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ANIMAL NUTRITION |
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* Prairie Swine Centre Inc., Saskatoon, Saskatchewan, Canada S7H 5N9; and and
Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A8
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: carcass composition • growth • net energy • maintenance energy • nutrient deposition • weaned pig
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The literature is limited on the impact of changing energy intake through the control of daily feed intake in the weanling pig. However, we can infer from studies using the growing pig (e.g., Bikker et al., 1995
; Quiniou et al., 1995) that both protein-dependent and energy-dependent phases of growth probably exist. The hypothesis tested in this experiment was that there would be a difference between the response of the weanling pig resulting from changes in dietary NE concentration vs. the response resulting from changes in feed (energy) intake.
The main objective of this experiment was to determine whether an interaction would be observed between daily energy intake and dietary NE concentration on BW gain and on tissue (protein, lipid, ash, water) accretion rates and ratios and on plasma IGF-I concentration. A secondary objective was to determine whether measured DE intake (DEi) or calculated NE intake (NEi; based on Centraal Veevoederbureau, 1994, 1998) is more effective in predicting animal growth performance.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. The experiment involved 3 all-in–all-out nursery rooms equipped with automatic light timers (12-h light:12-h dark cycle) and integrated controllers (Model PEC, Phason, Winnipeg, Manitoba, Canada) regulating the heating and ventilation systems. Room temperature was initially set at 29°C at weaning and gradually decreased by 1.5°C/wk. All pens (1.27 x 1.04 m) were equipped with fully slatted floors, a single nipple drinker, and an adjustable multiple-space dry feeder. The feeders were checked daily for proper feed flow to minimize wastage, and the drinkers were checked for adequate water flow.
Animals, Treatments, and Experimental Design
A growth and comparative slaughter trial was conducted with the castrated male offspring of C-22 females x 337 sires (PIC Canada Ltd., Winnipeg, Manitoba, Canada). The experiment was conducted in 3 replicates of 27 pigs each plus the initial slaughter group (ISG; n = 6). This provided a total of 87 barrows used in this experiment. Treatments were arranged as a 3 x 3 factorial, with 3 diets and 3 feed intake levels. Diets were formulated to contain 2.21, 2.32, and 2.42 Mcal of NE/kg (as-fed basis; Centraal Veevoederbureau, 1998). Three feed intake levels were used, corresponding to 100, 80, or 70% of ad libitum feed intake. These levels of restriction, and the nature of the design, were validated in a previous experiment (Oresanya, 2005).
Feed intake levels in the limit-fed pigs (80 or 70% of ad libitum feed intake) were based on the intake of pigs fed on an ad libitum basis within replicate. Before the first weighing period, completed on d 4 of the experiment, the feed intake of the control pigs was unavailable, so for this period alone, the restricted intakes were based on the data derived from other experiments using the same age of pig within the same barn (Oresanya et al., 2003, 2007). For limit-fed pigs, the total daily feed allowance was provided in a single morning feeding.
Before the beginning of the experiment, pigs were allowed ad libitum access to a pelleted commercial phase-1 starter diet (Ultrawean 21, Coop Feeds, Saskatoon, Saskatchewan, Canada) for the first 6 d post-weaning, followed by a pelleted phase-2 starter diet (GI MAX 21, Coop Feeds, Saskatoon, Saskatchewan, Canada) for the next 4 d. All available pigs were weighed on d 7 postweaning, and the most uniform animals, based on BW, weight per day of age, and postweaning ADG (8.5 ± 0.9 kg, 0.298 ± 0.041 kg/d, 0.164 ± 0.080 kg/d, respectively; mean ± SD) were selected. Pigs were blocked and randomly assigned to experimental treatments, and the ISG was selected based on BW.
Experimental Diets
Experimental diets (Table 1) were formulated to contain increasing levels of NE, based on Centraal Veevoederbureau (1998) NE values of the ingredients. The target NE concentration was 2.21 to 2.42 Mcal/kg; on analysis, the calculated NE concentrations were 2.15 (low), 2.26 (medium), and 2.37 (high) Mcal of NE/kg. Differences in NE concentration were achieved by a gradual reduction of CP concentration from 29.0 to 24.7% and crude fiber from 3.0 to 2.4%. Fat concentration was increased from 3.5 to 5.4% in the low- to high-NE diets. The diets contained celite added at 0.5% as a source of exogenous acid insoluble ash, to serve as an indigestible marker. The calculated and analyzed nutrient composition of the experimental diets is reported in Table 2.
