J. Anim Sci. 2006. 84:1926-1934. doi:10.2527/jas.2005-203
© 2006 American Society of Animal Science
Body composition of breeding gilts in response to dietary protein and energy balance from thirty kilograms of body weight to completion of first parity1
B. P. Gill2
Meat and Livestock Commission, PO Box 44, Winterhill House, Snowdon Drive, Milton Keynes, UK, MK6 1AX
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Abstract
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The relationships among BW, backfat depth, and body physical and chemical composition were evaluated in response to dietary protein and DE balance in breeding gilts from 30 kg of BW to weaning of the first litter. Large White (sire) x Landrace (dam) F1 hybrid (White; n = 75) and Landrace (sire) x (Meishan x Large White; dam) (Meishan; n = 19) hybrid gilts were received at 30 kg of BW. Five gilts were taken as the initial slaughter group at 30 kg of BW, and the remaining gilts were fed diets differing in total lysine to DE ratio, high (H) vs. low (L), from 30 kg of BW to mating (rearing), and during gestation and lactation, allowing factorial investigation of dietary treatment effects and interactions during rearing, gestation, and lactation. Gilts were slaughtered at approximately 50 and 90 kg of BW, and at mating, farrowing, and weaning. Gilts fed L diets during rearing were lighter at mating (117.9 vs. 133.6 kg of BW, P = 0.035) due to a reduction in gain (592 vs. 720 g/d, P = 0.002) and a restriction in protein accretion (83 vs. 117 g/d, P = 0.001). During rearing, lipid accretion did not differ between L- and H-fed gilts (208 vs. 198 g/d, P = 0.60), but the ratio of lipid to protein accretion was about 1.5-fold greater in L-fed gilts, where lipid mass expressed as a percentage of BW was increased at mating (26.0 vs. 21.9%, P = 0.005). Effects of L diets on lipid accretion during rearing were transient; no residual effects on body lipid mass (P > 0.17) were found at farrowing or weaning. Overall, Meishan hybrids carried greater lipid mass (P < 0.001) than White hybrid gilts. Whereas the rate of body lipid and protein accretion and body lipid and protein mass can be nutritionally influenced and can vary according to growth stage, reproductive status, and genotype, this study established that body protein mass expressed as a proportion of the lipid free empty BW remains inflexible. A value for this measure of 0.188 ± 0.0052 was found in White and Meishan hybrid gilts ranging from 28 to 203 kg of BW and 3 to 36 mm backfat depth, covering growth, pregnancy, and lactation, and offered diets differing in protein and energy balance. Body protein mass can be predicted as approximately 0.2 of the lipid free empty BW once body lipid mass is estimated accurately from physical measurements, such as backfat depth (P2, mm) and BW (kg), by regression using lipid (kg) = – 8.14 (SE, 1.302) + 0.167 (SE, 0.010) BW + 0.883 (SE, 0.065) P2 (residual SD = 3.51; R2 = 0.912).
Key Words: body composition • breeding gilt • energy • nutrition • pig • protein
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INTRODUCTION
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A number of studies have placed importance on the relationship between body composition of gilts during development and subsequent reproductive performance (O’Dowd et al., 1997
; Cia et al., 1998; Stalder et al., 2000). Particular emphasis has been given to body fat content with recommended targets for backfat thickness at mating and parturition in gilts and primiparous sows to optimize litter productivity and lifetime reproductive output (Whittemore, 1996). With continued selection for increased leanness, it is uncertain if previous maternal targets for fatness can be achieved in modern genotypes by nutritional manipulation of the dietary energy and protein balance. Additionally, the relationships between BW and backfat thickness, and the physical and chemical composition of first litter gilts and sows have been presented in a number of previous studies (Whittemore and Yang, 1989; Mullan and Williams, 1990; Everts and Dekker, 1995a), and it is appropriate to review these relationships and estimates of body composition with the continued development of modern genotypes. The objective of this study was to investigate changes in the physical and chemical composition of breeding gilts from 30 kg of BW to weaning of the first litter in response to dietary protein and DE balance.
