Download A genetic upper limit to whole-body protein deposition in a strain of

Published December 8, 2014
A genetic upper limit to whole-body protein deposition
in a strain of growing pigs1
P. J. Moughan,*2 L. H. Jacobson,3 and P. C. H. Morel†
*Riddet Centre, Massey University, Palmerston North, New Zealand; and †Institute of Food,
Nutrition and Human Health, Massey University, Palmerston North, New Zealand
ABSTRACT: The genetic upper limit to daily wholebody protein deposition (Pdmax) is an important constraint on pig growth. The Pdmax was determined for
a specified pig genotype using N balance and serial
slaughter techniques. A traditional N-balance study,
involving 36 and 90 kg of BW Large White × (Landrace
× Large White) entire male pigs, was first conducted to
demonstrate that a highly digestible, nutrient-dense
diet (1.54% Lys; 18 MJ of DE/kg, air-dried basis) was
able to support the attainment of Pdmax within the
constraints of pig appetite. Animals were allocated to
set levels of feed intake [set proportions of ad libitum
DE intake (DEi), 50 to 100%]. Nitrogen retention increased linearly with DEi up to 25.3 and 35.2 MJ of
DE/d for the 36 and 90 kg of BW pigs, respectively,
then showed a departure (P < 0.05) from linearity. For
DEi of the experimental diet above the latter intakes,
which were approximately 80% of a determined ad libitum DEi, the pigs deposited protein at a rate approaching Pdmax. When a linear plateau response
model (accepted a priori) was fitted, Pdmax values of
189.9 g/d at a DEi breakpoint of 28.3 MJ of DE/d at 36
kg of BW and 186.4 g/d at a DEi breakpoint of 37.3 MJ of
DE/d at 90 kg of BW were found. In the serial slaughter
study, 18 female and 18 entire male pigs were allocated
to 5 slaughter BW (25, 45, 65, 85, and 110 kg) such
that there were 5, 3, 3, 3, and 4 animals of each sex at
each slaughter weight, respectively. Animals were fed
the experimental diet ad libitum, and whole-body protein was determined at slaughter. Growth data were
analyzed by differentiating and combining continuous
mathematical functions for BW and body composition.
The ad libitum DEi were 27.4 and 50.7 MJ/d at 36 and
90 kg of BW for the entire males and were assumed,
based on the N-balance results, sufficiently high to
allow expression of Pdmax. There was an effect (P <
0.05) of sex on Pdmax vs. time (days on trial). Over the
BW range of 25 to 85 kg, Pdmax was constant for the
entire male and female pigs at 170 and 147 g/d, respectively. Above 85 kg of BW, Pdmax was no longer constant for either sex.
Key words: body composition, genetics, growth, pig
©2006 American Society of Animal Science. All rights reserved.
INTRODUCTION
It has been demonstrated empirically (Dunkin et al.,
1986; Campbell, 1988; Quiniou et al., 1995) that at high
dietary nutrient intakes and within the constraints imposed by appetite, at least some genotypes of pig have
a limit to daily protein deposition (Pdmax) and deposit
excess dietary protein and nonprotein energy as body
lipid. The Pdmax is an important constraint on porcine
1
We wish to acknowledge the advice of the late W. C. Smith and
the expert technical assistance of G. Pearson and C. Sosas.
2
Corresponding author: [email protected]
3
Current address: Novartis Institutes for BioMedical Research,
Novartis Pharma A.G., Basel, Switzerland.
Received May 24, 2005.
Accepted August 2, 2006.
J. Anim. Sci. 2006. 84:3301–3309
doi:10.2527/jas.2005-277
growth (Kielanowski, 1969; Rerat, 1972; Whittemore
and Fawcett, 1976).
Serial slaughter studies have been undertaken to determine Pdmax during the growth period in pigs (Siebrits et al., 1986; Rao and McCracken, 1990; Schinckel,
1999), but in most cases animals have been fed nutrientdense diets under favorable production conditions and
it has been assumed that determined protein deposition
(Pd) equates to Pdmax. With the exception of the work
of Mohn and de Lange (1998), very few studies have
shown independently that the dietary conditions were
in fact appropriate for the determination of Pdmax.
Traditionally, in serial slaughter studies, Pd rates are
determined by calculating differences in body protein
content between pairs of BW and dividing these differences by the respective number of days taken to bridge
the BW difference. This latter approach gives a mean
deposition rate over the BW range, which may not accu-
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Moughan et al.
rately reflect the true pattern of Pd, especially when
BW ranges are large.
The aim of the present work was to study the chemical
composition of the whole body of pigs and to provide
observations on Pdmax for a specified strain of growing
pig under conditions whereby the attainment of Pdmax
was shown independently to be unconstrained by dietary nutrient intake. We were interested in examining
whether Pdmax is constant over the BW range of 25 to
110 kg. Differential calculus was used to determine Pd
rate as a continuous variable over a BW range.
