78) and additional regression models were developed. Using more observations, urinary N excretion (g/d) was predicted by 1.68 + 2.30 x plasma urea concentration (mmol/L; R2 = 0.71) using blood collected at 4 h postfeeding. The RSD was 2.20 g/d and the 95% prediction interval ranged from 8.7 to 17.6 g/d for the average plasma urea concentration of 5.0 mmol/L, indicating that although R2 increased and RSD decreased by including more observations, the prediction of daily urinary N excretion by plasma urea remained inaccurate. Plasma urea could thus be used to detect differences among dietary protein treatments in the present study, but should not be used to predict protein deposition rate of grower pigs with restricted access to feed.
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Footnotes |
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2 Supported by project funding from the Agriculture and Agri-Food Canada/Natural Sciences and Engineering Research Council of Canada-Research Partnership Program and the Alberta Agriculture Research Institute, and program funding from Saskatchewan Agriculture & Food and the pork producers of Saskatchewan, Manitoba, and Alberta. S. Zervas received a scholarship from the State Scholarship Foundation of Greece. The authors acknowledge Degussa AG for amino acid assays.
Received for publication August 21, 2001. Accepted for publication August 5, 2002.
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
), or high-protein diet (
).