J. Anim. Sci. 2003. 81:705-716
© 2003 American Society of Animal Science
Effects of protein deprivation on subsequent growth performance, gain of body components, and protein requirements in growing pigs
K. Y. Whang*,
S. W. Kim
,
S. M. Donovan
,
F. K. McKeith and
R. A. Easter,1
* Department of Animal Science, Korea University, 136-701 Seoul;
and
Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409; and
and
Department of Animal Sciences andDivision of Nutritional Science, University of Illinois, Urbana 61801
1 Correspondence:
122 Mumford Hall, 1301 W. Gregory Dr. (phone: 217- 333-0460; fax: 217-244-2871; E-mail:
r-easter{at}uiuc.edu).
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Abstract
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Forty-eight barrows were used in a 2 x 6 factorial arrangement to test a hypothesis that feeding a protein-deficient diet affects subsequent growth response by altering the efficiency of protein utilization. Barrows were individually fed either a 9% crude protein (CP) diet or an 18% CP diet from 20 to 30 kg of body weight (BW) (depletion phase). From 30 to 45 kg BW (realimentation phase), pigs were fed one of six experimental diets with CP levels of 11.8, 13.1, 14.3, 15.6, 18.8, and 21.8%. Four pigs were slaughtered at 20 kg BW to determine initial body composition. Four pigs from each treatment in depletion phase (a total of eight) were slaughtered at 30 kg BW, and all pigs from each treatment in realimentation phase (a total of 36) were slaughtered at 45 kg BW for subsequent compositional analysis. Pigs were bled at 20, 30, and 40 kg BW for blood urea nitrogen (BUN), insulin-like growth factor (IGF)-I, and IGF-binding protein (IGFBP) assays. Pigs were given three times the maintenance digestible energy requirement (3 x 120 kcal BW-0.75•d-1) in three equal meals daily. The feed allowance was adjusted every 3 d. During the depletion phase, pigs fed the 18% CP diet grew faster and more efficiently (P < 0.01) and gained more (P < 0.01) water and protein than did pigs fed the 9% CP diet. Pigs fed the 18% CP diet showed higher (P < 0.01) BUN values, IGF-I concentrations, and IGFBP ratios than pigs fed the 9% CP diet. During the realimentation phase, pigs fed the 9% CP diet during the depletion phase grew faster (P < 0.05), tended to grow more efficiently (P = 0.066), gained more water (P < 0.01), and tended to gain more protein (P = 0.068) than pigs fed the 18% CP diet during the depletion phase. Pigs fed the 9% CP diet during the depletion phase tended (P = 0.069) to have a higher protein requirement during the realimentation phase than pigs fed the 18% CP diet during the depletion phase. When measured at 40 kg BW, pigs fed the 9% CP diet had a lower (P < 0.05) BUN than pigs fed the 18% CP diet during the depletion phase. However, the plasma IGF-I concentration and IGFBP ratio at 40 kg BW were not affected by dietary CP level fed during the depletion phase. This study indicates that pigs fed a protein-deficient diet exhibit compensatory growth. During the period of compensatory growth, the requirement of CP for those pigs is higher than that of pigs previously fed an adequate diet. This study also suggests BUN can be used as an indicator of protein utilization efficiency and compensatory growth.
Key Words: Binding Proteins • Body Protein • Compensatory Growth • Insulin-Like Growth Factor • Pigs
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Introduction
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Following a period of nutrient intake restriction, pigs show compensatory growth (Whittemore et al., 1978; Campbell et al., 1983; Bikker et al., 1996b). Compensatory growth results from either increased voluntary feed intake of pigs following a period of restriction (Owen et al., 1971; Donker et al., 1986) or from improved feed efficiency (Campbell et al., 1983; Prince et al., 1983). Bikker et al. (1996a,b) indicated that compensatory protein growth was mainly due to an increase in organ protein. However, our previous study showed that contribution from organ growth was not the major component for the compensatory protein growth (Whang et al., 2000b). We also showed that there was no difference in voluntary feed intake between pigs showing compensatory growth and pigs showing normal growth. Whether compensatory growth is mainly caused by improved feed efficiency is not fully understood. Thus, the present study was conducted mainly to test the hypothesis that compensatory protein growth occurs not as a result of a simple increase in feed intake, but as a consequence of increased efficiency of protein utilization.