MATERIALS AND METHODS
Table 1. Ingredient composition of the experimental diet
Ingredient
Cooked wheat1
Sucrose
Lactic casein
Soybean oil
Skim milk powder
Dicalcium phosphate
Calcium carbonate
Sodium chloride
D,L Methionine
Ethoxyquin (antioxidant)
Vitamin-mineral premix2
%, air-dried basis
48.43
16.80
15.90
8.00
8.00
2.20
0.02
0.16
0.07
0.02
0.40
1
Weet-bix, Sanitarium, Palmerston North, New Zealand.
Vitamin-mineral premix provided the following (units per kg of
diet): 10,778 IU of vitamin A; 1,600 IU of vitamin D3; 32 mg of vitamin
E; 2.4 mg of vitamin K; 1.6 mg of thiamine; 4 mg of riboflavin; 2.4
mg of vitamin B6; 20 ␮g of vitamin B12; 12 mg of pantothenic acid;
24 mg of niacin; 80 ␮g of biotin; 0.8 mg of folic acid; 40 mg of choline
chloride; 80 mg of iron (sulfate); 8.6 mg of manganese (sulfate); 0.4
mg of cobalt; 0.24 mg of selenium (sodium selenite); 77 mg of zinc
(oxide); 40 mg of copper (sulfate); and 0.47 mg of iodine (potassium
iodate).
2
All procedures involving animals were approved by
the Massey University Animal Ethics Committee. The
study was composed of 2 parts, a preliminary N-balance
trial, the objective of which was to demonstrate the
adequacy of a dietary regimen designed to allow expression of a genetic upper-limit to Pd, and the main study,
in which the dietary regimen was applied in a serial
slaughter trial to determine values for Pdmax in a specified strain of growing pig.
Preliminary N-Balance Study
Animals and Housing. Large White × (Landrace ×
Large White) entire male pigs, 22 of 30 (±0.4) kg of BW
and 24 of 84 (±0.7) kg of BW, which were the progeny
of commercially available, selected female pigs and intensively selected, Large White boars, were sourced
from a multiplying unit in the North Island of New
Zealand. The animals were kept individually in metabolism cages, designed to allow for complete and separate
collection of urine and feces, at 22 ± 1°C for the 30 kg
of BW pigs and at 18 ± 1°C for the 84 kg of BW pigs.
An additional four 35 kg of BW and four 75 kg of BW
entire male pigs of the same breed were housed under
the same conditions for a 14-d period, whereby the ad
libitum DEi of the experimental diet (see the diet and
feeding section) was determined. The mean daily ad
libitum DEi of the 35 kg of BW pigs was 34.3 MJ of DE
(midtrial mean BW = 38 kg), whereas the comparable
intake for the 75 kg of BW pigs was 42.6 MJ of DE
(midtrial mean BW = 80 kg).
Diet and Feeding. It was our aim to formulate a
highly digestible, energy-dense, and nutritionally balanced diet, which, when given to the pigs at a suitable
intake level, contained adequate digestible, balanced
protein, and nonprotein energy to allow expression of
Pdmax during the growth period (25 to 110 kg of BW).
A base diet formulation of greater than 16 MJ of DE/
kg of air-dried weight and of 1.3% Lys (air-dried basis)
was chosen, with all other essential AA supplied in
excess of ideally balanced levels (ARC, 1981). The formulation was constrained to contain levels of macro
minerals, vitamins, and trace elements to exceed the
stated requirements (ARC, 1981; Standing Committee
on Agriculture, 1988), dietary crude fiber was formu-
lated to be less than 2%, and dietary fat to be greater
than 3%, on an air-dried weight basis. The ingredient
composition of the diet is given in Table 1. The diet was
fed to the animals as a slurry (3:1 water:diet, wt/wt) 3
times daily (0900, 1700, and 2030).
Experimental Procedure. After a 4-d acclimation period, pigs in each BW range were weighed, and pairs
of pigs were randomly assigned to set levels (proportions of predetermined ad libitum DEi, 50 to 100%) of
feed intake for an 11-d period. Over the final 7 d of the
11-d period, total pooled feces (collected twice daily)
and pooled urine were collected separately for each pig.
Urine was collected twice daily over acid (4 N H2SO4 at
2.5% of the urine volume), with cage floors and funnels
being sprayed with distilled water at the collection time.
After collection, urine was filtered through fine gauze.
For sixteen of the twenty-two 30 kg of BW pigs, daily
urine output was subsampled (50 mL) for each pig and
submitted for creatinine analysis to provide an indication of the completeness of urine collection. Urine samples were kept chilled (4°C) in airtight containers, and
feces were kept frozen (−20°C). All feed refusals and
feed spillages were collected and oven-dried. Representative samples of diet and feces were freeze-dried and
finely ground for chemical analysis.