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), such that the TID Lys/Mcal of DE ratio exceeded the requirement for this class of pig (Oresanya et al., 2007). Other AA were formulated to levels according to the ideal protein ratio for this BW class of pig (NRC, 1998). This ensured that the AA supply was nonlimiting for growth. Diet formulation was based on the assayed CP and AA composition of soybean meal, fish meal, and skim milk powder (Degussa Corporation, Amino Acid Laboratory, Allendale, NJ).
Data and Sample Collection
Pigs were weighed at the initiation of feeding of the experimental diets (31.5 ± 0.3 d of age and 9.5 ± 1.0 kg of BW) and twice weekly thereafter on Mondays and Thursdays before feeding. Feed disappearance was measured at each weigh day for the pigs fed on an ad libitum basis, and the daily feed allowances for the limit-fed pigs were adjusted based on the ad libitum access intake on each diet, calculated on a BW basis. Freshly voided feces were collected from each pig by using the grab method (Veum et al., 2004) over 3 d (d 15 to 17) and pooled per pig, to determine DE and to estimate NE concentration of diets from the digestible nutrient content. Fecal samples were frozen and stored at –20°C until they were lyophilized with a freeze-drier (Model 40-SUB, Virtis Co. Ltd., Gardiner, NY). Feed samples were taken at the time of feeding and pooled per diet. All samples were stored at –20°C until required for analysis.
A blood sample from each pig was taken at approximately 1100 on d 7 and again on d 21. Blood samples were collected via venipuncture into Vacutainer tubes containing 143 USP units of sodium heparin (Becton, Dickinson and Co., Oakville, Ontario, Canada). Plasma was harvested after centrifugation at 700 x g for 15 min (Model Centrific 228, Fisher, Nepean, Ontario, Canada) and stored at –20°C for later assay of IGF-I concentration. Plasma samples were analyzed for IGF-I by RIA as described previously (Kerr et al., 1990) after acid-ethanol extraction (Daughaday et al., 1980).
Slaughter Procedure and Carcass Measurement
The comparative slaughter procedure was applied to replicates 1 and 2. Replicate 3 was conducted to increase the number of pigs for the growth performance study only. Pigs assigned to the ISG were killed at the commencement of the experiment (d 0). The rest of the pigs remained on the experimental treatments until they reached 25 ± 1 kg of BW, at which time they were killed to determine body composition.
Pigs were euthanized by CO2 asphyxiation followed by exsanguination (Hoenderken, 1983; Gregory et al., 1987). The carcass was split down the ventral midline from the groin to the chest cavity, and the entire viscera were removed from the carcass. The bladder was aspirated of its contents by using a syringe and was left attached to the carcass. The gastrointestinal tract was separated from the viscera and weighed, emptied of all digesta, patted dry, and reweighed. The liver, kidneys, heart, lungs, and spleen were weighed individually. All individual weights included the associated fat (i.e., mesenteric, renal, and pericardial fat). The weights of the organ fraction and blood were recorded as total organ weight and are referred to herein as "noncarcass." The weight of the eviscerated carcass (including head and feet) was recorded and is referred to as "carcass." The empty BW (EBW) of the pig was taken as the sum of the weight of the carcass and the noncarcass. The noncarcass fraction and blood were pooled and stored separately from the carcass.
The carcass and noncarcass were frozen at –20°C until further processing. The frozen carcasses were cut into quartiles and passed through a 10-mm die 4 times in a commercial grinder (Model 801 GHP-25, Autio Company, Astoria, OR). After the final pass through the grinder, subsamples of the carcass were collected and manually blended to produce a 250-g sample, which was then placed in a previously weighed aluminum container for later analysis. The noncarcass fraction was passed through the die once and mixed thoroughly before several subsamples were placed in a previously weighed aluminum container. All samples were weighed immediately after collection and kept frozen until freeze-drying to a constant weight.
Chemical Analyses
Feed and lyophilized fecal samples were prepared for chemical analyses by air-equilibration and passed through a 1-mm screen (Retsch Model ZM1, Brinkman Instrument of Canada Ltd., Rexdale, Ontario, Canada).
The acid insoluble ash content of the diet was used as an indigestible marker and was measured in feed and feces (McCarthy et al., 1974) to determine the apparent total tract digestibility of DM and other nutrients. Pure celite standard samples were assayed to confirm the accuracy of the analytical procedure, and a recovery of 99.9 ± 0.01% was attained.
