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ANIMAL NUTRITION |
* Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, N1G 2W1 Canada and
and
Institute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand
Abstract
Two experiments were conducted to determine independent effects of BW and DE intake on body composition and the partitioning of retained body energy between lipid and protein in pigs with high lean tissue growth potentials and when energy intake limited whole-body protein deposition. In a preliminary N-balance experiment involving 20 entire male pigs at either 30 or 100 kg BW, it was established that whole-body protein deposition increased linearly (P < 0.05) with DE intake at both BW. These results indicate that DE intake controlled whole-body protein deposition and that these pigs did not achieve their maximum whole-body protein deposition when fed semi-ad libitum. In the main serial slaughter experiment, 56 pigs, with a BW of 15 kg, were assigned to one of four DE intake schemes and slaughtered at 40, 65, 90, or 115 kg BW. Within DE intake schemes, DE intake was increased linearly (P < 0.05) with BW, allowing for an assessment of effects of DE intake and slaughter BW on chemical and physical body composition (carcass, viscera, blood). Between 15 and 90 kg BW, average DE intake of 16.1, 20.9, 25.2, and 28.8 MJ/d supported average BW gains of 502, 731, 899, and 951 g/d, respectively. The proportion of whole-body protein present in the carcass increased with BW and decreased with DE intake (P < 0.05), whereas the distribution of whole-body lipid between carcass and viscera was not influenced by BW and DE intake. A mathematical relationship was developed to determine the relationship between DE intake at slaughter (MJ/d) and chemical body composition in these pigs: whole-body lipid-to-protein ratio = 1.236 - 0.056 x (DE intake) + 0.0013 x (DE intake)2, r2 = 0.71. The data suggests that absolute DE intake alone was an adequate predictor of chemical body composition in this population of entire male pigs over the BW and DE intake ranges that were evaluated, simplifying the characterization of this aspect of nutrition partitioning for growth in different pig populations.
Key Words: Body Weight • Digestible Energy • Energy Partitioning • Growing Pig • Protein Deposition
Introduction
In modern pig types, energy intake may limit the expression of high lean tissue growth potentials for much of the growing-finishing period (Rao and McCracken, 1991
; de Greef, 1992; Bikker, 1994). The relationship between energy intake and lean tissue growth varies between groups of pigs and appears to be influenced by various factors, including pig genotype (Campbell and Taverner, 1988; Quiniou et al., 1995), BW (de Greef, 1992; Quiniou et al., 1995), nutritional history (de Greef, 1992; Bikker, 1994), and environment (Black et al., 1995; Le Bellego et al., 2002). Various approaches have been used to represent mathematically the relationships between energy intake and lean tissue growth or body protein deposition (PD), closely associated with lean tissue growth. Approaches include empirical relationships between energy intake and PD (Black et al., 1986; Quiniou et al., 1995; NRC, 1998), effects of dietary protein-to-energy ratios on the efficiency of utilization of balanced protein for PD (Kyriazakis and Emmans, 1992b), constraints on body lipid deposition (LD)-to-PD ratios (minimumLD/PD) (de Greef, 1992; Bikker, 1994), or constraints on ratios of whole-body lipid to whole-body protein (minimum lipid/protein) (Moughan et al., 1987; Schinckel and de Lange, 1996). Among these approaches, constraints on minimum lipid/protein may be preferred. It implies that growth and voluntary feed intake are determined by the body composition that pigs try to achieve and allows for the representation of compensatory growth (Emmans and Kyriazakis, 1997). However, these approaches are all rather empirical and require at least three parameters, which have generally been derived from data sets in which energy intake and BW are confounded or in which BW and energy intake effects are not considered simultaneously.
The objective of this study was to establish the effects of BW and energy intake on chemical and physical body composition in pigs with high lean tissue growth potentials, and lipid-to-protein ratio in particular.
Materials and Methods
Animals and General Management
Two experiments were conducted using 97 growing-finishing purebred Yorkshire entire male pigs from the University of Guelph herd. The first experiment, an N-balance study, was conducted to establish the relationship between energy intake and PD in this population of pigs at low (30 kg) and high (100 kg) BW. This information was used to better define the treatments for the more elaborate serial slaughter experiment.