Chemical Analysis. The diet was analyzed for total
N, AA, and GE. Feces and urine were analyzed for N,
and a subset of the urine samples was analyzed for
creatinine. Total N was determined in triplicate using
the Kjeldahl method (Kjeltec Auto 1030 Analyzer, Tecator AB, Sweden). Amino acids were determined in duplicate after hydrolysis in 6 N HCl with 1% added phenol
under vacuum for 24 h at 110 ± 1°C. The AA were
separated by ion exchange chromatography (HPLC,
Waters Associates, Milford, MA) and detected based on
the fluorescence of o-phthalaldehyde (OPA) postcolumn
Genetic upper limit to protein growth in pigs
derivates of the AA. Cysteine was determined in duplicate after performic acid oxidation and hydrolysis.
Tryptophan was determined in duplicate after alkaline
hydrolysis in Teflon screw-top tubes. Hydrolysis was in
4 N LiOH for 24 h at 110 ± 1°C under 1.05 kg/cm2 of
pressure with oxygen-free N. Cysteine and Trp peaks
were quantified after HPLC separation and detection
of the fluorescence of OPA derivatives. Gross energy
was determined in duplicate using an adiabatic bomb
calorimeter (Gallenkamp Autobomb, Loughborough,
UK) using benzoic acid as an internal standard. Urinary creatinine was determined by the Jaffé reaction
using a Cobas Fara II auto analyzer (Roche Products,
Basel, Switzerland).
Digestibility of Dietary Energy and Amino Acids.
The apparent fecal digestibility of GE in the experimental diet was determined using 8 entire-male pigs of 70
kg of BW of the same genotype as used in the N balance
study. The DE was determined using standard procedures based on a total collection of feces.
Apparent ileal digestibilities of AA in the experimental diet were determined using 9 Sprague Dawley rats
of 150 g of BW. Ileal digesta samples (terminal 10 cm
of ileum) were obtained by killing the animals and pooling the digesta for 3 lots of 3 rats each. Digestibility
coefficients (n = 3) were determined by reference to
the indigestible marker, chromic oxide, using standard
procedures (Moughan et al., 1984).
Statistical Analysis. The daily N balance for each
pig was determined using the following equation:
N retained = N ingested − (N feces + N urine).
Daily body protein deposition was determined by
multiplying the N balance by the factor 6.25. The relationships between Pd and dietary DE intakes (DEi)
were examined by fitting linear and piecewise regression models (Hudson, 1966; Matlab Software, Mathworks Inc., Natick, MA). Whenever the adjusted R2
value was lower for the piecewise model, indicating
departure from linearity, a linear-plateau model
(Campbell, 1988) was fitted to the data, with Pdmax
equating with Pd at the plateau.
Serial Slaughter Study
Experimental Procedure. The animals and diet were
as described for the preliminary N balance study. In the
study, 18 entire male and 18 female pigs were randomly
allocated to 5 slaughter weights (25, 45, 65, 85, and 110
kg of BW) such that there were 5, 3, 3, 3, and 4 animals
of each sex at each of the respective BW. At the beginning of the study, the animals were treated for internal
and external parasites by oral administration of ivermectin (Ivomec 0.4%, Merck, Sharp and Dohme Ltd.,
Auckland, New Zealand). The pigs were individually
penned in an insulated building maintained at 21 ± 1°C
and were given ad libitum access to fresh diet throughout the study. Fresh water was available at all times.
3303
Every 3 d, the animals were weighed at 0900 and
feed consumption was measured. Upon reaching its allocated slaughter weight, each pig was sedated with an
intramuscular injection (0.2 mL/kg of BW) of azaperone
(Stresnil 40 mg/mL, Smith, Kline and French Ltd.,
Auckland, New Zealand) and then anaesthetized with
halothane gas (Fluothane, Imperial Chemical Industries, Ltd., Cheshire, UK) administered via a face mask.
The animals were euthanized by an intracardiac injection (0.5 mL/kg of BW) of sodium pentobarbitone (Pentobarb 300 mg/mL, South Island Chemicals Ltd.,
Christchurch, New Zealand). The body was weighed
immediately, and then the contents of the digestive
tract, bladder, and gallbladder were removed and discarded.
After the digestive tract was rinsed and blotted dry,
the body, including the digestive tract, was reweighed,
and this was designated as empty BW. The empty body
was sealed in a plastic bag and stored frozen (−20°C).
Each empty body was weighed in the frozen state and
then cut into transverse sections using a band saw. The
body sections were ground 3 times through a 10-mm
aperture plate with thorough mixing of the contents
between each grinding. Twelve 200-g subsamples of
material were taken at random and mixed. This pooled
material was ground twice through a 6-mm aperture
plate. A single 200-g sample of the resulting material
for each animal was mixed for 1 min in a high-speed
blender, and the paste was submitted for analysis of
DM, ash, lipid, and N.