The moisture contents of feed and freeze-dried fecal samples were determined by drying at 135°C in an airflow-type oven for 2 h (method 930.15; AOAC, 1990). Nitrogen in feed and fecal samples was measured by combustion (method 968.06; AOAC, 1990) with a Leco protein/nitrogen apparatus (Model FP-528, Leco Corp., St. Joseph, MI). Calibration was conducted with an EDTA standard (nitrogen concentration 9.57 ± 0.02%; Leco Corp.). On analysis, the nitrogen concentration of EDTA was 9.56 ± 0.02%. Crude protein was calculated as nitrogen x 6.25.
Gross energy was measured in an adiabatic bomb calorimeter (Model 1281, Parr Instruments, Moline, IL). Benzoic acid (6,318 kcal/kg; Parr Instruments) was used as the standard for calibration and was determined to be 6,317 ± 2 kcal/kg at assay. Crude fat in feed samples was determined after ether extraction (method 920.39; AOAC, 1990) in an extractor apparatus (Labconco Corp., Kansas City, MO) and in fecal samples after acidification with 9 N HCl to allow quantification of saponified fatty acids, followed by ether extraction. Feed and fecal samples were analyzed for crude fiber by using an Ankom fiber analyzer (Ankom Technology Co., Fairport, MI). Ash was determined by incineration in a muffle furnace at 600°C for 12 h.
Feed samples were passed through a 0.5-mm screen and analyzed enzymatically for starch (method 996.11; AOAC, 2002) by using a total starch assay kit (AA/ AMG, Megazyme International Ireland Ltd., Bray, Co. Wicklow, Ireland). Feed samples were analyzed for total carbohydrates, total nonstarch polysaccharides, and free sugars based on the methods of Englyst and Hudson (1987) and Englyst (1989), respectively. Total sugars were calculated as total carbohydrates (starch + total nonstarch polysaccharides). According to Graham et al. (1986) and Bach Knudsen and Hansen (1991), apparent total tract digestibility of starch and sugar were assumed to be 100%; therefore, starch and sugar were not determined in the fecal samples.
Freeze-dried carcass and noncarcass samples were prepared for chemical analyses by blending in a grinder (Retsch Grindomix, Model GM200, F. Kurt Retsch GmbH and Co. KG, Haan, Germany). Samples were analyzed for DM, GE, crude fat, and ash as described above. Nitrogen was measured with the Leco apparatus (method 992.15; AOAC, 2002) and CP was expressed as nitrogen x 6.25. All chemical analyses were carried out in duplicate and were repeated when the intraassay CV exceeded 3%.
Calculations and Statistical Analyses
Apparent digestibility values of N, energy, and other nutrients were determined by using the following equation:
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where DADN is the apparent digestibility value of a nutrient N, ID is the percentage index marker concentration in the assay diet, AF is the percentage nutrient concentration in feces, AD is the percentage nutrient concentration in the assay diet, and IF is the percentage index marker concentration in feces, all on a DM basis. The equation given by Noblet and Perez (1993) was used to calculate ME, and NE was estimated from digestible nutrients according to Centraal Veevoederbureau (1994):
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and
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where NE is expressed in kilocalories per kilogram (as-is), DCP is digestible CP, DEE is digestible ether extract, ST is starch, SG is sugar, and DRES is digestible residuals, calculated as digestible OM – (DCP + DEE + ST + SG + digestible crude fiber).
Digestible energy intake was calculated from the measured DE concentration x ADFI. Similarly, ME intake (MEi) was quantified from the calculated ME concentration x ADFI, and NEi was quantified from the calculated NE concentration x ADFI.
Digestible energy intake for maintenance (DEim) was calculated as 0.110 Mcal/(kg of BW0.75 x d) (NRC, 1998) and NE for maintenance (NEim) was calculated as 0.078 Mcal/(kg of BW0.75 x d) (Just, 1982). Digestible energy and NE available for growth (DEig and NEig, respectively) were calculated as DEi – DEim or NEi – NEim.
Energy efficiency for gain (expressed as Mcal/kg) was calculated as DEig/ADG or NEig/ADG, where DEig or NEig was expressed in megacalories per day and ADG was the ADG in kilograms. Energy partitioning into protein and lipid deposition (expressed as g/Mcal) was calculated as protein deposition (PD) [or lipid deposition (LD)]/DEig or PD (or LD)/NEig, where PD and LD were the respective determined deposition rates (g/d) of the slaughtered experimental pigs, calculated as described below.
