Effects of dietary protein and oathull fiber on nitrogen excretion patterns
and postprandial plasma urea profiles in grower pigs1,2
S. Zervas*† and R. T. Zijlstra*3
*Prairie Swine Centre Inc., Saskatoon, SK, Canada S7H 5N9 and
†Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, SK, Canada S7N 5A8
ABSTRACT: 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. Three
dietary protein concentrations (high, 19.7; medium,
16.9; low, 13.8%) and two fiber levels (high, 5.0; low,
3.6% crude fiber) were tested in a 3 × 2 factorial arrangement. Diets (wheat, barley, soybean meal; oathulls as
fiber source) were formulated to 3.25 Mcal of digestible
energy (DE)/kg and 2.2 g of digestible lysine/Mcal DE
for low- and medium-protein diets, and 2.4 g/Mcal of DE
for high-protein diets, and supplemented with lysine,
methionine, tryptophan, threonine, isoleucine, or valine to meet an ideal amino acid profile. Pigs (32 ± 3.4
kg; n = 42) were housed in metabolism crates for 19 d.
On d 10 or 11, catheters were installed by cranial vena
cava venipuncture. Daily feeding allowance was adjusted to 3× maintenance (3 × 110 kcal DE/kg body
weight0.75), and was fed in two equal meals. Feces and
urine were collected from d 15 to 19. Five blood samples
were collected in 2-h intervals on d 16 and 19. Fecal,
urinary, and total N excretion was reduced linearly
with a reduction of dietary protein (P < 0.001); the reduction was greater for urinary (48%) and total N excretion (40%) than for fecal N excretion (23%). Similarly,
the ratio of urinary to fecal N was reduced linearly with
a reduction of dietary protein (P < 0.001). Retention
of N (g/d) was reduced linearly, but N retention as a
percentage of N intake was increased linearly with a
reduction of dietary protein (P < 0.001). The addition of
oathulls did not affect N excretion patterns and plasma
urea (P > 0.10). Dietary treatments did not affect average daily gain or feed efficiency (P > 0.10). A dietary
protein × time interaction affected plasma urea (P <
0.001). For medium- and high-protein diets, plasma
urea increased postprandially, peaking 4 h after feeding, and then decreased toward preprandial levels (P
< 0.05). Plasma urea did not alter postprandially for
the low-protein diet (P > 0.10). Urinary N excretion (g/
d) was predicted by 3.03 + 2.14 × plasma urea concentration (mmol/L) at 4 h after feeding (R2 = 0.66). Plasma
urea concentration is indicative of daily urinary N excretion and reduction of dietary protein is effective to
reduce total and urinary N excretion.
Key Words: Excretion, Fibers, Nitrogen, Pigs, Protein, Urea
2002 American Society of Animal Science. All rights reserved.
J. Anim. Sci. 2002. 80:3238–3246
Introduction
1
Presented in part at the ASAS Midwestern Section Mtg., Des
Moines, IA, March 13 to 15, 2000 (J. Anim. Sci. 78 [Suppl. 1]:66).
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.
3
Correspondence: P.O. Box 21057, 2105 8th St. E. (phone: 306373-9922; fax: 306-955-2510; E-mail: [email protected]).
Received August 21, 2001.
Accepted August 5, 2002.
Nitrogen originating from swine manure may impact
the environment inside and outside the swine barn. Dietary manipulations may alleviate the environmental
impact of swine production by manipulating N excretion,
and these manipulations may thus enhance sustainable
swine production. Emitted ammonia, a volatile N waste,
originates mainly from urea excreted in urine, and dietary manipulations reducing urinary N excretion will
thus reduce ammonia emission (Canh et al., 1998a).
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
3238
Protein affects plasma urea and urinary nitrogen
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.
Materials and Methods
Experimental Protocol
The animal protocol was approved by the University
of Saskatchewan Committee on Animal Care and Supply
(protocol # 990041) and followed principles established
by the Canadian Council on Animal Care (CCAC, 1993).
Three dietary protein concentrations (high, 19.7; medium, 16.9; and low, 13.8%) and two fiber levels (high,
5.0; low, 3.6% crude fiber) were tested in a 3 × 2 factorial
arrangement for a total of six treatments. The main dietary ingredients were barley, wheat, soybean meal, corn
starch, canola oil, and synthetic AA; chromic oxide was
included as an indigestible marker (Table 1). 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 lowand medium-protein diets, and 2.4 g/Mcal DE for highprotein 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).