Chemical Analysis. Dry matter and ash were determined (n = 6) by the method described by Harris (1970),
with the exception that a temperature of 70°C was used
for drying to ensure that fat was not lost. Lipid content
was obtained by petroleum ether-extraction of duplicate, freeze-dried samples of the whole-body tissue
(AOAC, 1975). Total N was determined in duplicate
using the Kjeldahl method on ground (1 mm sieve),
lipid-extracted material, and CP was estimated using
a factor of 6.25.
Statistical Analysis. Body weight (y, kg) was regressed against time (x, days on trial) using linear (y =
α + β1 x + e), quadratic (y = α + β1 x + β2 x2 + e), and
cubic (y = α + β1 x + β2 x2 + β3 x3 + e) regression models,
and the statistical significance of sex and the time ×
sex interaction effects were determined using the PROC
GLM procedure (SAS Inst. Inc., Cary, NC). The BW vs.
time functions were differentiated with respect to time
to obtain the growth rate at a given time. The cubic
model gave untenable growth rates at the extreme BW
and was thus discarded.
Empty BW was regressed against BW (simple linear
regression), and the relationships between whole-body
protein, ash, and lipid contents and empty BW were
determined by fitting allometric, linear, and polynomial
regression functions using SAS. The relationship describing empty BW as a function of BW was then substituted into the respective equations describing wholebody protein, ash, and lipid as a function of empty BW,
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Moughan et al.
Table 2. Analyzed nutrient composition of the experimental diet1
Component
%, air-dried basis
DM
N
Lys
Met + Cys
Trp
His
Phe + Tyr
Thr
Leu
Ile
Val
91.45
4.02
1.54
0.94
0.32
0.67
2.06
0.83
1.78
0.82
1.04
1
The GE content was determined to be 20.9 MJ/kg of air-dried diet.
resulting in equations giving the chemical body components as functions of BW.
The latter equations (whole-body chemical components as a function of BW) were differentiated with
respect to BW, to give amounts (kg) of protein, ash, and
lipid per kilogram of BW. Finally, each growth rate
(BW per unit of time) function was multiplied by the
function best describing protein, ash, and lipid content
per kilogram of BW to yield predicted growth rates
for protein, ash, and lipid. Differences were considered
significant at P < 0.05.
RESULTS
Preliminary N-Balance Study
Nutrient Composition of the Experimental Diet.
The determined nutrient composition of the experimental diet is given in Table 2. The determined Lys content
was high in comparison with that formulated (1.54 vs.
1.3%). All other essential AA were close to formulated levels.
Digestibility of Dietary Energy and Amino Acids.
The mean (n = 8) apparent fecal digestibility of GE was
94.2 (±0.22)%. The mean DE content of the diet was
19.7 (±0.05) MJ/kg of DM or 18.0 (±0.01) MJ/kg of airdried feed. The mean (n = 3) apparent ileal AA digestibility for the dietary essential AA ranged from 81% for
Thr to 92% for Met. The mean apparent ileal digestibility value for Lys was 91%.
N-Balance Trial. The pigs in the N-balance trial
consumed the experimental diet readily with minimal
feed spillage. One pig from the lower BW cohort and 2
pigs from the higher BW group were excluded from the
study because of difficulties with urine collection. Small
amounts of feed were refused on the ad libitum treatment, which were collected and corrected for. Mean BW
at the beginning and end of the 7-d collection periods
were 29.5 (±0.46) and 39.4 (±1.00) kg, respectively, for
the lower BW group and 84.2 (±0.68) and 94.3 (±0.73)
kg for the higher BW group.
Respective mean BW at the midpoints of the collection periods were 36 and 90 kg. Mean daily urinary
Figure 1. Linear-plateau regression model describing
protein deposition (Pd) in the N-balance study for entire
males pigs of 36 (▲) and 90 kg of BW (䊐) for an improved
genotype given increasing dietary energy intakes of an
experimental diet.
creatinine excretion expressed on a metabolic BW basis
(mg of creatinine/kg of BW0.75) for 16 pigs from the 36
kg of BW group ranged from 98 to 124 mg/kg0.75 (overall
mean, 110 mg/kg0.75), and the CV of daily urinary creatinine excretion between days within pig ranged from
1.6 to 15.2% with an overall mean CV of 7.8%.
When the N balance data were fitted to the linear
and piecewise regression models, the piecewise model
gave the highest adjusted R2 values at both BW (0.81
vs. 0.79 and 0.42 vs. 0.41 for the 36 and 90 kg of BW
animals, respectively) and the lowest SE of the estimate
(SEE) values (16.4 vs. 17.4 and 20.5 vs. 20.8 for the 36
and 90 kg of BW animals, respectively). The slopes
of the respective second regression lines (i.e., the line
deviating in slope from the initial regression line) for
the piecewise regression models were not different from
zero. For the 36-kg of BW pigs, the abscissa at the join
point (α) for the piecewise regression model corresponded to a DEi of 25.3 MJ/d. For the 90 kg of BW pigs,
the comparable value was 35.2 MJ of DE/d.