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MATERIALS AND METHODS
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Animals used in this study were managed in line with the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (Consortium, 1999). Research proposals were submitted and approved by the Research Steering Committee of the Meat and Livestock Commission.
Animals and Dietary Treatments
Large White (sire) x Landrace (dam) F1 hybrid (White; n = 75) and Landrace (sire) x (Meishan x Large White; dam) (Meishan; n = 19) hybrid gilts were received from UK pig breeding companies at approximately 30 kg of BW. Four companies supplied the White hybrid gilts in approximately equal numbers, and the Meishan hybrid gilts were supplied by one of these companies. Five gilts, 1 from each of the 4 companies plus another from the single sourced population of Meishan hybrids, were taken as the initial slaughter group at approximately 30 kg of BW. As described in Figure 1, the remaining gilts were fed diets differing in total lysine to DE ratio, high (H) vs. low (L), from 30 kg of BW to mating (or rearing), and during gestation and lactation, allowing factorial investigation of the effects of dietary treatments during rearing, gestation, and lactation on body composition. The ratio of other essential amino acids to total lysine content was balanced to meet or exceed the recommended profile for ideal protein published by ARC (1981). To determine the effects of dietary treatments on physical and body composition during growth and reproduction, gilts were slaughtered at approximately 50 and 90 kg of BW, at mating, 24 h after farrowing (to allow colostrum intake and fostering of piglets to nonexperimental sows), and at weaning.
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Figure 1. Diagram illustrating the dietary treatments fed to experimental gilts from 30 kg of BW to weaning of the first litter. The high (H) or low (L) designation indicates a high or low total lysine to DE ratio in the diet from 30 to 50 kg of BW, from 50 kg of BW to mating, from mating to 24 h postfarrowing, and from 24 h postfarrowing to weaning on d 25 of lactation. Actual diet formulations for each phase are given in Table 1 .
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General Management
Details of the diets used are presented in Table 1. Gilts were fed ad libitum for 16 wk after entry at approximately 30 kg of BW. During wk 1 to 7, they were penned in groups of 10, balanced for BW, breeding company, and genotype, and during wk 8 to 16 they were housed in individual pens. After wk 16 and until mating, gilts were penned in groups of 8 and fed 2.5 kg/d using individual feeding stalls. Gilts were mated using within breeding company boars at third or fourth estrus. During pregnancy, gilts were penned in groups of 3 and fed 2.25 kg/d using individual feeding stalls. Gilts were moved to the farrowing house between 5 and 7 d before their expected date of parturition. They were housed in farrowing crates and fed the experimental diets, increasing by 0.5 kg/d, from 2.5 kg after farrowing to 7 kg on d 10 of lactation. Thereafter, the daily allowance was adjusted according to litter size, reaching between 9.5 and 11.5 kg/d immediately before weaning. Litter size was standardized to 8 to 12 piglets nursed per sow by crossfostering within 24 h of farrowing. Piglets were weaned at approximately 25 d (SD = 2.95) of age.
Physical and Chemical Analyses
After weighing (live) and stunning, blood was collected at exsanguination, weighed, and sampled. Intestines were emptied of their contents and weighed in separate parts representing the stomach, small intestine, and large intestine. The visceral fraction included the heart, lungs, diaphragm, liver, gallbladder, pancreas, kidneys, spleen, liver, gallbladder, empty bladder, ovaries, reproductive tract, mesenteric tissue, mammary tissue, and empty intestines. Empty BW (EBW) was calculated as the weight of the hot eviscerated carcass plus the weights of the blood and the visceral fraction.
Eviscerated carcasses were weighed and chilled overnight at 1°C. Carcasses from gilts slaughtered at approximately 30 kg of BW were minced whole. For all other carcasses, the whole head was removed by cutting, at a right angle to the back, through the joint between the atlas and the skull. The tail was removed between the fifth sacral and the first coccygeal vertebrae. The hind feet were removed by a cutting at a right angle to the tibia and fibula, at the level of the medial edge of the calcaneus, and forefeet were removed between the ulna and radius and the carpal bones. Hair was removed after scalding. All these components (head, tail, feet, and hair) were weighed and retained as a miscellaneous fraction.