The relationship between live BW and EBW at slaughter was determined for the ISG. This was used together with the data from chemical analysis of the carcass and noncarcass of the ISG to estimate the initial body composition of the experimental pigs. The gains in protein, lipid, ash, water, and energy were estimated as
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Empty body GE content was estimated in 2 ways, by bomb calorimeter analysis conducted on carcass and noncarcass and by calculation based on the analyzed protein and lipid concentrations and using the factors 5.66 and 9.46 Mcal/kg for protein and lipids, respectively (Ewan, 2001). Similarly, energy retained as protein (ERP) and energy retained as lipids (ERL) were calculated as PD (in g/d) x 5.66 kcal/g and LD (in g/d) x 9.46 kcal/g, respectively.
Statistical Analyses.
Data were analyzed by using the MIXED procedure (SAS Inst. Inc., Cary, NC) with the individual pig as the experimental unit and initial BW as a covariate for performance data. The statistical model included the effect of diet, feeding level, and the diet x feeding level interaction. Plasma IGF-I concentration data were analyzed by using repeated measures and appropriate covariance structures (Littell et al., 1998; Wang and Goonewardene, 2004). The statistical model included the effect of day, diet, feeding level, and the following interactions: diet x day, and diet x feeding level. Regression analyses within SAS were used to evaluate the efficiency of utilization of measured DE and calculated NE within diets for growth and nutrient deposition. Differences in the slopes of the regression lines were evaluated according to the methods of Zar (1984).
Pearson correlation coefficients between DEi, NEi and performance, and carcass variables were analyzed by using the correlation procedure of SAS. Least squares means are reported, and differences were considered significant at P < 0.05. Trends (0.05 < P < 0.10) are reported, and P > 0.10 was considered nonsignificant.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) with increasing feeding level. Dietary NE concentration and feeding level did not affect G:F. However, a NE x feeding level interaction (P = 0.031) on G:F was observed, because pigs fed the intermediate NE concentration diet at the 80% feeding level exhibited decreased G:F compared with the other treatments.
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). Net energy intake and NEig increased with increased dietary NE concentration (P < 0.001). As expected, DEi, DEig, NEi, and NEig increased with increasing feeding level (P < 0.001).
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). The partition of DEig and NEig to PD (g/Mcal) tended to decline (P = 0.052) and declined (P < 0.001), respectively, as dietary NE concentration increased. In contrast, the partitions of DEig and NEig to LD (g/Mcal) were increased with increased dietary NE concentration and feeding level (P < 0.05). However, a NE x feeding level interaction (P < 0.05) was observed because of a greater lipid deposition per megacalorie of DEig and NEig in pigs allowed ad libitum access to the high-NE concentration diet compared with those receiving other treatments (P < 0.05).
Regression analyses within dietary NE concentration showed differences in the slope of the linear relationship between ADG and NEig (P < 0.05; Figure 1). Similar analyses revealed differences in the slope (P < 0.05) of the linear relationship between PD, LD, and the LD:PD ratio and NEig (Figure 1).
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). Carrying this one step further, total retained energy (RE), ERP, and ERL were correlated with DEi by using DEi (kcal/kg of BW0.75 per d) as the independent variable and RE, ERP, and ERL (kcal/kg of BW0.75 per d) as dependent variables. By setting RE = 0, the quantity of DE required for maintenance (DEim) was estimated at 118 kcal/kg of BW0.75 per d. In the same manner, the ME required for maintenance (MEim) was estimated to be 116 kcal/kg of BW0.75 per d and the NE required for maintenance (NEim, the MEi at zero RE) was estimated to be 71 kcal/kg of BW0.75 per d.
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) and CP was increased (P < 0.05) with increased dietary NE concentration. Apparent GE and DM digestibility increased up to 2%, whereas CP, ash, and crude fat digestibility were increased up to 1, 7, and 34%, respectively, with increased dietary NE concentration. Conversely, apparent crude fiber digestibility decreased (P < 0.001) by as much as 23% with increased dietary NE concentration.
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Body Chemical Composition
Protein concentration in carcass and empty body declined up to 3% with increased dietary NE concentration (P < 0.05; Table 7). There was a tendency (P < 0.10) for protein concentration in carcass and empty body to decline with increasing feeding level.