Blood urea nitrogen (BUN) can be used to estimate lean growth potential and efficiency of nitrogen utilization during compensatory growth (Hancock et al., 1988; Hayden et al., 1993; Whang et al., 2000a). It is not clear whether IGF-I and IGFBP can be used as indicators of lean accretion in swine. This study was conducted to see if IGF-I and IGFBP are valid indicators of lean growth in swine.
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Materials and Methods
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Animals and Feeding
A total of 48 barrows (Pig Improvement Co., Franklin, KY; Camborough 15 x line 326; initial weight = 18.0 ± 0.2 kg) were used. Paired littermate barrows with a similar BW from 16 litters were allotted to each treatment. Pigs were individually housed in partially slotted, concrete-floor pens with a stainless-steel feeder and a nipple waterer. The ambient temperature was maintained at approximately 22°C.
Pigs were given ad libitum access to a 17% CP diet from 18 to 20 kg BW. Four pigs were slaughtered to determine initial body composition at 20 ± 0.1 kg BW. The remaining 44 pigs were allocated between two treatments representing either an 18% or 9% CP diet, which was fed to pigs from 20 to 30 kg BW (depletion phase, Table 1
). At the end of the depletion period, four pigs from each dietary treatment were slaughtered for body composition assessment. As a second factor, the remaining 18 pigs in each treatment were individually fed one of six experimental diets with six CP levels (11.8, A; 13.1, B; 14.3, C; 15.6, D; 18.8, E; and 21.8%, F) from 30 to 45 kg BW (realimentation phase, Table 2). At 45 kg BW, all the remaining 36 pigs (3 pigs x 2 x 6 treatments) were slaughtered for estimation of gains in body components.
To ensure a constant balance of amino acids, diets A to E were prepared by dilution of diet F with cornstarch and crystalline cellulose. The ME level was the same in all diets. The ME level was adjusted by changing the ratio of cornstarch to soybean oil in a diet. Pigs over the weight range of 20 to 45 kg BW were fed three times the maintenance DE requirement (3 x 120 kcal•BW-0.75•d-1) in three equal meals daily at 0600, 1200, and 1800. The feeding level was adjusted every 3 d based on the change in BW.
The animal care protocol was approved (#A4R273) by the Laboratory Animal Care Committee of the University of Illinois at Urbana-Champaign, and all pigs were available for monitoring by the institutional veterinarian or his representative.
Bleeding and Plasma Analyses
Pigs were bled at 20, 30, and 40 kg BW for assays of BUN, IGF-I, and IGFBP. The blood sampling procedure for BUN has been previously described (Whang and Easter, 2000). The animals were fasted for 12 h and fed a BUN test diet (Table 3) based on an equation of kg BW0.75 x 0.58 g nitrogen. Pigs were then bled 2, 3, and 4 h after the meal to obtain the peak BUN value. Pigs were also bled between meals at 1200 and 1800 for IGF-I and IGFBP assays. Pigs were bled by ear vein puncture. A 10-mL heparinized blood sample was obtained and centrifuged at 2,000 x g for 20 min. Harvested plasma was frozen at -70°C until analysis.
The BUN levels of individual samples were measured on an autoanalyzer (Boehringer Mannheim Diagnosis, Indianapolis, IN) employing a method described by Skeggs (1957) and Marsh et al. (1965).
Plasma IGF-I concentration was measured by specific RIA according to Zhao et al. (1995). A 500--L aliquot of plasma was chromatographed in 0.25 mol/L formic acid on a 0.9 x 100-cm column containing Sephadex G-50 (Pharmacia LKB, Piscataway, NJ) to separate IGF-I from IGFBP. Between 47 and 72 mL of eluent that contained IGF-I was collected, lyophilized, and then resuspended with RIA buffer (0.03 M sodium phosphate, 2.5 g/L of BSA, 0.2 g/L of sodium azide, pH 7.5) to reconstitute to the original volume (500 -L). More than 90% of IGF-I was recovered from the column following the procedure of Zhao et al. (1995). The IGF-I concentration was measured by detecting [125I]IGF-I as the radioligand and a polyclonal anti-rabbit somatomedin-C-IGF-I antibody. The antibody was distributed through the Hormone Distribution Program from the National Institute of Diabetes, Digestive and Kidney Diseases to the National Hormone and Pituitary Program. The assay was conducted using duplicates of 25-, 50-, and 100--L samples, and binding was determined by a 
-counter (COBRA Auto-Gamma 5000, Packard Instrument, Meriden, CT). Samples were analyzed within a single assay with an intraassay CV of 6% for IGF-I.