In both experiments, pigs were housed individually in fully slatted floor pens (1 m x 1.75 m) in environmentally controlled rooms (minimum 22°C) and given free access to water supplied by nipple drinkers (Möhn and de Lange, 1998). The pigs were weighed weekly and fed twice daily in equal portions. Feeding levels were adjusted weekly according to BW.
Experiment 1: Relationship Between Energy Intake and Whole-Body Protein Deposition at Two Body Weights
In this experiment, N-balances and dietary energy digestibility were determined in pigs exposed to four levels of energy intake when they weighed approximately 30 (low BW) and 100 kg (high BW). Pigs were obtained at approximately 20 and 85 kg BW having been fed well-balanced commercial pig diets ad libitum.
Twenty-one low-BW (21.8 ± 1.4 kg) pigs, derived from 11 litters, were assigned randomly to energy intake levels (n = 5 per energy intake level, but n = 6 for the highest energy intake level). Littermates were assigned to different treatments, whereas average initial BW was kept similar across treatments. In a similar manner, twenty high BW (84.3 ± 2.7 kg) pigs, derived from 20 litters, were assigned randomly to energy intake levels.
The target energy intake levels were either 100%, 90%, 74%, or 59% of voluntary daily digestible energy intake (DEi) according to NRC (1998; DEi (MJ/d) = 55.07 x (1 - e-0.0176 x BW(kg))) for low-BW pigs, and 95%, 78%, 66%, or 54% of DEi for high-BW pigs. These DEi were referred to as semi-ad libitum, or mild, moderate, and severe energy intake restriction, respectively. Actual feeding levels were established based on targeted daily DEi divided by the estimated diet DE contents.
Diets were formulated to ensure that dietary levels of vitamins, minerals, and lysine exceeded NRC (1998) requirements for growing-finishing pigs with high lean tissue growth potentials; lysine was formulated to be the first-limiting amino acid in all diets (Table 1). This was done to ensure that DEi determined pig growth performance. To maintain diet consistency across treatments, at each of the two BW only diets for the two extreme feeding levels were manufactured (basal diets), whereas diets for the intermediate feeding levels were prepared by blending the basal diets (Table 1).
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. For every successful 24-h urine collection, urine was weighed and a 5% subsample was taken and stored at 4°C. The 5% subsamples were pooled for each pig and N-balance period before analysis. Feces and wasted feed were collected quantitatively and pooled for each pig over the entire N-balance period. Feces was stored at -11°C until further analysis.
Experiment 2: Independent Effects of Energy Intake and Body Weight on Body Composition
Fifty-six pigs (17.1 ± 1.1 kg), consisting of eight groups of four littermates and eight groups of three littermates, were used in the serial slaughter experiment. Pigs were assigned to one of four slaughter BW (40, 65, 90, and 115 kg) and to one of four energy intake schemes. Littermates were assigned to different slaughter BW but to the same DEi at slaughter, except for the two extreme DEi where two littermates were assigned to each slaughter BW. When assigning pigs to treatments, BW was considered in such a manner that the mean initial BW was similar across projected slaughter BW.
Within energy intake schemes, DEi was increased linearly with BW, in such a manner that the three lowest DEi at a specific slaughter weight were the same as the three highest DEi at the previous slaughter weight (Figure 1). This allowed us to assess effects of BW and DEi on body composition, even though DEi increased with BW. The four energy intake schemes, which were derived from the results of the preliminary experiment, were high, medium-high, medium-low, and low. Energy intake levels were adjusted weekly based on the mean anticipated BW for the next week. Of the 16 DEi x BW at slaughter combinations (4 x 4), two treatments (40-kg low and 115-kg high) were eliminated, as there were no pigs exposed to similar energy intake levels at slaughter in any of the other treatments, resulting in five levels of energy intake at slaughter across the four slaughter BW (19.0, 24.2, 29.3, 34.4, and 39.5 MJ/d; Figure 1).