The remaining carcass was split down the vertebral midline, and each side was weighed separately. The left side was utilized as follows: perirenal (flare) fat was removed and weighed, and the side was cut at the last rib. The following measurements were taken on the cut surface: maximum LM width (A) and depth (B), depth of subcutaneous fat and skin at 6.5 cm from the midline and at right angles to the skin (P2). The hind and forequarters were then separated into primal cuts, and the ham primal was weighed and dissected into skin, lean, subcutaneous fat, intermuscular fat, bone, and trimmings. Dissected tissues were weighed. The carcass fraction was regrouped to include left side primals, ham dissected tissues, and kidney fat. All fractions, including blood samples were stored at – 20°C. The visceral, carcass, and miscellaneous fractions were subsequently minced, mixed, and sampled. Blood and minced samples were freeze-dried and ground and analyzed for N (Ebeling, 1968), lipid (Pettinat et al., 1983), and ash (MAFF, 1982) contents.
Statistical Analysis
Physical and body composition data from gilts slaughtered at 50 and 90 kg of BW, mating, farrowing, and weaning were subjected to ANOVA using a Generalized Linear Model (Minitab Statistical Software, Release 14, State College, PA), with each gilt classified as the experimental unit. Model inputs for the analysis of White hybrid gilt data included company of origin and dietary treatment (L vs. H), and additionally for gilts slaughtered at weaning, main treatment effects during rearing, gestation, and lactation, and all factorial interactions. Statistical determination of dietary treatment effects was not included in the model inputs for Meishan hybrid gilt data due to lack of replication. Differences in the body composition of White and Meishan hybrid gilts during rearing and at mating, farrowing, and weaning were compared using 1-way ANOVA. Predictive equations for body lipid mass were established by multiple least squares regression analysis, with BW (kg), EBW (kg), backfat depth (P2, mm), and ham subcutaneous fat (kg) as predictor variables.
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RESULTS
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Feed intake during rearing and lactation according to dietary treatment is presented in Table 2. Least square means for ADG and daily rates of lipid and protein accretion from approximately 30 kg of BW to mating are presented in Table 3. Least square means for the physical and chemical composition according to dietary treatment for gilts slaughtered at approximately 30, 50, and 90 kg of BW and at mating, farrowing, and weaning are presented in Tables 4 and 5.
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Table 4. Physical and chemical composition (least square means) of gilts during rearing and at mating according to dietary treatment1
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Table 5. Physical and chemical composition (least square means) of gilts at farrowing and weaning according to dietary treatment
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Dietary Treatment
No effects of dietary treatment on feed intake were observed from entry at approximately 30 kg of BW to wk 7 of the rearing stage; but over wk 8 to 16, gilts fed the L diet had a reduced intake (Table 2, P = 0.003). No differences were observed (P = 0.98) in the feed intake of lactating gilts fed the H and L diets during lactation or any residual effects of dietary treatments during rearing and gestation. Litter size at birth averaged 9.9 ± 0.28 born alive and was standardized by cross fostering resulting in an average of 10.2 ± 0.16 piglets weaned, and litter weaning weight averaged 60.0 ± 1.5 kg, with no differences associated with dietary treatment during lactation (data not shown).
Gilts fed L diets during rearing were lighter in BW (P = 0.035) at mating (Table 3) due to a reduction in daily gain (P = 0.002) associated with the restriction in daily protein accretion (P = 0.001). Although daily lipid accretion during rearing (P = 0.60) did not differ between L- and H-fed gilts, the ratio of lipid to protein accretion was about 1.5-fold greater in L-fed gilts (Table 3). Whereas L diets increased body lipid mass (P = 0.011) up to and around 90 kg of BW, at mating body lipid mass was similar (P = 0.51) for L- and H-fed gilts (Table 4). However, because L-fed gilts were lighter at mating, lipid mass expressed as a percentage of BW (P = 0.005) was greater in L-fed gilts (Table 4). Therefore between 30 kg of BW and mating, at an equivalent BW, lipid mass (P < 0.001) was increased in gilts reared on L compared with H diets (Figure 2).