A linear/plateau model was fitted to the data (Figure
1), indicating Pdmax values of 189.9 g/d at a feed DEi
breakpoint of 28.3 MJ of DE/d at 36 kg of BW and 186.4
g/d at a DEi breakpoint of 37.3 MJ of DE/d at 90 kg
of BW.
Serial Slaughter Study
All pigs consumed the experimental diet readily and
remained healthy with the exception of 1 male pig that
developed diarrhea and was removed from trial. Daily
feed intake was best described by a quadratic equation,
with sex being a significant (P < 0.05) model parameter.
Feed intake relationships for the female and entire
male pigs were
Females: FI = 0.023 + 0.052 BW
− (2.7 × 10−4) BW2 (RSD = 0.251); and
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Genetic upper limit to protein growth in pigs
Table 3. Regression parameters for the models describing pig BW (kg) as a function of
time and pre- and postdifferentiation of the BW vs. time functions1,2
Equation parameter
Model
Sex
Linear
Female
Male
Female
Male
Quadratic
α
β1
β2
24.28
23.57
23.21
23.44
0.960
1.093
1.049
1.009
—
—
−0.0011
0.0012
Sex
Female
Male
Female
Male
Quadratic
3.98
3.77
3.93
3.74
Equation parameter
Original model (postdifferentiation)
Linear
RSD3
α
β1
0.960
1.093
1.049
1.009
—
—
−2.2 × 10−3
2.4 × 10−3
1
For the models y = α + β1 x + e (linear) and y = α + β1 x + β2 x2 + e (quadratic), where x = time (day of
trial) when BW = 25 kg at d 0, α = intercept, β1 = regression coefficient for the linear effect of time, and
β2 = regression coefficient for the quadratic effect of time. All equation parameters were significant (P <
0.01).
2
Initial BW (mean ± SD) were 26.6 ± 1.16 and 26.9 ± 1.48 for the males and females, respectively.
3
RSD = Residual SD.
Entire males: FI = 0.038 + 0.048 BW
−4
− (1.9 × 10 ) BW (RSD = 0.263),
2
where FI = feed intake (kg/d), BW = BW (kg), and RSD =
residual SD.
The regression of BW against time (days on trial)
showed an effect (P < 0.05) of sex for the linear and
quadratic models; hence, separate relationships were
derived for females and entire males. Model parameters
are given in Table 3, along with model parameters after
the respective BW vs. time functions had been differentiated with respect to time, to give predictions of mean
ADG (kgⴢd−1). Figure 2 shows plots of predicted ADG
vs. BW for females and entire males, as derived from
the 2 models.
For the whole-body compositional data, fitting of an
allometric equation (y = a xb) gave high negative correlations between the a and b parameters for protein, ash,
and lipid. Because the parameters were highly correlated, a linear model (b = 1) was fitted. A linear model
gave the best fits to the data (lowest residual standard
deviations, RSD) for protein and ash vs. empty BW,
and there were no sex or interaction effects. For the
lipid and DM components of empty BW, the best fit to
the data was found with a quadratic model. There were
no statistically significant interactions, but there was
an effect of sex (P < 0.05), and separate equations were
derived for each sex of pig for lipid and DM. Equation
parameters and RSD are given in Table 4. The relationship between empty BW and BW was best described
by a linear model (RSD = 0.780); Empty BW (kg) =
−0.736 + 0.952 BW (kg), where BW = live BW.
Relationships for body protein and lipid as a function
of BW were derived (by substitution of the empty BW
with BW function). The latter functions were differentiated with respect to BW and were multiplied by the
different BW daily gain functions for each sex to give
daily deposition functions for protein and fat for each
sex of pig for each BW/time function (Table 5). Daily
body protein and body lipid deposition (Ld) rates as
derived from the linear and quadratic equations are
shown in Figures 3 and 4, and body Ld:Pd ratios as
derived from the linear equations are given in Figure 5.
The linear model predicted that Pd was a constant
145 g/d between 25 and 110 kg of BW for female pigs
and a constant 165 g/d for entire males. For the males,
Ld increased from 209 g/d at 25 kg of BW to 448 g/d at
110 kg of BW, whereas comparable values for the females were 208 and 535 g/d, respectively. The quadratic
model predicted that Pd for the entire males was 153
g/d at 25 kg of BW, increasing to 182 g/d at 110 kg of
Figure 2. Average daily BW gain as derived from linear
or quadratic functions describing the BW increase over
time for entire male and female pigs of an improved
genotype between 25 and 110 kg of BW (– – – linear function, male; . . . . . quadratic function, male; – ⴢ – ⴢ – linear
function, female; —— quadratic function, female).