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). In order to minimize the number of diets that needed to be prepared and to maintain diet consistency across treatments, four basal diets were manufactured from which the experimental diets were prepared by blending (Table 3). Diets were analyzed for Ca and Na content to confirm accuracy of blending.
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). Blood was collected quantitatively and weighed. Visceral organs (kidneys, spleen, liver, lungs, and heart) were weighed individually. The full gastrointestinal tract (including the anus) was weighed, emptied, and reweighed to determine gut fill. The gastrointestinal tract was then added to the visceral organs and placed in a plastic bag and frozen at -11°C. The empty carcass, which included head, feet, hair, and skin, was weighed, placed in plastic bags, and stored at -11°C.
Homogenization and sampling of carcass and viscera for chemical analyses was performed according to Tuitoek et al. (1997) with slight modifications according to Möhn et al. (2000).
Analytical Methods
The dry matter contents in diet samples were determined, in duplicate, by oven drying for 2 h at 135°C (AOAC, 1990); those in feces, carcass, and viscera by freeze-drying followed by oven drying for 2 h at 135°C. The N content in fresh diet and urine samples and in the freeze-dried feces, carcass, and viscera samples were determined in triplicate using the Kjeldahl method (AOAC, 1990). Crude fat (as ether extract) and ash contents in fresh diet and freeze-dried carcass and viscera samples were determined in duplicate according to AOAC (1990).
In the first experiment, GE contents were determined in feed and feces according to AOAC (1990). In the second experiment, the amino acid contents in the basal diets were determined using ion exchange chromatography with postcolumn derivatization with ninhydrin (Llames and Fontaine, 1994; Degussa A. G., Hanau, Germany).
Calculations and Statistical Analysis
In Exp. 1, N retention was calculated as diet N allowance minus N in wasted feed, urine, and feces. The rate of PD (in grams per day) was calculated as daily N retention x 6.25. To assess whether amino acid intake determined PD, lysine disappearance was calculated as apparent ileal digestible intake minus lysine retention in PD, minus estimated integumental lysine losses (Möhn et al., 2000).
In Exp. 2, within energy intake schemes and for individual pigs, PD (in grams per day) was calculated as the determined protein (in grams) at a specific slaughter weight, minus estimated protein at a previous slaughter weight divided by time (in days). Estimated protein was derived from BW and the mean determined protein content (a percentage) in BW of pigs at the appropriate BW and energy intake level combination. Composition of the empty body was calculated from weights of the three body fractions (carcass, viscera, and blood) and their respective compositions. Blood was assumed to contain 81.5% water, 0.6% fat, 0.7% ash (McDonald et al. 1966), and 17.5% CP (Möhn et al. 2000). The lipid-to-protein ratio was calculated from total-body protein and total-body lipid weight.
Statistical procedures were performed using the GLM procedure and SAS Version 8 (SAS Inst., Inc., Cary, NC). Treatment means were compared using the Tukey honestly significant difference test (Snedecor and Cochran, 1989). Regression analysis using GLM was conducted to evaluate linear and quadratic effects of DEi and BW on the various response criteria. In Exp. 1, the difference in PD response to energy intake between the low- and high-BW pigs was evaluated by assessing the interaction between DEi (covariable) and BW group (class variable; high vs. low), and mean BW during the N-balance period was used as a covariable.
In Exp. 2, responses were analyzed statistically using target DEi at slaughter (n = 5), litters (nested within energy intake levels at slaughter; n = 16; random effect in mixed model), target slaughter BW (n = 4), and actual DEi and EBW at slaughter (covariables) as sources of variation. The interactive effect of target DEi at slaughter and target slaughter BW was considered as well. Within target slaughter BW, responses were analyzed with the same model but omitting the target slaughter BW effect. Levine’s test was performed to confirm the equality of variance of the response variables between the various DEi and slaughter BW combinations using SAS Version 8. In Exp. 2, both absolute DEi and DEi over maintenance (DEim; MJ/d) were considered using estimates of DEim derived from NRC (1998) (0.458 x BW0.75) and Noblet et al. (1999) (1.02 x BW0.60). The DEi values used were those determined for the day preceding slaughter.