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Figure 2. Relationship between body lipid mass and BW in gilts during rearing (30 kg of BW to mating). Regression lines for body lipid mass in gilts fed diets that were high (H) or low (L) in total lysine to DE ratio differed (P < 0.001). Regression slopes for H- and L-fed gilts were 0.244 (P < 0.001) and 0.321 (P < 0.001), with residual errors of 4.675 and 2.630, respectively.
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Dietary effects during rearing on protein accretion and body protein mass were observed as differences in some physical measurements relating to muscle size and lean content at mating (Table 4), where gilts fed the L diet had reduced LM width (P = 0.017) and depth (P = 0.019) and ham lean content (P = 0.002). Although L diets restricted protein accretion during rearing, resulting in reduced daily gain and BW at mating, the relationship between body protein mass and the lipid free EBW (LFEBW) was not nutritionally influenced; body protein mass remained a constant 0.212 ± 0.0074 of the LFEBW for L- and H-fed gilts (Figure 3).
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Figure 3. Relationship between body protein mass and the lipid free empty BW in gilts during rearing (30 kg of BW to mating). Regression lines for body protein mass in gilts fed diets that were high (H) or low (L) in total lysine to DE ratio did not differ (P > 0.05). Regression slope and residual error for pooled data were 0.212 (P < 0.001) and 1.492, respectively.
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The effects of L diets on lipid accretion during rearing were transient as there were no residual effects on lipid mass (P > 0.18) at farrowing and at weaning the first litter (Table 5). No consistent effects of L and H diets were observed during gestation on physical and body composition at farrowing in gilts representing the extremes (HH vs. LL) of the rearing x gestation factorial dietary treatment interactions (HH, HL, LH, and LL).
No consistent residual effects of dietary treatments were observed during rearing and gestation on physical and chemical composition at weaning (Table 5). However, feeding diet L during lactation reduced BW at weaning (166.4 vs. 175.6 kg, P = 0.026) resulting from a reduction in body protein mass (22.2 vs. 24.4 kg, P = 0.013). Dietary effects during lactation on body protein mass were observed as differences in some physical measurements relating to muscle size and lean content at weaning (Table 5), where gilts fed the L diet had reduced LM width (103.4 vs. 107.7 mm, P = 0.043) and ham lean content (7.23 vs. 7.88 kg, P = 0.056).
Genotype
The small sample size for Meishan hybrid gilts did not allow statistical determination of dietary treatment effects. The main objective was investigation of any fundamental differences in body composition and physical measurements between Meishan and White hybrid gilts at different stages of growth and first litter production. The results (Table 6) indicate that Meishan hybrid gilts carried a greater mass of body lipid than White hybrid gilts, giving increased backfat depth (P < 0.015) at approximately 90 kg of BW, mating, farrowing, and weaning.
Estimates of Body Composition by Regression
It is clear that body lipid can be influenced by nutrition and genotype, but the relationship between body protein mass and LFEBW is fixed over a wide BW range and cannot be easily shifted (Figure 4). Body protein mass can therefore be predicted as proportionally 0.188 ± 0.0052 or approximately 0.2 of the LFEBW once body lipid mass is estimated accurately from physical measurements, such as backfat depth and BW. Prediction equations for body lipid mass using the resource data from this study are presented in Table 7.
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Figure 4. Relationship between body protein mass and the lipid free empty BW in all gilts. Regression slope and residual error were 0.188 (P < 0.001) and 1.726, respectively.
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Table 7. Regressions of body lipid mass (kg) against BW, empty BW (EBW), backfat depth (P2), and ham subcutaneous fat content (HSF)1
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DISCUSSION
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Results from this study confirm that lipid mass per unit of BW can be increased in gilts by restricting protein accretion before mating by changing the dietary protein and energy balance through reduced protein supplementation. The consequences of this approach include a reduction in ADG and lower BW at mating. However, dietary induced increases in body lipid content with resulting increases in backfat depths from 30 kg of BW to mating were transient as no residual effects were present at farrowing and weaning the first litter. Whereas there may be scope for manipulating body lipid and fatness in gilts before mating by reducing dietary protein content, protein mass per unit of the LFEBW could not be nutritionally influenced and remained constant at 0.212 between 30 kg of BW and mating.