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A NE x feeding level interaction was detected in water, lipid, and GE concentrations in carcass and empty body (P < 0.01; Table 7). The interaction was illustrated by a reduced water concentration and by increased lipid and GE concentrations in the carcass and empty body of pigs given ad libitum access to the high-NE concentration diet.
Nutrient Deposition Rates and Ratios
The rates of water, protein, lipid, and ash deposition and RE in noncarcass were not affected by dietary NE concentration (Table 8) but were increased with increasing feeding level (P < 0.001). Ash:protein and water:protein ratios were not affected by NE concentration and feeding level in the noncarcass fraction.
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The results of empty body deposition rates of water, protein, lipid, ash, LD:PD ratio, water:protein ratio, ash:protein ratio, and RE are shown in Table 9. Reflective of the effect on carcass and noncarcass, the rates of water, protein, and ash deposition, and the protein:water and ash:protein ratios in empty body were not affected by dietary NE concentration. In contrast, water, protein, and ash deposition rates were increased with increasing feeding level (P < 0.01). The ash:protein ratio decreased with increasing feeding level (P < 0.05), whereas the water:protein ratio was unaffected by feeding level. A NE x feeding level interaction was detected in RE:energy intake ratios (RE:GE, RE:DE, RE:ME, and RE:NE; P < 0.01; Table 9).
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). The interaction of NE and feeding level on LD, the LD:PD ratio, and RE in carcass and empty body illustrates the increased LD, LD:PD ratio, and RE in the carcass and empty body of pigs given ad libitum access to the high-NE concentration diet as compared with lower energy diets.
Physical Body Composition at Slaughter
Carcass, noncarcass, and empty BW were not affected by NE concentration. In addition, carcass, non-carcass, and empty BW expressed as a percentage of live BW were not affected by NE concentration (Table 10). In addition, carcass BW, EBW, and EBW as a percentage of live BW were not affected by feeding level. Noncarcass weight and noncarcass weight as a percentage of live BW increased up to 10 and 7%, respectively, with increasing feeding level (P < 0.05). In contrast, carcass weight as a percentage of live BW declined up to 2% with increasing feeding level (P < 0.05).
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). There were 7, 14, and 16% increases in empty digestive tract, kidney, and liver weights, respectively, with increasing feeding level (P < 0.05). These organ weights as a percentage of EBW tended to increase in a manner similar to the level of intakes (P < 0.06). Blood, heart, lung, and spleen weights (and as a percentage of empty body) were not affected by dietary NE concentration and feeding level.
Plasma IGF-I Concentrations
There was no effect of dietary NE concentration on plasma IGF-I concentrations (Table 11). However, IGF-I concentrations increased with increasing feeding level and were greater on d 21 than on d 7 (P < 0.001), indicative of the increasing ADG over this time frame.
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. Average daily gain and ADFI were positively correlated with DEi and NEi (P < 0.001), but not with G:F. Furthermore, empty body lipid concentration, LD, PD, and the LD:PD ratio were all positively correlated with DEi and NEi (P < 0.001). Empty body CP concentration was weakly negatively correlated with NEi (P < 0.05).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, Quiniou et al. (1995), and Weis et al. (2004) studied the effect of varying energy supply, achieved by controlling daily feed intake, on growth performance and body composition in growing pigs, but we could find no references in the literature to similar studies in the weanling pig. In contrast, there have been numerous studies on the effect of energy concentration on the growth performance of weanling pigs. For example, Tokach et al. (1995), Reis de Souza et al. (2000), and Levesque (2002) evaluated increasing ME, GE, or DE, respectively, on performance. However, no one has investigated the interaction of dietary energy concentration and daily feed intake.
In studies of energy utilization, it is important that the AA supply is adequate and that marginal deficiencies do not confound experimental results. We confirmed in a preliminary trial (Oresanya et al., 2006) that the experimental model used here could avoid confounding by the AA supply if daily feed intake restriction was less than 30%. Furthermore, the optimal total Lys:DE ratio used in the current study was 5% greater than the required ratio determined in a previous experiment (Oresanya et al., 2007) using the same genetics in the same barn. As a final guard against AA insufficiency, selected ingredients were preassayed for AA content prior to diet formulation. We also confirmed, by using a factorial approach to determining the Lys requirement, that AA intake was not limiting performance.