Results and Discussion
Experiment 1: Relationship Between Energy Intake and Whole-Body Protein Deposition at Two Body Weights
Upon commencement of the N-balance observations, the average BW was 31.1 ± 1.9 kg and 103.1 ± 1.9 kg for low- and high-BW pigs, respectively. During the adaptation period, one of the low-BW pigs died. All remaining pigs appeared to be healthy and readily consumed the experimental diets.
In the low-BW pigs, fecal digestibility of diet energy was not influenced by feeding level (overall mean, 90.5 ± 0.7%). Across feeding levels, determined diet DE content was 6.1% higher than calculated diet DE content. For relating PD to DEi, the calculated diet DE contents were used (Figure 2). The PD increased linearly (P < 0.05) with DEi, whereas the quadratic effect of DEi on PD was not significant. Calculated lysine disappearance did not differ across DEi, with treatment means ranging between 22.8 and 25.6% of apparent ileal digestible lysine intake.
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). Calculated lysine disappearance did not differ across DEi, with treatment means ranging between 20.0 and 23.5% of apparent ileal digestible lysine intake.
Calculated lysine disappearance was marginally larger than the minimal lysine disappearance observed in N-balance experiments conducted under similar conditions (Möhn et al. 2000; 19%). Apparently, lysine, the first-limiting amino acid in the experimental diets, did not limit PD as was intended by the dietary formulations. Because intakes of essential nutrients (amino acids, vitamins, and minerals) exceeded requirements and PD increased with energy intake level, it was concluded that DEi limited the expression of lean tissue growth potentials in this population of entire male pigs and within this DEi range.
The slopes of the linear relationship between PD and DEi (Figure 2) represent the marginal partitioning of energy intake between PD and LD. Per megajoule of extra DEi, PD increased by 7.9 ± 1.0 and 5.1 ± 0.6 g for the low- and high-BW pigs, respectively. The slope decreased (P < 0.05) with BW, suggesting that pigs need to deposit more lipid per unit of PD as BW increases. The latter is in agreement with observations made by others (Black et al., 1986; Dunkin, 1990; Quiniou et al., 1995).
Experiment 2: Effects of Energy Intake and Body Weight on Body Composition
For various reasons that were not related to experimental treatments (clinical signs of disease, temporary loss of appetite), data from five pigs were excluded from the data set (Tables 4, 5, and 6). The remaining pigs showed no signs of clinical disease or abnormal behavior during the experiment and readily consumed their feed allowances.
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). These systematic errors reflect an underestimation of nutrient contents in some of the ingredients or a systematic error in analytical procedures. The calculated CP and sodium content levels in the blended diets were within 5% and 10% of predicted values based on analyzed nutrient contents in the basal diets, respectively (data not shown), indicating that diets were prepared properly. Given the variability in determined dietary DE contents in the preliminary N-balance study, calculated dietary DE contents were used to establish DEi in the serial slaughter study.
The design of the current study allowed for the assessment of energy intake and BW effects on chemical and physical body composition. In previous studies that were of similar nature (e.g., Campbell and Taverner, 1988; de Greef et al., 1994; Bikker et al., 1996; Quiniou et al., 1995; Le Bellego et al., 2002), fewer treatments were evaluated. For example, Quiniou et al. (1995) studied the effects of body composition at four feeding levels (0.71, 0.80, 0.90, and 1 x ad libitum) at only two different BW at slaughter (45 and 100 kg), whereas de Greef et al. (1994) reported on the effects of two levels of energy intake (12.6 and 16.3 MJ/d above maintenance) at five slaughter weights (25, 45, 65, 85, and 105 kg). It should be noted that serial N-balance and indirect calorimetry measurements (e.g., Quiniou et al., 1995) allow for the evaluation of BW and energy intake effects on the partitioning of energy intake but not on body composition.