Reducing dietary protein content during gestation did not influence maternal BW and composition at farrowing, though Everts and Dekker (1995b) reported a decrease in protein and increase in lipid mass in the EBW of gilts on d 108 of pregnancy when dietary protein levels during gestation were reduced from 178 to 120 g of DM/kg. Using the deuterium oxide dilution technique, Shields et al. (1984, 1985) reported greater levels of maternal lipid mass from d 70 to 105 of pregnancy in first litter gilts when protein content in semisynthetic diets was reduced from 140 to only 50 g/kg. Reduction in backfat depth resulting from body fat mobilisation at the onset of lactation may explain the lack of difference in body lipid mass at farrowing (i.e., after parturition) between gilts fed H and L diets during gestation. The reduction in dietary CP content during gestation was not as severe as that employed in the study of Shields et al. (1985), and this may explain the lack of a treatment response in maternal protein mass at farrowing in this study. Furthermore, dietary protein and total lysine content and intake in gilts fed the low protein diet during gestation met the protein and lysine requirements for breeding gilts recommended by NRC (1998) or were close to BSAS (2003) recommended requirements for first litter gilts. This would suggest that dietary protein levels and intake during gestation would need to be greatly reduced to achieve major and significant shifts in maternal body lipid and protein mass at parturition.
Few slaughter-based studies have been conducted on maternal body composition at weaning in response to dietary protein and energy supply during lactation; most published work has focused on litter productivity and changes in BW and fatness from sow BW and ultrasonic measurements of backfat thickness at farrowing and weaning (O’Dowd et al., 1997). The reduction in protein mass and BW at weaning in response to dietary protein reduction during lactation observed in the current study was also observed by Shields et al. (1984, 1985) using the deuterium oxide dilution technique with diets containing 50, 140, and 230 g of CP/kg. The reduction in body protein mass at weaning in response to reduced dietary lysine content during lactation is not unexpected because in the L diet a total lysine content of 0.7% at a mean daily intake of 6.81 kg/d is below the lysine requirements for lactating sows recommended by NRC (1998) and BSAS (2003). Conversely, Kotarbinska (1983) was unable to influence body protein mass in first litter gilts at weaning by feeding protein free diets during lactation over 29 d.
Whereas the rate of body lipid and protein accretion and body lipid and protein mass can be nutritionally influenced and can vary according to growth stage, reproductive status, and genotype, this and other studies of body composition have established that body protein mass expressed as a proportion of the LFEBW is extremely inflexible to nutritional influences and remains relatively constant and independent from BW, reproductive status, and genetic makeup. In the current study, a value of 0.188 was found in White and Meishan hybrid gilts ranging from 28 to 203 kg of BW and 3 to 36 mm of backfat at P2, covering growth, pregnancy, and lactation and offered diets differing in protein and energy balance. A value of 0.211 can be obtained from Whittemore and Yang (1989) based on the chemical composition of first and fourth litter sows slaughtered at mating and weaning and representing wide differences in fatness at farrowing and feeding level, and litter size imposed during lactation. Similarly, Everts and Dekker (1995b) reported a value of 0.22 in gilts at mating, which did not change on d 108 of pregnancy after dietary protein treatments of 178 to 120 g of DM/kg. In sows Everts and Dekker (1995a) reported mean values ranging between 0.209 and 0.215 at the end of the third lactation after a series of dietary treatments differing in protein content during pregnancy (120 vs. 178 g of DM/kg) and lactation (178 vs. 204 g of DM/kg) over the preceding parities. A value of 0.199 was reported by Mullan and Williams (1990) for first litter gilts during lactation and subjected to different feeding levels before mating (ad libitum vs. 1.8 kg/d), during pregnancy (2.7 vs. 1.5 kg/d), and lactation (to appetite vs. 2 kg/d). A value of 0.204 can be calculated from the results of a study by Kotarbinska (1983) on the body composition of mulitparous sows and growing, pregnant, and first parity gilts subjected to a wide range of nutritional treatments. Differences in the body composition of Meishan and Yorkshire breeds of growing pigs were compared by White et al. (1995) and between 71 and 260 d of age; their results indicate that body protein mass as a proportion of the lipid free BW averaged 0.216 and did not differ between the 2 breeds.
