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A genetic upper limit to whole-body protein deposition in a strain of growing pigs¹

P. J. Moughan^*,2, L. H. Jacobson³ and P. C. H. Morel

^* Riddet Centre, Massey University, Palmerston North, New Zealand; and and Institute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand

	Abstract

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The genetic upper limit to daily whole-body 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 x (Landrace x 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

	INTRODUCTION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
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 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 nutrient-dense 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 accurately 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

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
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 x (Landrace x 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 formulated 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).

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 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:

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, Math-works 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 iver-mectin (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.

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 = + ß₁ x + e), quadratic (y = + ß₁ x + ß₂ x² + e), and cubic (y = + ß₁ x + ß₂ x² + ß₃ x³ + e) regression models, and the statistical significance of sex and the time x 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 whole-body protein, ash, and lipid as a function of empty BW, 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

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
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.

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 creatinine excretion expressed on a metabolic BW basis (mg of creatinine/kg of BW^0.75) for 16 pigs from the 36 kg of BW group ranged from 98 to 124 mg/kg^0.75 (overall mean, 110 mg/kg^0.75), 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 () 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.

View larger version (8K): [in this window] [in a new window]   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 () for an improved genotype given increasing dietary energy intakes of an experimental diet.

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 (kg·d^–1). Figure 2 shows plots of predicted ADG vs. BW for females and entire males, as derived from the 2 models.

View larger version (8K): [in this window] [in a new window]   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; – • – • – linear function, female; —— quadratic function, female).

View larger version (8K): [in this window] [in a new window]   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; – • – • – linear function, female; —— quadratic function, female).

View larger version (7K): [in this window] [in a new window]   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; – • – • – linear function, female; —— quadratic function, female).

View larger version (7K): [in this window] [in a new window]   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.

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

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

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
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 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 nutrient-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 oven-dried 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 near-complete 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 N-balance 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 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 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.

	Footnotes

 
¹ We wish to acknowledge the advice of the late W. C. Smith and the expert technical assistance of G. Pearson and C. Sosas.

³ Current address: Novartis Institutes for BioMedical Research, Novartis Pharma A.G., Basel, Switzerland.

² Corresponding author: p.j.moughan{at}massey.ac.nz

Received for publication May 24, 2005. Accepted for publication August 2, 2006.

	LITERATURE CITED

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 


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Mohn, S., and C. F. M. de Lange. 1998. The effect of body weight on the upper limit to protein deposition in a defined population of growing pigs. J. Anim. Sci. 76:124–133.[Abstract/Free Full Text]

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