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Moughan et al.
Table 4. Model parameters and residual SD (RSD) for
regressions describing whole body protein and ash (linear) and lipid and DM (quadratic) as functions of empty
BW for pigs of 25 to 110 kg of BW1
Body
component
Protein
Ash
Lipid
DM
Sex
β1
β2
RSD
Female and male
Female and male
Female
Male
Female
Male
0.159
0.031
0.125
0.124
0.310
0.305
—
—
0.0018
0.0014
0.0019
0.0015
0.676
0.165
1.392
1.249
0.627
1.030
1
β1 = Regression coefficient for the linear effect of empty BW (kg);
and β2 = regression coefficient for the quadratic effect of empty BW.
For each model, the intercept was not significant.
BW. Comparable values for the female pigs were 158
g/d at 25 kg, decreasing to 128 g/d at 100 kg of BW.
For Ld, the quadratic model predicted values of 193 g/
d at 25 kg of BW for the entire males, increasing to 537
g/d at 110 kg, and for the females 227 g/d at 25 kg of
BW increasing to 472 g/d at 110 kg of BW. Irrespective
of the BW vs. time equation, the Ld:Pd ratios were
always greater than 1.0 for females and entire males
of the genotype of pig studied and over the BW range
investigated. For females at 25 kg of BW, the body
Ld:Pd ratio was 1.44:1.0 and increased to 3.67:1.0 at
110 kg. For the entire males at 25 kg of BW, the ratio
was 1.27:1.0 and increased to 2.95:1.0 at 110 kg of BW.
Investigation of the BW range whereby the quadratic
model parameter became significant (P < 0.05, i.e., a
significant departure from linearity) using a mixed
model analysis based on the respective slaughter
groups showed BW to increase with time in a linear
fashion up to a BW of 85 kg for both sexes, after which
the quadratic effect became significant. Linear regression equations describing BW (kg) as a function of time
(D days on trial) for the 25 to 85 kg of BW range were
Females: BW =
25.21 + 0.97D (R2 = 0.995; RSD = 1.34); and
Entire males: BW =
26.73 + 1.12D (R2 = 0.952, RSD = 4.17).
Figure 3. Body protein deposition (Pd) as derived from
linear or quadratic functions describing the BW increase
over time for entire male and female pigs of an improved
genotype between 25 and 110 kg of BW (– – – linear function, male; . . . . . quadratic function, male; – ⴢ – ⴢ – linear
function, female; —— quadratic function, female).
Whole-body protein deposition as determined using
the linear model from 25 to 85 kg of BW was 147 g/d
for the females and 170 g/d for the entire male pigs.
DISCUSSION
The Preliminary N-Balance Study
In designing the study, the objective was to formulate
a diet capable of supporting the genetic upper limit
to protein deposition (Pdmax), within appetite, for a
selected strain of pig. In the preliminary study, the
experimental diet was shown to be a palatable, highly
digestible diet supporting a high voluntary feed DEi.
Considerable care was taken in formulating the diet to
ensure that no nutrient was limiting for normal growth
and development. It was predicted, based on a validated
simulation model of N digestion and metabolism in the
growing pig (Moughan et al., 1987b), that the formulated diet would provide absorbed balanced protein (Pg)
to the sites of protein synthesis, in excess of 200 g/d for
entire male pigs heavier than 30 kg of BW and fed at
greater than 80% of their ad libitum feed intake capacity (for a 30-kg male pig fed at 80% ad libitum intake,
the simulated value for Pg was 202 g/d, whereas at
Table 5. Prediction equations for daily body protein and lipid deposition (kg/d) derived
from linear and quadratic relationships describing BW as a function of time (days on
trial) for pigs of 25 to 110 kg of BW
Model
Sex
Body protein deposition
Linear
Female
Male
Female
Male
0.145
0.165
[1.049 − (2.2 × 10−3 D)] × 0.151
[1.009 + (2.5 × 10−3 D)] × 0.151
Quadratic1
1
D = Day of trial when BW at d 0 = 25 kg.
Body lipid deposition
0.960
1.093
[1.049
[1.009
× (0.117 + 0.004 BW)
× (0.116 + 0.003 BW)
− (2.2 × 10−3 D)] × (0.117 + 0.004 BW)
+ (2.5 × 10−3 D)] × (0.116 + 0.003 BW)
Genetic upper limit to protein growth in pigs
Figure 4. Body lipid deposition (Ld) as derived from
linear or quadratic functions describing the BW increase
over time for entire male and female pigs of an improved
genotype between 25 and 110 kg of BW (– – – linear function, male; - - - - - quadratic function, male; – ⴢ – ⴢ – linear
function, female; —— quadratic function, female).