Composition of plasma IGFBP-1, -2, and -3 was characterized by Western ligand blotting as described by Zhao et al. (1995) with duplication. Thirty microliters of a 1:10 dilution of plasma was prepared for SDS-PAGE by the addition of sample buffer (Laemmli, 1970), applied to a 4% stacking gel, and separated through a 12% gel under nonreducing conditions overnight at 50 V, 4°C. Following the electrophoresis, proteins were electrotransferred onto 0.45--m nitrocellulose membranes (Micron Separations, Westborough, MA) using a Buchler semidry electrophoresis transfer unit (Labconco, Kansas City, MO) at 200 mA for 1 h. Nitrocellulose membranes were sequentially blocked with Tris-buffered saline (0.15 M sodium chloride, 0.01 M Tris HCl, pH 7.5) containing 30 g/L Tergitol NP-40, Tris-buffered saline containing 10 g/L BSA and Tris-buffered saline containing 1 g/L Tween. Membranes were incubated overnight at 4°C with [125I]IGF-I in Tris-buffered saline containing 10 g/L bovine serum albumin and 1 g/L Tween. Membranes were washed with Tris-buffered saline, air-dried, and IGFBP was visualized by exposure to Kodak X-Omat AR film (Rochester, NY) with an intensifying screen for 7 d at -70°C.
Body Composition
The whole body was ground and analyzed for water, protein, fat, and ash as described by Whang et al. (2000b). Pigs were slaughtered by electrical stunning and exsanguinated. Heads were separated from bodies, and the head, body, and emptied viscera were separately ground three times through a 1.6-cm die. Between grinds, the material was thoroughly mixed. A composite, whole-body sample was prepared by mixing proportional subsamples of the ground head, body, viscera, and blood. Samples were frozen at -20°C until analyzed for DM, ash, protein (nitrogen x 6.25), and fat (AOAC, 1990). The water content was calculated by weight loss after drying at 105°C for 15 h in a forced-air oven. Samples were then extracted in 87% chloroform/13% methanol (vol/vol) in a Soxhlet apparatus for 24 h, and fat content was calculated by weight difference. Dry ashing was accomplished by heating at 550°C for 24 h. A sample mass balance of 99 to 101% was accepted. If this was not achieved, the analysis was repeated.
Statistical Analysis
All data were analyzed by ANOVA using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Single degree of freedom orthogonal contrasts were used to test linear and quadratic responses to dietary protein levels given during the realimentation phase. The GLM procedure (PROC GLM) of SAS was programmed to perform the statistical analysis used to compare the response slopes between the treatments (Littell et al., 1997) as previously described by Kim and Easter (2001). Regression equations were obtained between the body component gains and the dietary protein level during the realimentation phase from pigs previously received either 9 or 18% CP diets during the depletion phase. Slopes from regressions were compared in order to obtain the relative response of body component gains of pigs previously fed a protein-deficient diet (9% CP) to pigs previously fed a protein-adequate diet (18% CP).
Where appropriate, response criteria were fitted to a rectilinear "broken-line" model using the NLIN procedure of SAS as described by Robbins (1986). This model was used to estimate the crude protein requirement as follows:
where Y is the dependent variable, x is the dietary CP level, b is the slope of the regression equation, c is the breakpoint, and a (when x > = c) and a + bc (when x < c) are the Y intercepts. Separate regression analyses were conducted for pigs fed the 9 and 18% CP diets during the depletion phase, and response criteria were regressed on dietary CP concentration during the realimentation phase. The breakpoints for ADG, gain:feed, average daily water gain, and average daily protein gain were determined using a model involving one linear spline and plateau and further analyzed to detect the statistical differences.