A total of 42 crossbred barrows (Camborough-15 × Canabrid, Pig Improvement Canada, Acme, AB, Canada;
initial BW 32 ± 3.4 kg) were selected in three groups
(two groups of 12 and one group of 18), for a total of
seven observations per treatment. The 19-d experimental period consisted of a 14-d adaptation to diets and
metabolism crates and a 5-d collection of feces and urine.
Pigs were housed individually in confinement-type metabolism crates (0.71 × 1.83 m), which allowed separate
collection of urine and feces, in an environmentally-con-
3239
trolled room with an average temperature of 21°C. Lights
were on from 0700 to 1900. Diets were fed in wet-mash
form, in a 1:1 water:mash ratio. Daily feed allowance
was adjusted to 3× maintenance (3 × 110 kcal DE/kg
BW0.75; 1.57 kg feed/d; NRC, 1998), which was fed in two
equal meals at 0800 and 1600. Water was supplied ad
libitum through a nipple drinker.
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
3240
Zervas and Zijlstra
Table 1. Ingredient composition of experimental diets (low, medium,
and high protein), as-fed basis
Low-fiber diet
Ingredient, %
Barley
Wheat
Soybean meal
Cornstarch
Oathulls
Canola oil
Mineral premixa
Vitamin premixb
Salt
Chromic oxide
Limestone
Dicalcium phosphate
L-Lysine-HCl
L-Threonine
DL-Methionine
L-Isoleucine
L-Valine
L-Tryptophan
High-fiber diet
Low
Medium
High
Low
Medium
High
49.4
21.8
10.0
13.8
—
0.5
0.5
0.5
0.4
0.4
1.130
0.897
0.363
0.158
0.094
0.020
0.035
0.020
40.5
28.3
17.2
9.9
—
—
0.5
0.5
0.4
0.4
1.400
0.740
0.143
0.055
0.012
—
—
—
40.0
25.7
24.8
5.6
—
—
0.5
0.5
0.4
0.4
1.400
0.650
—
—
—
—
—
—
50.1
20.0
10.0
7.4
5.0
3.0
0.5
0.5
0.4
0.4
1.113
0.878
0.361
0.157
0.096
0.022
0.029
0.021
49.9
21.3
16.7
—
5.0
3.3
0.5
0.5
0.4
0.4
1.150
0.673
0.147
0.054
0.017
—
—
—
43.5
20.4
25.1
—
5.0
2.5
0.5
0.5
0.4
0.4
1.150
0.556
—
—
—
—
—
—
a
Supplied per kilogram of diet: Zn, 100 mg as zinc sulfate; Fe, 80 mg as ferrous sulfate; Cu, 50 mg as
copper sulfate; Mn, 25 mg as manganous sulfate; I, 0.5 mg as calcium iodate; Se, 0.1 mg as sodium selenite.
b
Supplied per kilogram of diet: vitamin A, 8,250 IU; vitamin D3, 825 IU; vitamin E, 40 IU; niacin, 35 mg;
D-pantothenic acid, 15 mg; riboflavin, 5 mg; menadione, 4 mg; folic acid, 2 mg; thiamine, 1 mg; D-biotin,
0.2 mg; vitamin B12, 0.025 mg.
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 totaltract 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 × 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. Re-
peated-measures analysis was used to evaluate the effect
of time after feeding on plasma urea. Values are reported
as least squares means.
Results
Health problems did not occur during the experiment,
and feed refusals were not observed. Analyzed dietary
AA concentrations were close to calculated values, and
diets were thus balanced according to an ideal digestible
AA ratio (Table 2). 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.