100% ad libitum intake simulated Pg was 256 g/d; comparable values for a 90-kg male pig were 261 and 334
g/d, respectively). It was also predicted by growth modeling, based on simulated lipid to protein ratios in the
whole body tissue gain and assuming Pdmax values for
the strain of 200 g/d or less, that dietary nonprotein
energy supply (above 90% ad libitum DEi) was more
than sufficient to support maximal rates of body Pd.
Therefore, in the simulation of growth, Pg and nonprotein energy intake were predicted to be generously
oversupplied relative to the amounts required to sustain Pdmax. Given that the pigs studied here were
thought to have Pdmax values lower than 200 g/d, the
diet was considered to provide adequate levels of balanced protein to allow the expression of Pdmax for pigs
fed ad libitum and that growth would not be constrained
nutritionally. As well as receiving a well-balanced nu-
Figure 5. Body lipid deposition to protein deposition
ratio (Ld/Pd) as derived from a linear function describing
body composition changes over time for entire male (bold
line) and female (nonbold line) pigs of an improved genotype between 25 and 100 kg of BW.
3307
trient-rich diet, the pigs in the N balance study were
housed under optimal environmental conditions. The
ambient room temperatures were within the thermoneutral ranges for the respective BW groups and at
the respective levels of feed intake (Holmes and
Close, 1977).
Although it seemed, based on the simulation exercise,
that the proposed dietary regimen for the main serial
slaughter study (i.e., when the experimental diet was
given ad libitum) would support the attainment of
Pdmax for the strain of pig, this proposition was tested
empirically by the conduct of the in vivo N-balance
study.
The traditional N-balance technique, whereby body
N retained is determined as the difference between dietary N intake and fecal plus urinary N output, may
be criticized due to incomplete collection of excreta N
leading to an overestimate of retention (Just et al.,
1982). Ammonia can be lost from feces especially when
these are oven dried, and also from urine, particularly
when the pH of the urine is greater than 5 and when
urine collecting vessels are not airtight. In the current
study the loss of nitrogenous components was minimized by collecting the feces twice daily and conducting
chemical analysis on freeze-dried rather than ovendried samples. Urine was collected twice daily over acid
and stored in airtight containers. At each collection
time, the floor of the metabolism crate and sides of the
collection funnel were sprayed with distilled water in
an attempt to maximize urine collection. Moreover, the
completeness of urine collection in the current study
was assessed by determining the variation in daily urinary creatinine excretion (Moughan et al., 1987a). Das
and Waterlow (1974) reported a mean CV of daily urinary creatinine excretion of 7% when urine collection
from rats was known (radioactive marker) to be complete. The overall mean coefficient of variation for daily
urinary creatinine excretion for pigs in the current
study was 7.8%, and it was thus concluded that a nearcomplete collection of urine was achieved.
Although considerable care was exercised in the present work, the general inaccuracy of N-balance trials
(leading to an overestimation of true N-balance) was
recognized, and the data were interpreted relatively
rather than as absolutes. The pattern of change of Nbalance with different amounts of nutrient intake was
emphasized rather than the absolute N-balance attained. Of importance in relation to the objectives of
this study was to establish that the nutritional conditions were adequate to allow demonstration of the
achievement of Pdmax. If Pd for a given strain of pig
can be shown to increase with increasing amounts of
DEi, to reach a plateau whereby further increases in
intake of the protein adequate diet do not lead to increases in Pd but rather increases in body fatness (Morris, 1983; Curnow, 1986; Campbell, 1988), then it is
reasonable to assume that Pd at plateau approximates
Pdmax (Whittemore et al., 1988). This latter response
model was accepted a priori in the present work, and
3308
Moughan et al.
the objective was thus to test whether there was a statistically significant departure from linearity in the response data.
In the current study, the N-balance data were statistically analyzed using linear and piecewise regression
models. The piecewise regression model iteratively minimized the residual sums of squares with respect to the
vector of observations describing the 2 linear phases
and gave the lowest residual sums of squares. The latter
model, which best described the experimental data, indicated a statistically significant departure from a linear response of Pd to dietary energy intake at energy
intakes considerably lower than the ad libitum DEi.
The second slope on each piecewise regression was not
statistically significant different from zero.
A linear plateau relationship is commonly used to
describe the theoretical relation between body Pd and
dietary energy intake (Campbell, 1988). When such a
relationship was fitted to the N balance data, Pdmax
for the genotype investigated was found to be close to
190 g/d at both BW. The results from the linear plateau
model supported the appropriateness of the dietary regimen for the application in the main serial slaughter
study. The linear plateau model indicated that Pdmax
was attained at DEi higher than 28.3 MJ of DE/d at
36 kg of BW and 37.3 MJ of DE/d at 90 kg of BW.