According to the results of a previous study (Oresanya et al., 2007) and those reported by Levesque (2002), measured DE and calculated NE values may differ from formulated values. For a more accurate interpretation of results and to confirm that dietary energy concentration was indeed varied, NE was determined by using the actual digestibility of individual components of the diet and applying the Centraal Veevoederbureau (1994) prediction equation. Although the calculated NE values of diets were lower than formulated, the range of NE among the diets was virtually identical to those intended (220 and 210 kcal, respectively). The range in measured DE differed from formulated DE (140 vs. 90 kcal), illustrating once again the importance of measuring dietary energy concentration directly in studies focused on the utilization of dietary energy.
It has long been recognized that energy concentration is an important determinant of voluntary feed intake of pigs (NRC, 1987; Lewis, 2001), with feed intake declining as energy concentration increases. However, in the current study, increasing dietary NE concentration did not affect feed intake. Reis de Souza et al. (2000) reported no effect on feed intake with DE concentration increasing from 3.24 to 3.50 Mcal/kg in weaned pigs between 7 to 25 kg of BW. Conversely, Levesque (2002) showed a 6.3% decline in feed intake of weaned pigs between 7 to 20 kg of BW when the measured DE concentration increased from 3.18 to 3.59 Mcal of DE/kg. In our previous study (Oresanya, 2005), increasing the NE concentration reduced the feed intake of pigs. It is generally accepted that dietary energy concentration is not the only factor affecting feed intake in the weaned pig (Patience et al., 1995).
Certain dietary factors, for example, bulkiness (Whittemore et al., 2001) and fat content (e.g., Xing et al., 2004), exert direct or indirect physiological effects that may reduce feed intake. Because dietary fat is suggested to reduce the digesta passage rate (Azain, 2001), elevated dietary fat concentration may act as a constraint on feed intake, and may explain part of the reduction in feed intake observed in other studies when dietary energy concentration was increased (Van Lunen and Cole, 1998; Smith et al., 1999; Levesque, 2002). Dietary fat concentration was increased from the low to the high energy diet by 13.1, 6.8, and 8.2%, respectively, in the aforementioned studies compared with only 5.4% in the current study. The results of the current study suggest that because feed intake was not highly correlated with dietary energy concentration, the physiological effect of dietary components may be an important factor in determining feed intake above the simple NE concentration.
Similar to feed intake, feed efficiency was not affected by dietary NE concentration. This is contrary to our previous studies in which feed efficiency was improved with increasing DE concentration (Levesque, 2002; Oresanya et al., 2007). Feed efficiency would be expected to increase either when a reduction in feed intake occurs without changes in growth rate (Pettigrew and Moser, 1991; Xing et al., 2004) or an increase in growth rate occurs without any change in feed intake. However, the end point in the current study was a constant BW, whereas most growth studies are conducted to a constant length of feeding period. Thus, if pigs require more time to reach a given BW end point, this will inevitably affect G:F because longer feeding periods are associated with increased maintenance feed requirements, for example, more days on test.
Because the pigs did not reduce their feed intake as dietary NE concentration increased, NEi also increased (Table 4). This observation is consistent with the suggestion that weaned pigs have a limited ability to regulate energy intake based on energy density (NRC, 1987). However, it must be noted that DEi calculated from the measured DE concentration in the diet was not affected by dietary treatment. This would suggest that the response of weaned pigs to energy concentration should be expressed in terms of the total available energy equivalent (i.e., calculated NEi). This is supported by a stronger correlation between empty body composition and nutrient deposition rates with NEi as compared with DEi.
The observation that NEi increased with increased dietary NE concentration, without increasing the growth rate, strongly suggests that pigs fed the low-NE diet were able to consume sufficient energy to maximize growth rate. The adequacy of the low-NE diet was further confirmed by the decreased slope observed when comparing ADG against NEi among the 3 diets (Figure 1); the increase in ADG per unit of NEi was decreased on the high-NE diet.
Contrary to the response of ADG to changes in NE concentration, increasing daily feed intake did increase ADG. However, body protein concentration tended to decline with increased feed intake, whereas NE concentration interacted with feed intake on both body water and lipid content in the empty body. As discussed below, the results of empty body composition and nutrient deposition rates further support the adequacy of the low-NE diet.