Expressed as percentage of N intake, urinary and total
N excretion decreased linearly, whereas N retention increased linearly with decreasing dietary protein (Table 3;
3241
Protein affects plasma urea and urinary nitrogen
Table 2. Nutrient composition of experimental diets, as-fed basis
Low-fiber diet
Nutrients
Protein:
Calculated
DE, Mcal/kg
CP, %
Ether extract, %
Crude fiber, %
ADF, %
NDF, %
Calcium, %
Phosphorus
Total, %
Available, %
Apparent ileal digestible AA, %a
Lysineb
Threonine
Methionine
Tryptophan
Total AA, %
Lysine
Threonine
Methionine
Tryptophan
Analyzed
DE, Mcal/kg
CP, %
Ether extract, %
Crude fiber, %
ADF, %
NDF, %
Total AA, %
Lysine
Threonine
Methionine
Tryptophan
High-fiber diet
Low
Medium
High
Low
Medium
High
3.25
13.60
1.87
3.50
5.00
12.10
0.65
3.25
16.50
1.48
3.50
5.13
11.86
0.75
3.25
19.50
1.53
3.70
5.57
12.20
0.75
3.25
13.60
4.42
5.00
7.19
16.00
0.65
3.25
16.50
4.77
5.30
7.71
16.74
0.65
3.25
19.50
3.93
5.28
7.85
16.30
0.65
0.50
0.28
0.50
0.27
0.52
0.27
0.50
0.28
0.50
0.26
0.50
0.25
0.71
0.47
0.26
0.13
0.71
0.47
0.22
0.14
0.78
0.52
0.25
0.17
0.71
0.47
0.26
0.13
0.71
0.47
0.23
0.14
0.78
0.52
0.25
0.17
0.84
0.60
0.29
0.17
0.86
0.63
0.26
0.19
0.96
0.71
0.29
0.23
0.84
0.60
0.30
0.17
0.87
0.63
0.27
0.19
0.97
0.71
0.30
0.23
3.23
13.6
1.9
3.4
4.0
11.5
3.17
16.8
1.5
3.7
3.8
12.1
3.22
19.1
1.4
3.8
4.3
12.8
3.11
14.2
4.3
5.0
5.6
15.9
3.17
17.0
4.7
4.9
5.3
15.7
3.14
20.2
3.8
5.1
5.4
15.8
0.83
0.64
0.30
0.20
0.92
0.67
0.27
0.25
0.99
0.74
0.30
NDc
0.86
0.65
0.31
0.19
0.88
0.68
0.29
0.23
1.01
0.76
0.31
ND
a
Calculated to meet the ideal pattern of apparent digestible AA (% of lysine): threonine, 61; methionine,
28; tryptophan, 17; isoleucine, 55; valine, 66; phenylalanine, 59 (NRC, 1998).
b
Apparent digestible lysine content was formulated to 2.2 g/Mcal DE for low- and medium-protein diets
and 2.4 g/Mcal DE for high-protein diets.
c
ND = not determined.
Table 3. Effects of dietary protein (high, medium, low) and fiber level on N balance
and digestibility in grower pigsa
Low-fiber diet
Item
N variable, g/d
N intake
Fecal N
Urinary N
Total N excretion
N retention
Urinary/fecal N
N variable, % of N intake
Fecal N
Urinary N
Total N excretion
N retention
N digestibility, %
a
P-value for
P-value for Protein
Low
Medium
High
Low
Medium
High
Pooled
SEM
34.1
6.1
9.5
15.6
18.5
1.6
42.1
6.8
14.4
21.2
20.9
2.2
47.9
8.0
17.6
25.6
22.3
2.3
35.6
6.7
8.8
15.5
20.0
1.3
42.6
7.5
13.8
21.2
21.4
1.9
50.6
8.6
17.6
26.2
24.4
2.1
0.02
0.4
0.8
0.7
0.7
0.2
0.001
0.058
NS
NS
0.029
0.051
0.001
NSb
NS
NS
NS
NS
0.001
0.001
0.001
0.001
0.001
0.004
0.001
NS
NS
NS
NS
NS
17.8
27.9
45.7
54.3
82.9
16.1
34.3
50.3
49.7
83.9
16.7
36.7
53.4
46.6
83.6
18.8
24.8
43.7
56.3
81.2
17.5
32.3
49.8
50.2
82.5
17.0
34.6
51.8
48.2
82.9
1.0
1.8
1.8
1.8
0.9
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
0.001
0.001
0.001
NS
NS
NS
NS
NS
NS
Based on seven pigs per treatment.
NS = not significant (P > 0.10).
b
High-fiber diet
Fiber
Protein × fiber
Linear
Quadratic
NS
NS
c
b
a
Based on seven pigs per treatment for energy and performance variables and plasma urea on d 19, and on four pigs per treatment for plasma urea on d 16.
Measured over 8 d.
NS = not significant (P > 0.10).