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Results
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Depletion Phase (from 20 to 30 kg Body Weight)
During the depletion phase, pigs that received the 18% CP diet grew faster (P < 0.01) and more efficiently (P < 0.01) than those that received the 9% CP diet (Table 4). Pigs fed the 9% CP diet took more (P < 0.01) time (27.3 ± 0.5 d) to reach 30 kg BW than pigs fed the 18% CP diet (17.0 d). When compared on the basis of ADG (g/d) of body components, pigs that received the 18% CP diet also gained more (P < 0.01) water and protein than pigs fed the 9% CP diet, but fat gains were not different (P = 0.38) between treatments. Pigs that received the 18% CP diet had greater (P < 0.01) water content (%) and less (P < 0.01) fat content (%) than those that received the 9% CP diet. However, protein content (%) did not differ (P = 0.47) between treatments (Table 4). Pigs that received the 9% CP diet tended (P = 0.09) to have smaller viscera than pigs that received the 18% CP diet (Table 5). During the realimentation phase, the weight of empty viscera from pigs that received the 9% CP diet during the depletion phase linearly increased as the CP level increased (P = 0.021).
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Table 4. Growth performance, gain in body components during the depletion phase (20 and 30 kg body weight), and body composition at 30 kg body weight
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The changes of BUN levels during the depletion phase in individual pigs were calculated. The change in BUN during the depletion phase in pigs fed the 18% CP diet (-0.87 ± 0.47 mg/dL) was smaller (P < 0.01) than that in pigs fed the 9% CP diet (-5.39 ± 0.41 mg/dL) (Figure 1a).
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Figure 1.
Characteristics of plasma constituents of pigs fed diets with different protein levels during 20 to 30 kg BW. (a) The changes in blood urea nitrogen from 20 to 30 kg BW, (b) concentration of plasma IGF-I measured at 30 kg BW, and (c) the ratio of (IGFBP-3 to IGFBP-1 and -2 at 30 kg BW. Data points represent means ± SE of three pigs. Pigs fed the 18% CP diet from 20 to 30 kg BW ( ); (diagonal lines): pigs fed the 9% CP diet from 20 to 30 kg BW.
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The concentration of IGF-I in pigs fed the 18% CP diet (138.5 ± 10.5 ng/mL) was greater (P < 0.01) than that in pigs fed the 9% CP diet (81.9 ± 7.1 ng/mL) (Figure 1b
). The ratio of IGFBP-3 to IGFBP-1 and -2 was higher (P < 0.01) in pigs fed the 18% CP diet (1.70 ± 0.12) than in pigs fed the 9% CP diet (1.18 ± 0.05) (Figure 1c).
Re-alimentation Phase (30 to 45 kg Body Weight)
During the realimentation phase, pigs previously fed the 9% CP diet during the depletion phase grew faster (P < 0.05), took fewer days to reach 45 kg BW, and tended (P = 0.065) to grow more efficiently than pigs previously fed the 18% CP diet (Table 6). The number of days to reach 45 kg BW decreased linearly (P < 0.01) and ADG increased linearly (P < 0.01) as the CP level during the realimentation phase increased for both groups of pigs previously fed 9 and 18% CP diets. Gain:feed was the same among pigs fed different CP diets during the realimentation phase as it was in those previously fed the 9% CP diet. However, gain:feed increased linearly (P < 0.01) as the CP level during the realimentation phase increased in pigs previously fed the 18% CP diet (Table 6).
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Table 6. Days to reach 45 kg body weight, average daily gain, average daily feed intake, and gain:feed of pigs during the realimentation phase (30 to 45 kg BW)
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During the realimentation phase, pigs previously fed the 9% CP diet during the depletion phase had greater (P < 0.01) water gain and tended to have greater (P = 0.068) protein gain than pigs previously fed the 18% CP diet (Table 7). Average daily fat gain did not differ between the pigs previously fed the 9 and 18% CP diets. Average water gain and average protein gain increased linearly (P < 0.01) as the CP level during the realimentation phase increased for the both groups of pigs previously fed the 9 and 18% CP diets. Different dietary CP levels during the re-alimentation phase did not affect average daily fat gain of pigs (Table 7).
At the end of the realimentation phase (45 kg BW), pigs previously fed the 9% CP diet during the depletion phase had a lower water content, a lower protein content, and a higher fat content than pigs previously fed the 18% CP diet (Table 8).
At 45 kg BW, there was no difference in viscera weights between the groups of pigs previously either fed the 9 or 18% CP diets during the depletion phase (Table 5). However, within pigs previously fed the 9% CP diet during the depletion phase, viscera weights linearly increased (P < 0.05) as dietary CP levels during the realimentation phase increased, whereas viscera weights were not affected by the CP levels during the realimentation phase in pigs previously fed the 18% CP diet during the depletion phase.