It is concluded that when DEi from the experimental
diet exceeded 28 and 37 MJ of DE/d for the 36 and 90
kg of BW pigs, respectively, the pigs in the current
study deposited body protein at Pdmax. These dietary
energy intakes were well within the previously determined ad libitum dietary energy intakes of the genotype
of pig investigated. Although the N-balance trial involved entire male pigs, it was assumed (because
Pdmax is lower for females than entire male pigs, Whittemore, 1993) that the determined DEi would also allow
expression of Pdmax in females.
Because of the recognized deficiencies of the N-balance method for determining absolute values of Pdmax
(Just et al., 1982), the conditions determined in the
present N-balance study as suitable for the expression
of Pdmax were replicated in a serial slaughter study.
The Serial Slaughter Study
The high quality nutrient-dense diet fed to the growing pigs was shown in the preliminary N-balance study
to support the attainment of Pdmax in the given strain
of pig, as long as feed DEi exceeded 28 and 37 MJ of DE/
d at 36 and 90 kg of BW, respectively. The comparable
actual feed DEi found in the serial slaughter study were
27.4 and 50.7 MJ of DE/d for the entire male pigs.
Although the DEi at 36 kg of BW was slightly lower than
that determined to maximize Pd (i.e., attain Pdmax), by
fitting the linear/plateau model in the N-balance study,
it was assumed that dietary nutrient intake was unlikely to limit Pd in the strain of pig studied. This assumption was supported by the body Ld:Pd ratios determined for the pigs. Regardless of BW, the ratios were
greater than 1:1, and the ratio increased with time on
trial. The lipid to protein ratios demonstrated that the
animals were relatively fat (Whittemore, 1993) indicating that dietary energy was being supplied at a rate in
excess of that needed to meet the energy requirement
for protein and essential lipid synthesis and other vital
body functions, and the excess nutrients were being
deposited as adipose tissue.
Different response models applied in the present
work led to different interpretations concerning Pdmax,
and the choice of statistical model has an important
effect upon the conclusions drawn concerning the relationship between Pdmax and BW. The linear model,
after differentiation and inclusion of the whole body
protein composition term, specified a constant Pdmax
with BW. For female pigs this was 146 g/d and for
entire male pigs 165 g/d. The quadratic model, after
differentiation and inclusion of the whole body protein
content term, resulted in an increasing rate of Pd with
increasing BW for entire males and a decreasing rate
of Pd with increasing BW for females. According to this
model, at 25 and 110 kg of BW, entire male pigs had
Pdmax values of 153 and 182 g/d, respectively, and at
the same BW female pigs had Pdmax values of 157 and
128 g/d, respectively. The values for Pdmax determined
in the current study fell within the higher region of the
range of published values for Pdmax of 90 to in excess
of 200 g/d (Whittemore, 1983; Campbell, 1985; and
Whittemore et al., 2001).
Simply maximizing goodness of fit is not a suitable
criterion for accepting a particular model. This is well
exemplified in the present reported study. The linear
and quadratic models gave similar fits to the data but
led to different predictions. The linear model has the
advantage of simplicity, and in this study we were particularly concerned to test whether Pdmax was constant
between 20 and 110 kg of BW.
Further analysis with the linear model showed that
there was a departure (P < 0.05) from linearity at 85
kg of BW for both sexes. The increase in body protein
from 25 to 85 kg was best described by a linear function,
and Pdmax for this genotype of pig would be a constant
between 25 and 85 kg of BW. Beyond 85 kg of BW, the
relationship was no longer constant. The latter finding
gives support to the recent conclusion of Whittemore
et al. (2001) and that of Mohn and de Lange (1998) that
adoption of a single value for Pdmax over the BW range
20 to 90 kg may be suitable for purposes of practical
nutrition. The present result has implications for the
choice of suitable models (e.g., Gompertz function; logistic model) for describing the relationship between
Pdmax and age. Such models should allow for a constancy in Pdmax over a considerable portion of the
growth phase.
The current study provides evidence that for a specified improved genotype of pig, Pdmax was achievable
within appetite when the pigs were given a carefully
formulated high quality experimental diet. Over the
BW range 25 to 85 kg of BW, Pdmax was constant
Genetic upper limit to protein growth in pigs
for entire male and female pigs at 170 and 147 g/d,
respectively. Above 85 kg of BW, maximal Pd was no
longer a constant for either sex. The actual shape of
the curve beyond 85 kg of BW requires further clarification and is likely to be important in practice where pigs
are killed at higher BW.
The methodology used in the current study, which
involved differentiation and then the combination of
continuous growth and body composition functions for
the growing pig, serves as an example of a different
approach to analyzing data from protein retention studies. That Pdmax was constant from 25 to 85 kg of BW
for the genotype of pig studied here has implications
for pig growth modeling and suggests that the approach
of using a single Pdmax value to describe pig genotype
is justifiable over the BW range 20 to 85 kg. At higher
BW a change in Pdmax with BW must be taken into
account.
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