Little is known about the interactive effects of dietary energy concentration and feed (energy) intake on the chemical composition of the body of the weaned pig. A previous study by Campbell and Dunkin (1983) reported a decline in empty body protein concentration with increasing energy intake in pigs growing between 7 and 19 kg of BW. Conversely, empty body lipid concentration increased with energy intake, but at a greater rate in pigs fed the low-CP diet compared with the high-CP diet (Campbell and Dunkin, 1983). In the current study, empty body protein concentration declined with increasing NE concentration, but not feeding level. In contrast, an interaction of NE concentration and feeding level in empty body water and lipid content resulted in decreased water content and greater lipid content in the empty body of pigs with unrestricted access to the high-NE diet compared with pigs on the other treatments.
The increase in noncarcass weight to increased feeding level in the current study was predominantly due to an increase in the weight of the gastrointestinal tract, liver, and kidneys (Table 10). Bikker (1994) indicated that the metabolically active organs (intestines, liver, kidneys, and pancreas) are very sensitive to the amount and type of nutrients ingested. The decline in carcass weight with increasing feeding level may be due to an increase in gut fill at greater feeding levels (Bikker, 1994). The effect of feeding level on noncarcass weight in the current study is consistent with those reported in growing pigs by other workers (Rao and McCracken, 1992; Bikker et al., 1995, 1996; Gomez et al., 2002).
Together, these results suggest that the adverse effects of increasing NE concentration, in terms of body chemical composition, but not in terms of physical composition, may explain the lack of improved BW gain in the weaned pig when dietary energy concentration is increased.
In the current study, protein, water, and ash deposition rates in the empty body were not affected by dietary NE concentration, but were increased with increasing feeding level. Research in growing pigs has reported a concomitant increase in protein, water, and ash with increasing feeding level (Bikker et al., 1995; Quiniou et al., 1995, 1996; Gomez et al., 2002).
As observed in other studies that evaluated the response of weaned pigs to energy intake (Gädecken et al., 1985; Kyriazakis and Emmans, 1992; Collin et al., 2001), the present results indicate that weaned pigs deposited more protein than lipids. Empty body PD increased with increasing feeding level but was not affected by dietary NE concentration. Similarly, empty body LD increased with increasing feeding level, but unlike PD, NE concentration interacted with feeding level on LD. Indeed, a 95% greater LD was observed in pigs given ad libitum access to the high-NE diet compared with the low-NE diet. The increase in empty body PD and LD with increasing feeding level observed in the current study is consistent with studies in growing pigs (Campbell et al., 1983; Bikker et al., 1995).
It is well recognized that PD increases linearly with increasing energy intake when the diet is limiting only in energy (de Lange et al., 2001). Assuming that increasing the dietary energy concentration is a way to increase lean growth, remove the limitation from physical gut capacity, or both, one might expect PD to increase with increased NE concentration. Data from the current study revealed that only LD, and not PD, increased with increasing dietary NE concentration. Contrary to the preceding assumption, this study demonstrates that increasing energy concentration may increase only LD in weaned pigs.
The lipid:protein ratio is an indicator of the associated variations of BW gain (Whittemore and Fawcett, 1976) and of lean growth. It assumes that below the maximum PD, the LD:PD ratio is constant and independent of BW (Whittemore and Fawcett, 1976) and energy intake (Whittemore, 1983). In the current study, the LD:PD ratio was increased concomitant with an increase in energy intake with increasing NE concentration (Figure 1). This demonstrates that the LD:PD ratio in the weaned pig is increased by both energy intake and energy concentration. Indeed, the LD:PD ratio in the empty body increased by 38 and 49% with increasing feeding level and energy concentration, respectively. Clearly, increasing the supply of utilizable energy through any means appears to increase the LD:PD ratio.
There was no relationship between NEi and LD:PD ratio on the low- and medium-NE diets. However, because of the increase in LD:PD ratio in pigs allowed unrestricted access to the high-NE diet, the LD:PD ratio increased linearly with NEig, that is, the quantity of NE consumed above maintenance (R2 = 0.78; P < 0.05; Figure 1). This further demonstrates a negative effect of increasing NE from 2.15 to 2.37 Mcal/kg on the lean growth of pigs.