Estimated disappearance of lysine, expressed as a percentage of apparent ileal digestible lysine intake, was 35.6% for pigs on low energy intake (65 to 115 kg BW). These values were 27.3, and 30.2% for pigs on medium-low and medium-high energy intakes, respectively (40 to 115 kg BW). For pigs on high intake, the value was 35.1% (40 to 90 kg BW). These values are considerably higher than the minimum value of 25.6% observed in a serial slaughter study conducted under similar conditions (Möhn et al., 2000) and indicate that lysine intake did not limit PD.
Actual DEi levels were similar to the targeted levels, except for pigs up to about 55 kg BW that were on the high energy intake treatment, which had frequent feed refusals. This was anticipated because targeted DEi for the high intake treatment was about 15% higher than the voluntary daily DEi according to NRC (1998) for pigs below approximately 55 kg BW. As BW increased, feed refusals in pigs on high energy intake declined gradually as the targeted DEi relative to the voluntary intake according to NRC (1998) declined.
Growth Performance.
Average daily gain increased linearly (P < 0.05) with average DEi (ADEi) at all BW ranges, whereas quadratic effects (P < 0.05) of ADEi on ADG were observed at the three lowest BW ranges. Similar responses have been observed in other studies (e.g., Campbell and Taverner, 1988; Bikker et al., 1996; Coudenys, 1998). The ADG of pigs at the highest levels of energy intake (high energy group for 15 to 90 kg BW, medium-high group for 15 to 115 kg BW) was at least 950 g/d, which was lower than that observed in the preliminary N-balance study (988 g/d between 27 and 44 kg BW; 1142 g/d between 96 and 118 kg BW). However, in the N-balance study, pigs were fed semi-ad libitum and consumed more feed than in the main study, especially at higher BW (2.99 vs. 2.74 kg/d).
Over the three 25-kg BW ranges (40 to 65, 65 to 90, and 90 to 115 kg) PD increased linearly (P < 0.05) and not quadratically with energy intake (Table 5). The observed linear effects of energy intake on PD are consistent with observations in the preliminary N-balance study and indicate that energy intake determined PD. This suggests, but does not confirm, that the observed lipid-to-protein ratio was minimum lipid/protein at all DEi and slaughter BW combinations.
Chemical Body Composition.
Weight and chemical composition of the main body components are summarized for the four slaughter BW in Table 6. Across all observations, the sum of chemical body constituents (protein, lipid, ash, and water) contributed to 98.8 ± 0.2% of empty BW, confirming the adequacy of sampling and analytical procedures. In the current study, head, tail, feet, hair, and skin were included in the carcass fraction. As a result, the proportion of whole-body protein and lipid present in the carcass is higher than that observed in other studies (e.g., Quiniou et al., 1995; Bikker et al., 1996).
Across slaughter BW, protein content (grams per kilogram) in the empty body was not influenced by energy intake level and slaughter BW, but the interactions between these two sources of variation was significant (P < 0.05). The latter is supported by the reductions (P < 0.05) in protein content with increasing DEi in pigs slaughtered at 65 and 90 kg BW, whereas no effects were observed at the other slaughter BW (Table 6). Energy intake tended to influence (P = 0.06) the proportion of whole-body protein present in the carcass: in pigs slaughtered at 65 and 90 kg BW, the proportion of whole-body protein was 3.6 and 2.1 percentage units lower (P < 0.05) at the highest energy intake level than at the lowest energy intake level, whereas no differences were observed at the other slaughter BW. The proportion of whole-body protein contained in the carcass increased curvilinearly with BW (P < 0.05), from 83.7% at 40 kg BW to 89.3% at 115 kg BW, whereas no differences were observed between the two highest slaughter weights (Table 6). Quiniou et al. (1995) and Bikker et al. (1996) reported similar effects of energy intake and BW on empty-body protein content and partitioning of body protein between carcass and viscera. These findings indicate that lowering energy intake and increasing BW at slaughter are means to increase the fraction of whole-body protein recovered in the carcass and thus in more valuable pork products.