The change in BUN from 20 to 40 kg BW was lower (P < 0.05) in pigs previously fed the 9% CP diet during the depletion phase than pigs previously fed the 18% CP diet (Figure 2). As dietary CP increased during the realimentation phase, the differences in BUN level measured at 20 and 40 kg BW were increased (linear; r2 = 0.700; P < 0.01).
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Figure 2.
Blood urea nitrogen (BUN) of pigs fed diets with different protein levels from 30 to 40 kg BW. The values are the differences of BUN levels that were measured at 20 and 40 kg BW. Responses to increasing dietary crude protein are shown in pigs fed the 18% CP diet from 20 to 30 kg BW ( ––-) and pigs fed the 9% CP diet from 20 to 30 kg BW (•––-•). Data points represent means ± SE of three pigs.
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The IGF-I level and IGFBP ratio at 40 kg BW were not affected by dietary protein level given during the depletion phase. However, the IFG-I level and IGFBP ratio were linearly increased (P < 0.05) as dietary protein level during the realimentation phase increased (Figure 3).
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Figure 3.
Concentrations of plasma IGF-I of pigs at 40 kg BW. Pigs were fed diets with different protein levels from 30 to 40 kg BW. Data points represent means ± SE of three pigs.
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Discussion
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This study was conducted to further characterize compensatory growth phenomena in growing pigs. During the realimentation phase that followed the depletion phase, pigs previously fed a protein-deficient diet grew faster and tended to grow more efficiently than pigs previously fed a protein-adequate diet when all pigs were fed the same fixed amount of diet on a BW basis. These pigs with compensatory growth tended to gain more body protein and had a higher protein requirement for maximal gain during the realimentation phase than pigs with normal growth.
Pigs fed the 9% CP diet during the depletion phase were clearly amino acid deficient compared to pigs fed the 18% CP diet, which showed normal growth rates (NRC, 1998). During the realimentation phase, each group of pigs showed different responses to dietary CP levels. Bikker et al. (1996a) demonstrated that compensatory growth mainly occurs by fat gain rather than by protein gain, and that protein gain mainly occurred in organs. However, our data show that pigs that previously received a protein-deficient diet gained more body protein and gained protein more efficiently during the following realimentation phase, whereas fat gain was not different from that of normal pigs. This indicates that compensatory growth does promote protein rather than fat gain. Bikker et al. (1996a) restricted feed intake of pigs during the depletion phase and increased feed intake during the realimentation phase, whereas our study mainly targeted protein restriction. This reflects previous findings that excess energy intake due to increased feed intake causes excess fat gain during the realimentation phase (Owen et al., 1971; Stamataris et al., 1991).
Pond and Mersmann (1990) showed that the major component of compensatory growth in pigs was organ growth, such as that of the gastrointestinal tract, and this result agrees with Koong et al. (1983) and Bikker et al. (1996a). However, our study does not show the different growth in organs, indicating that the animal compensatory growth response varies depending on the condition of restriction and realimentation given to the animal. Pond and Mersmann (1990) concluded from their compensatory growth study that the nature and degree of compensatory growth are affected by interrelated factors.
Increased feed intake was observed during compensatory growth in our previous study (Whang et al., 2000b). Restricted feeding eliminated a possibility that pigs would exhibit compensatory responses because of higher voluntary feed intake (compensatory feed intake) during the period of compensatory growth. In a situation where the feed intake of all pigs and dietary energy content were the same, but only dietary CP level was different, we could minimize the changes in the weight of organs due to the effect of feed intake and so could generate a situation where compensatory growth occurs mainly due to the changes in the carcass.
When a restriction was caused only by dietary protein level, pigs responded by growing more slowly. This was mainly due to decreased water and protein gains, not because of the fat gain, which caused the protein-restricted pigs to be smaller but fatter than the normal pigs by the end of the depletion phase. When pigs were fed a protein-sufficient diet after the depletion phase, protein gain in pigs was linearly increased by dietary protein content. Using the slope ratio technique of Kim and Easter (2001), the response of average daily protein gain to increased dietary protein content tended to be greater (P = 0.073) in pigs previously fed a protein-restricted diet than those fed a normal diet (6.17 vs 5.58 g daily protein gain/CP percentage in diets during the realimentation phase, respectively; Table 9). This result agrees with those of a previous study by Kyriazakis et al. (1991), which showed that the pigs with compensatory growth had a higher efficiency of protein gain than the pigs with normal growth.