In general, the effect of dietary NE concentration and feeding level on water and ash deposition rates in carcass, noncarcass, and empty body closely resembled the effect described for PD. In fact, these 3 chemical components in the body are known to be closely associated (Kotarbinska, 1971; de Greef, 1992). Thus, the empty body water:protein ratio was constant among dietary NE concentrations and feeding levels. Likewise, the ash:protein ratio was not influenced by dietary NE concentration (mean = 0.16). However, because empty body PD increased at a faster rate than ash, the ash:protein ratio declined with increasing feeding level. In contrast, Kyriazakis and Emmans (1992) showed that energy intake of pigs between 12 to 28 kg of BW did not change the empty body ash:protein ratio (0.19).
Increasing the NE concentration increased the amount of lipid deposited per megacalorie of energy (Table 4). This may be related to the increase in dietary fat content and intake. Chudy and Schiemann (1969) indicated that dietary fat is utilized with a greater efficiency for body lipid deposition than carbohydrates. This is due to lower heat losses associated with the incorporation of dietary fatty acids into body lipids. Daily LD was related to digestible fat intake (r = 0.71, P < 0.001). However, LD was equally related to starch intake (r = 0.88, P < 0.001). The decreased efficiency of energy utilization for growth with increasing NE concentration (Table 4 and Figure 1) and feeding level is clearly due to the changing body composition, as demonstrated by the decline in empty body protein and increase in lipid content. Because 1 kg of lean muscle contains 77 to 80% water, compared with only 5% for adipose tissue, the energy cost of lean growth is considerably less than adipose tissue deposition (NRC, 1998). Furthermore, mostly because of an increase in LD, there were increases in the RE:DE, RE:ME, and RE:NE ratios with increasing NE concentration and energy intake (interaction, P < 0.001; Table 9).
Our estimate of 118 kcal/kg of BW0.75 per d for DEim is similar to the 122 kcal/kg of BW0.75 per d for weaned pigs estimated by Campbell and Dunkin (1983) and the estimate of 110 kcal/kg of BW0.75 per d reported by the NRC (1998). Our estimated NE for maintenance (71 kcal/kg of BW0.75 per d) agrees with that reported by Robles and Ewan (1982) and is similar to the 78 kcal/kg of BW0.75 per d reported by Just (1982).
Insulin-like growth factors, and particularly IGF-I, mediate the growth-stimulating action of GH (Simmen et al., 1998) and GH-dependent increases in PD (Boyd and Bauman, 1989). Circulating IGF-I reflects endogenous GH secretion and overall growth in well-nourished humans and animals (Blum et al., 1993; Simmen et al., 1998). The effect of an increasing dietary energy concentration and feeding level on circulating concentrations of IGF-I in the weaned pig has not previously been established.
Feeding level, and not NE concentration, had a considerable effect on plasma IGF-I in the current study (Table 11). The lack of effect of dietary NE concentration on plasma IGF-I concentrations is consistent with that of Lee et al. (2002), who showed that increasing dietary DE concentrations from 2.95 to 3.50 Mcal/kg in growing pigs from 59 to 105 kg of BW did not affect serum IGF-I concentrations.
Buonomo et al. (1987) indicated that circulating concentrations of IGF-I are positively correlated with growth rate in pigs. Plasma IGF-I concentrations in the current study increased 105% from d 7 to 21, and are consistent with the increase in growth rate within that period.
On the basis of these results, we conclude that the response of the pig to changes in dietary energy concentration differ from those accruing from changes in daily feed intake. Consequently, one must be very careful in extrapolating conclusions about energy utilization obtained under restricted feed intake regimens to commercial conditions in which the dietary energy concentration is being changed. The few instances reported herein of interaction between changes in NE concentration and changes in feed intake suggest that in many respects, the outcomes as they relate to energy utilization are quite different. Because NE correlates more closely than DE with LD and LD:PD, we are also able to conclude that NEi offers an advantage over DEi in predicting body composition and rate of gain in weanling pigs.
The results of the current study indicate that increasing the dietary NE concentration increased energy intake, body lipid content, and deposition rate, but not protein deposition rate. Thus, in future studies of energy utilization, there is a clear need to consider both total BW gain and the composition of that gain. Furthermore, we must be careful when making adjustments to the energy concentration of practical diets based on the results of experiments using restricted feed intake, because the pig clearly responds differently to the 2 situations. As a final point, it should be noted that these results were obtained with 1 genetic population; the response to energy supply may differ among different genotypes.
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2 Current address: Cargill Animal Nutrition, 235-36th Street North, T1H 5R8, Lethbridge, AB, Canada.
3 Correspondence: john.patience{at}usask.ca
Received for publication January 3, 2007. Accepted for publication November 1, 2007.
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