Across BW at slaughter, body lipid content (grams per kilogram) showed no interactive effects of target BW and DEi at slaughter. Body lipid content increased (P < 0.05) with energy intake level; within BW at slaughter, this effect was significant only at 90 and 115 kg BW (Table 6). The observed energy intake effects at the higher BW are in agreement with de Greef and Verstegen (1993), Quiniou et al. (1995), and Bikker et al. (1996). Conversely, Rao and McCracken (1992) reported no effects of energy intake on the empty-body lipid content. These apparent discrepancies can be attributed to the duration of the experiments because energy intake effects on body lipid are observed only after pigs are exposed to different energy intake levels for extended periods of time. Body lipid content tended to increase (P = 0.09) with BW at slaughter. This indicates that increases in body lipid content with BW in pigs on typical feed intake schemes (de Greef et al., 1994; Bikker et al., 1995, 1996) largely reflect increases in energy intake, and not effects of BW per se. Neither energy intake level nor slaughter BW influenced the proportion of body lipid present in the carcass at slaughter. Conversely, Rao and McCracken (1992) and Bikker et al. (1996) reported that increases in the level of energy intake decreased the proportion of body lipid present in the carcass, even though absolute effects were small.
In general and within BW at slaughter, treatment effects on body water and body ash content (grams per kilogram) tended to be similar to treatment effects on body protein content (Table 6), reflecting the close association between these three chemical body components (Schinckel and de Lange, 1996).
According to Emmans and Kyriazakis (1995), whole-body water content is best expressed per kilogram of body protein based on an allometric relationship: a x P0.855. In the current study, the exponent value varied between 0.805 and 0.952 across energy intake schemes, which appears consistent with the value of 0.855. Whole-body water weight was 5.46 ± 0.06, 5.67 ± 0.05, 5.71 ± 0.05, and 5.70 ± 0.06 kg per kilogram of (protein)0.855 for the low, medium-low, medium-high, and high energy intake levels, respectively, and did not differ between the four energy intake schemes. Apparently, within pig types, body lipid content does not influence the relationship between body water and body protein (Emmans and Kyriazakis, 1995). Across treatments, body water weight was 5.62 ± 0.03 (n = 51) per kilogram of (protein)0.855, which is higher than mean values reported in literature reviews (4.89, ARC, 1981; 5.06, Emmans and Kyriazakis, 1995) but is only slightly higher than a value reported for entire male pigs with high lean tissue growth potentials (5.4, de Greef and Verstegen, 1993). Gender and genotype appear to contribute to the relationship between body water and body protein.
Across treatments, the ratio of whole-body ash weight to whole-body protein weight was 0.203 ± 0.003 and was not influenced by slaughter BW or by energy intake level at slaughter. This value is consistent with Kyriazakis and Emmans (1992a), Quiniou et al. (1995), and Coudenys (1998), who reported values of 0.192, 0.186, and 0.189, respectively.
Ratios of Whole-Body Lipid to Protein.
Given the close association of both whole-body water and whole-body ash with whole-body protein, the lipid-to-protein ration is an effective and simple means to characterize body composition in growing pigs. Moreover, constraints on lipid/protein (e.g., minimum lipid/protein) can be used to represent effects of pig type, BW, energy intake, and environmental factors on the partitioning of retained energy between body lipid and body protein (Schinckel and de Lange, 1996).
In this experiment and within energy intake schemes, energy intake was increased gradually with BW. As a result, observed treatment effects on body protein, lipid and lipid-to-protein ratio are closely confounded with effects on PD, LD, and LD/PD. Based on the experimental design, it thus cannot be assessed whether energy intake effects on the partitioning of retained energy are best represented based on constraints on body composition (minimum lipid/protein) or on composition of growth (minimumLD/PD).
In the current study and across BW at slaughter, DEi influenced (P < 0.01) the lipid/protein ratio. Within BW at slaughter, DEi effects on lipid/protein ratio were evident in pigs slaughtered at 65 kg or heavier (Table 6). In contrast, target BW at slaughter did not influence lipid/protein ratio (Figure 3). This indicates that the observed effects of BW on lipid/protein ratio (de Greef and Verstegen, 1993; Quiniou et al., 1995; Bikker et al., 1996) can largely be attributed to the confounding of BW with energy intake in experimental designs.