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Table 9. Comparison of slope ratio responses of selected parameters to the dietary protein levels during the realimentation phase in pigsa
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The gain:feed ratio tended to be greater in pigs previously fed a protein-deficient diet than in pigs previously fed a normal diet during the realimentation phase. When gain:feed was reevaluated only for protein gain and intake, protein gain:protein intake also tended to be higher (P = 0.056) in pigs previously fed a protein-deficient diet than in pigs previously fed a normal diet during the realimentation phase (Table 10). There was no clear trend in whole-body gain:feed related to responses to dietary protein level given during the realimentation phase. However, when gain:feed was reevaluated by this revised measure for protein (protein gain:protein intake), the efficiency of protein utilization was affected (quadratic effect, P < 0.05) by the level of dietary protein given during the realimentation phase (Table 10). The maximal efficiency of protein utilization in pigs previously fed a protein-deficient diet occurred when the dietary protein content during the realimentation phase was 14.7%. However, efficiency of protein utilization in pigs previously fed a normal diet was not affected by the protein content during the realimentation phase.
The effort to determine the differences in the protein requirements between the pigs previously fed either a protein deficient diet or a normal diet was investigated using the broken-line technique (Robbins, 1986) based on ADG (Figure 4a), gain:feed (Figure 4b), average daily water gain (Figure 5a), and average daily protein gain (Figure 5b) of pigs during the realimentation phase. Breakpoint estimates of the CP requirement of pigs during the realimentation phase based on ADG (16.64 vs 16.06%), gain:feed (16.50 vs 15.97%), average daily water gain (16.70 vs 16.11%), and average daily protein gain (17.06 vs 16.79%) were averaged for pigs fed the 9 and 18% CP diets during the depletion phase, respectively, and these average values from the analysis indicate that the pigs previously fed a protein-deficient diet tended (P = 0.069) to require higher dietary protein (16.73%) for maximal protein gain than the pigs previously fed a normal diet (16.23%). This finding agrees with other hypotheses (Kyriazakis et al., 1991) and our previous finding from a N balance trial that pigs exhibiting compensatory growth required more amino acids (Whang et al., 2000a).
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Figure 4.
Broken-line plots of (a) ADG and (b) gain:feed of pigs fed diets with different protein levels during 30 to 45 kg BW. Responses to increasing dietary CP are shown in pigs fed the 18% CP diet from 20 to 30 kg BW (––-) and pigs fed the 9% CP diet from 20 to 30 kg BW (•––-•). Data points represent means ± SE of three pigs. Breakpoints for ADG were 16.64% at 916.9 g for pigs fed the 9% CP diet (ADG = 916.9 - 52.37 x (16.64 - CP); P = 0.001) and 16.06% at 863.0 g for pigs fed the 18% CP diet (ADG = 863.0 - 63.3 x [16.06 - CP]; P = 0.001). Breakpoints for gain:feed (G/F) were 16.51% at 0.55 for pigs fed the 9% CP diet (G/F = 0.55 -0.03 x [16.51 - CP]; P = 0.006) and 15.97% at 0.53 for pigs fed the 18% CP diet (G/F = 0.53 -0.04 x [15.97 - CP]; P = 0.001).
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Figure 5.
Broken-line plots of (a) average daily water gain (ADWG) and (b) average daily protein gain (ADPG) of pigs fed diets with different protein levels during 30 to 45 kg BW. Responses to increasing dietary crude protein are shown in pigs fed the 18% CP diet from 20 to 30 kg BW (––-) and pigs fed the 9% CP diet from 20 to 30 kg BW (•––-•). Data points represent means ± SE of three pigs. Breakpoints for average daily water gain were 16.70% at 680.5 g for pigs fed the 9% CP diet (ADWG = 680.5 - 63.85 x [16.70 - CP]; P = 0.002) and 16.11% at 554.8 g for pigs fed the 18% CP diet (ADWG = 554.8 - 59.8 x [16.11 - CP]; P = 0.026). Breakpoints for average daily protein gain were 17.06% at 158.8 g for pigs fed the 9% CP diet (ADPG = 158.8 - 11.8 x [17.06 - CP]; P = 0.0001) and 16.79% at 148.3 g for pigs fed the 18% CP diet (ADPG = 148.3 - 11.49 x [16.79 - CP]; P = 0.001).