If body lipid mass can be predicted accurately, for example using BW and physical measurements of fatness such as backfat depth, then body protein mass can be taken as an approximation of 0.2 of the LFEBW. Using the resource data from this study, body lipid mass was estimated by regression using BW, EBW, backfat depth (P2), and the physically dissected weight of the ham primal subcutaneous fat as predictor variables. The EBW can be predicted from BW, and in this study, the regression of EBW on BW returned the following equation: EBW (kg) = 1.30 (SE = 1.168) + 0.924 (SE = 0.008) BW (residual SD = 3.87, R2 = 0.993). The regression coefficient for BW compares favorably with equations presented by Whittemore and Yang (1989), Mullan and Williams (1990), and Everts and Dekker (1995b) returning similar values for EBW at an equivalent BW.
In previous studies, ultrasonic measurements of backfat depth at the P2 position in the live animal together with BW have been used as good predictors of body lipid mass (Whittemore and Yang, 1989; Mullan and Williams, 1990), whereas in this study, all equations containing P2 as a predictor variable are based on caliper measurements of P2 on the carcass. Nevertheless ultrasonic and caliper measurements are highly correlated (r = 0.869 and 0.942) as shown by the regression between ultrasonic P2 and caliper P2 reported in first litter sows by King et al. (1986) and Mullan and Williams (1990).
The predicted lipid mass in the EBW of gilts using BW and backfat depth (P2) as predictor variables using the regression equation established in this study (Table 7) and regression equations reported by King et al. (1986), Whittemore and Yang (1989), Mullan and Williams (1990) and Everts and Dekker (1995a) were compared at BW of 130 kg (light) and 175 kg (heavy) at low (12 mm) or high (25 mm) backfat depths. The predictions (Table 8) compare favorably with Whittemore and Yang (1989), particularly for light and lean animals, but for fatter and heavier animals previous equations return greater values, especially for Mullan and Williams (1990) and Everts and Dekker (1995a). Thus there is likely to be good concordance in the prediction of body lipid mass using equations from various studies where the sample statistics for BW and P2 or any other predictor variable are not significantly different between populations. Predictive equations for body lipid mass derived from genotypes selected for reduced fatness should, therefore, not be applied to genotypes with much greater depths of subcutaneous fat.
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Table 8. Body lipid mass (kg) predicted in light- vs. heavy-BW gilts with low- vs. high-P2 backfat depths using regression equations with BW and backfat depths as predictor variables
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In conclusion, the rate of body lipid and protein accretion and body lipid and protein mass can be nutritionally influenced in pigs and can vary according to growth stage, reproductive status, and genotype. However, this and other studies of body composition have established that body protein mass as a proportion of the LFEBW is extremely inflexible to nutritional influences and remains relatively constant and independent from BW, reproductive status, and genetic makeup. In the current study, a value of 0.188 was found in White and Meishan hybrid gilts ranging from 28 to 203 kg of BW and 3 to 36 mm of P2, covering growth, pregnancy, and lactation and offered diets differing in protein and energy balance. Body protein mass can, therefore, be predicted as approximately 0.2 of the LFEBW once body lipid mass is estimated accurately from physical measurements, such as backfat depth (P2, mm) and BW (kg). Accordingly the equation lipid (kg) = – 8.14 (SE = 1.302) + 0.167 (SE = 0.010) BW + 0.883 (SE = 0.065) P2 (residual SD = 3.51, R2 = 0.912) was generated by regression in this study. Future studies attempting to influence body composition of pigs should consider the biological implications of the intransigent relationship between body protein mass and the LFEBW.
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Footnotes
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1 Financial support was provided by the Meat and Livestock Commission (MLC). The author thanks MLC staff for technical assistance and Julian Wiseman and John Corbett of Nottingham University for providing abattoir and laboratory resources. 
2 Corresponding author: pinder_gill{at}mlc.org.uk
Received for publication April 15, 2005.
Accepted for publication January 30, 2006.
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LITERATURE CITED
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Abstract
INTRODUCTION