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This model had a better predictive accuracy than the model in which the linear effect of DEi was considered only (r2 = 0.53).
The quadratic effect of energy intake on lipid/protein ratio in this study is not consistent with observed linear effects of energy intake on lipid/protein ratio in studies where pigs with high lean tissue growth potentials were exposed to varying energy intake levels and slaughtered at a similar weight across energy intake levels (Campbell and Taverner, 1988; Quiniou et al., 1995; Bikker et al., 1996). In the current study, the quadratic DEi effect is primarily a consequence of the lipid/protein ratio values observed in pigs on the extreme DEi at slaughter (Figure 3). It is possible that the observed lipid/protein ratio in pigs at the highest energy intake level exceeded minimum lipid/protein. In these pigs, energy intake may have exceeded energy intake required to maximize PD, which could have resulted in rates of LD that were higher than those determined by minimum lipid/protein. However, based on observed PD, calculated lysine disappearance, and results from the preliminary N-balance study, it is unlikely that energy intake did not determine PD and that observed lipid/protein ratio differs from minimum lipid/protein in pigs at the highest DEi at slaughter. Moreover, the observed difference in lipid/protein ratio between the two highest DEi at slaughter is consistent with effects of energy intake on lipid/protein ratio observed by others in entire male pigs with high lean tissue growth potentials (Campbell and Taverner, 1988; de Greef et al., 1994). Alternatively, observed lipid/protein ratio at the lowest energy intake level may have been higher than the minimum lipid/protein. It can be hypothesized that pigs on the lowest energy intake level had insufficient opportunity to adjust to the low energy intake level. Subsequent to the current study, we observed a lipid/protein ratio of 0.72 ± 0.05 (n = 4) in ad libitum-fed entire male pigs at 16.1 ± 0.3) kg BW. This observation was made with pigs from the same population and reared under similar conditions as those used in the current study, indicating that lipid/protein ratio in young growing pigs can be quite high and may be higher than minimum lipid/protein (Kyriazakis and Emmans, 1992a,b). It is, therefore, quite likely that the lipid/protein ratio in pigs at the lowest energy intake level would have been lower than 0.6 if energy intake restrictions had been initiated earlier.
When evaluating energy intake and BW effects on the partitioning of retained energy between protein and lipid maintenance energy requirements may be considered because only energy intake above maintenance requirements is available to support PD and LD. However, maintenance energy requirements are difficult to predict accurately and are known to vary with pig genotype, BW, and environmental conditions (e.g., Schinckel and de Lange, 1996; Noblet et al., 1999; Le Bellego et al., 2002). When comparing models based on either absolute daily energy intake or energy intake above assumed maintenance requirements (NRC, 1998; Noblet et al., 1999), the effects of sources of variation on observed lipid/protein ratio were similar. However, the degree of fit was lower for models based on energy intake above maintenance requirements. For example, r2 values were 0.59 and 0.60 for models that included linear and quadratic effects of DEi minus DEim at slaughter on lipid/protein ratio and when maintenance energy requirements were estimated according to NRC (1998) and Noblet et al. (1999), respectively. This indicates that it is more accurate and simpler to predict lipid/protein ratios based on models that include absolute daily energy intakes than on those that include daily energy intakes above assumed maintenance requirements. It also suggests that the partitioning of retained energy between PD and LD is determined by absolute energy intake, whereas the actual PD and LD are determined by energy intake above maintenance requirements.
Conclusions and General Discussion
In the current study, the proportion of whole-body protein present in the carcass increased with BW and decreased with DEi, whereas BW and DEi did not influence the proportion of whole-body lipid present in the carcass. This is consistent with previous studies (Quiniou et al., 1995; Bikker et al., 1996) and indicates that manipulating BW and energy intake at slaughter are means to increase the fraction of whole-body protein recovered in the carcass.