0.001
0.001
NS
NS
NS
NS
0.4
0.3
4.8
6.3
3.7
5.0
3.3
3.5
4.0
4.9
2.9
3.4
5.5
6.7
NS
NS
0.001
0.001
NS
NS
NS
NS
0.5
0.4
4.2
6.4
3.6
4.7
2.1
2.5
3.5
4.3
3.1
3.3
4.6
5.9
NS
NS
NS
NS
NS
NS
NS
NS
0.026
0.018
0.579
0.395
0.558
0.380
0.537
0.366
0.543
0.370
0.550
0.368
0.581
0.396
NSc
0.014
0.040
0.082
0.081
0.001
0.001
0.001
0.5
0.020
80.3
3.239
80.6
3.211
High
Medium
Low
80.5
3.109
84.1
3.274
84.2
3.249
Quadratic
Linear
Protein × fiber
Fiber
P-value for
High
Medium
Low
86.1
3.473
Energy
Digestibility, %
DE, Mcal/kg
Performance
ADG, kg/db
Feed efficiency
Plasma urea on d 16, mmol/L
Prefeeding
4 h postfeeding
Plasma urea on d 19, mmol/L
Prefeeding
4 h postfeeding
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,
Item
Plasma Urea
Pooled
SEM
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 × 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).
Energy digestibility increased linearly (Table 4; 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 × 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).
High-fiber diet
Energy and Nitrogen Digestibility
and Animal Performance
Low-fiber diet
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 highfiber 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).
P-value for Protein
Zervas and Zijlstra
Table 4. Effects of dietary protein (low, medium, high) and fiber level on energy digestibility, DE content,
plasma urea concentration, and performance in grower pigsa
3242
3243
Protein affects plasma urea and urinary nitrogen
Figure 1. Effects of dietary protein concentration (high,
19.7; medium, 16.9; and low, 13.8%) and time postfeeding
on plasma urea concentration (average for d 16 and 19;
day effect, P > 0.10). A protein × time postprandial interaction affected plasma urea concentration (P < 0.001). Values
without a common superscript differ among dietary protein and time postfeeding (P < 0.05).
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 ×
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 lowprotein diets (P > 0.10).
Plasma urea was correlated positively to daily urinary
N excretion at all sampling times for d 16 and 19 (P <
0.01). The specific correlation coefficients for prefeeding
and 2, 4, 6, and 8 h postfeeding were 0.59, 0.78, 0.81,
0.81, and 0.81 for d 16, and 0.62, 0.68, 0.81, 0.77, and 0.79
for d 19, respectively. Preprandial plasma urea therefore
had the worst prediction for daily urinary N excretion
among sampling times (Figure 2; R2 = 0.39). Preprandial
plasma urea ranged from 3.4 to 7.4 mmol/L for highprotein, from 2.3 to 4.8 for medium-protein, and from
1.9 to 4.7 for low-protein diets.
Plasma urea at 4 h after feeding had the highest correlation to daily urinary N excretion among sampling
times, coinciding with the maximal plasma urea concentrations (Figure 1). 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 lowprotein diets. Daily urinary N excretion (g/d) was predicted by 3.03 + 2.14 × 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 inter-
Figure 2. Relationship of daily urinary N excretion with
preprandial plasma urea concentration on d 19 (R2 = 0.39;
n = 42). Each data point represents one pig fed a low(䊉), medium- (䊏), or high-protein diet (▲).
val 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.
Discussion
In the present study, reduction of dietary protein reduced fecal, urinary, and total N excretion. The reduced
urinary N excretion coincided with a reduced plasma
urea concentration. The inclusion of oathulls in the diets
did not affect N excretion patterns or plasma urea.
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
Figure 3. Relationship of daily urinary N excretion with
plasma urea concentration at 4 h postprandial on d 19
(R2 = 0.66; n = 42). Each data point represents one pig fed
a low- (䊉), medium- (䊏), or high-protein diet (▲).
3244
Zervas and Zijlstra
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 highprotein diet. The discrepancy between ADG and N reten-
tion 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 higherthan-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 × 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
3245
Protein affects plasma urea and urinary nitrogen
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 × 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 mediumprotein diets, but not for the low-protein diet, causing
the protein × 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 × 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.
Implications
Reducing dietary protein content will reduce urinary
N and total N excretion in grower pigs. A reduction in
total N excretion may reduce the land base required for
sustainable manure application if other nutrients do not
become a limiting factor. The reduction of urinary N may
reduce ammonia emission. Plasma urea concentration
was related to urinary N excretion, suggesting that daily
urinary N excretion can be predicted from plasma urea.
However, the prediction of daily urinary N excretion may
not be accurate enough to predict N status or protein
deposition for grower pigs.
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