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Although water and fat percentages of the whole body at 30 kg BW were different between groups, body protein percentage was not different. The tendency for pigs to keep the same percentage of body protein regardless of protein intake has been observed but not clearly explained. This finding, however, agrees with Campbell et al. (1983), who reported that feeding level affected the percentages of body fat and body water, but not body protein. The lack of variation in body protein content was confirmed during the realimentation phase in this experiment. The dietary protein level during the realimentation phase did not affect protein percentage of pigs within a group previously fed the same diet.
The change in BUN level (from 20 to 40 kg BW) was lower in the pigs previously fed a protein-deficient diet than pigs previously fed a normal diet. This result is in agreement with the observation that animals fed protein-deficient diets have reduced body N breakdown and N excretion (Garlick et al., 1973; Roobol and Alleyne, 1974; Whittemore et al., 1978). This also confirms our previous N balance experiment (Whang et al., 2000a).
A relationship between BUN and efficiency of N utilization was found with PST-treated pigs (Miller and Baldwin, 1989a,b). They reported that PST-treated pigs showed a linear decline of BUN in a dose-dependent manner. During the realimentation phase, the BUN was linearly correlated with dietary CP (P < 0.01). Many experiments using BUN to estimate AA requirement (Hahn et al., 1995) and protein quality (Eggum, 1970; Orok and Bowland, 1975; Bassily et al., 1982) showed the BUN changes in a curvilinear fashion. In our study, the amount of any excessive AA among treatments was proportionally same because the AA pattern of all experimental diets used during the realimentation phase was constant.
It is well established that IGF-I is related to growth and nutritional status. Pigs that grow faster and have less body fat have a higher plasma IGF-I concentration (Lund-Larsen and Bakke, 1975). In swine, intact males have both a higher IGF-I concentration and protein deposition rate than barrows (Taylor et al., 1992). These authors also reported that serum IGF-I was positively correlated with protein deposition at 30, 60, and 90 kg BW. Our study confirms the positive correlation between IGF-I level, rates of growth, and protein deposition. This also agrees with Whang et al. (2000a) who reported the positive correlation between IGF-I level and N retention. The relationship between growth and IGFBP ratio is also well established (Coleman and Etherton, 1991; Owen et al., 1999). The IGFBP-3 level is elevated in porcine somatotropin-treated swine (Evock et al., 1990). Fasting and weaning decreases IGFBP-3, but increases IGFBP-1 (White et al., 1991). The ratio of IGFBP-3 to IGFBP-1 and -2 should be positively correlated with growth and protein deposition (Owens et al., 1991; 1999). The ratio of IGFBP of pigs fed the 18% CP diet in the present experiment was higher than in pigs fed the 9% CP diet during the depletion phase, indicating that the pigs with greater protein gains show the higher ratio of IGFBP-3 to IGFBP-1 and -2.
However, the concentration of IGF-I and IGFBP ratio at 40 kg BW were not affected by feeding regime during the depletion phase. The compensatory growth measured in ADG, gain:feed, water gain, protein gain, and BUN were not correlated with IGF-I concentration or IGFBP ratios. The changes in IGF-I concentration or IGFBP ratio during the realimentation phase did not respond to compensatory growth. The IGF-I level and IGFBP ratio may have recovered to normal levels quickly. However, the IGF-I concentration (Figure 3) and IGFBP ratio increased (linearly; P < 0.01) as dietary CP was increased during the realimentation phase. It may be that the regulatory responses to realimentation occurred before the samples were obtained at 40 kg BW.
This study indicates that pigs exhibiting compensatory growth grow faster and tend to gain more protein, have a higher efficiency of protein utilization, and a higher protein requirement than normally growing pigs.
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Implications
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Pigs that have previously been fed a protein-deficient diet may need increased amounts of amino acids to support potentially higher protein gain during compensatory growth. Because these pigs have a higher efficiency of protein utilization during a realimentation phase, they have a higher protein requirement. Thus, providing higher protein diets to pigs showing compensatory growth may improve their lean growth.
Received for publication August 17, 2001.
Accepted for publication October 8, 2002.
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Literature Cited
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Abstract
Introduction