No effects of energy intake on the relationships between body water, body ash and body protein weight were observed in this study consistent with previous observations (ARC, 1981; de Greef and Verstegen, 1993; Kyriazakis and Emmans, 1992a; Emmans and Kyriazakis, 1995). However, the relationship between body water and body protein weight is known to vary with pig genotype (Emmans and Kyriazakis, 1995). Within pig populations, whole-body lipid/protein ratio is a simple and effective means of characterizing body composition.
The current study confirmed that energy intake is insufficient to maximize lean tissue growth in modern pig genotypes with high lean tissue growth potentials. This result stresses the need to characterize the relationship between energy intakes and lean tissue growth in different pig populations and when the intake of essential nutrients does not limit lean tissue growth or PD (Schinckel and de Lange, 1996; Emmans and Kyriazakis, 1997).
Whittemore (1983) and Whittemore (1986) suggested that, when energy intake limits expression of lean tissue growth potentials or maximum PD, the partitioning of energy intake above maintenance requirements between PD and LD was constant and independent of BW and energy intake level. Later, Rao and McCracken (1992) suggested that the distribution of retained energy between lipid and protein is not affected by energy intake level. More recently, studies reported by de Greef and Verstegen (1993), Quiniou et al. (1995), Bikker et al. (1996), and Coudenys (1998) indicate that this concept is not correct because whole-body lipid content and minimum lipid/protein increase with BW or energy intake level. The latter is consistent with energy intake effects on the efficiency of using balanced protein for growth (Kyriazakis and Emmans, 1992a,b). These recent studies and the results of the current study suggest that pigs become fatter when they eat more or when they grow heavier, even if energy intake is insufficient to maximize PD or lean tissue growth. The latter has been represented mathematically in various pig growth models (Black et al., 1986; NRC, 1998).
The results of the current study suggest that BW effects on the partitioning of retained energy between LD and PD, or lipid and protein, in growing pigs are largely a reflection of increases in energy intake with BW. Therefore, characterizing effects of energy intake on this partitioning and lean tissue growth can be simplified because there is no need to consider BW effects (Schinckel and de Lange, 1996; Emmans and Kyriazakis, 1997).
Additional studies are needed to identify whether constraints on the partitioning of retained energy apply to the composition of the growth (minimumLD/PD) and/or to body composition (minimum lipid/protein). If constraints apply to body composition, as well as the actual nutrient intake, previous nutrition also influences composition of growth (LD/PD). In this case, the dynamics of LD and PD used to achieve minimum lipid/protein need to be characterized. This is closely associated with the phenomenon of compensatory growth (Bikker et al., 1996) and may explain the quadratic effect of energy intake on lipid/protein ratio observed in the serial slaughter experiment.
Implications
Lowering energy intake and increasing body weight at slaughter are means of increasing the fraction of whole-body protein mass recovered in the carcass and thus in more valuable pork products. In growing-finishing pigs with high lean tissue growth potentials, energy intake should generally be maximized. The characterization of energy intake and body weight effects on energy partitioning in growing pigs with high lean tissue growth potentials can be simplified, as body weight effects on this partitioning can be attributed largely to increases in energy intake with body weight.
Footnotes
1 This work was supported by the Ontario Ministry of Agriculture, Food and Rural Affairs; Ontario Pork; Agribrands Purina; and the Natural Sciences and Engineering Research Council of Canada. We would like to thank A. Gillis, E. Jeaurond, and M. Lee for their assistance in data collection and laboratory analyses. 
2 The experimental protocol was reviewed and approved by the University of Guelph Animal Care Committee.
3 Correspondence—phone: 519-824-4120, ext. 56477; fax: 519-836-9873; e-mail: cdelange{at}uoguelph.ca.
Received for publication September 9, 2002. Accepted for publication September 11, 2003.
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) from lowest to highest energy intake were 33.2 (5), 34.0 (5), 36.0 (5), and 35.1 (6) kg, respectively. Mean BW (and number of observations) for high-BW pigs (
) from lowest to highest energy intake were 103.7 (5), 108.8 (5), 106.4 (5), and 106.3 (5) kg, respectively.
; 65 kg,