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Changes in weight and composition in various tissues of pregnant gilts and their nutritional implications¹

F. Ji^*, G. Wu^*,, J. R. Blanton, Jr.^* and S. W. Kim^*,2

^* Texas Tech University, Department of Animal and Food Sciences, Lubbock 79409; and and Texas A&M University, Department of Animal Science, College Station 77843

	Abstract

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The objectives of this study were to characterize the quantitative changes in various body tissues of high-lean type gilts during gestation and to determine the protein needs of pregnant gilts based on changes in tissue contents. Thirty-five gilts (158.2 ± 8.3 kg) were housed in individual gestation crates with six unbred gilts randomly selected and slaughtered to provide data for d 0 of gestation. The remaining gilts were bred and assigned randomly to one of six slaughter groups: d 45, 60, 75, 90, 102, and 112. Gilts were fed 2 kg (as-fed basis) of gestation diet daily (3.1 Mcal/kg of ME and 0.56% lysine). Carcass soft tissue, bone, gastrointestinal tract, spleen, pancreas, kidney, liver, uterus, fetus, mammary gland, and the remaining viscera were separated and weighed. Carcass soft tissue, liver, remaining viscera, uterus, and gastrointestinal tract were ground, freeze-dried, and analyzed for composition. Body weights of the gilts increased quadratically (P < 0.001) during gestation. Weights of carcass soft tissue and uterus, including placenta, increased linearly (P < 0.001) during gestation. Weights of individual fetuses, fetal litters, individual mammary glands, and the entire mammary glands increased cubically (P < 0.001) during gestation. Crude protein in carcass soft tissue increased cubically (P < 0.01), whereas DM and ether extract (EE) in carcass soft tissue increased linearly (P < 0.01). The DM, CP, and EE in the entire mammary glands increased quadratically (P < 0.001) during gestation. The DM, CP, and EE in fetal litter increased cubically (P < 0.01) as gestation progressed. The accretion rates of the conceptus, fetal litter, individual fetus, individual mammary gland, and CP in fetal litter differed (P < 0.05) before and after d 70 of gestation. The CP daily gain from all maternal and fetal tissues was 40 and 103 g/d before and after d 70 of gestation, respectively, suggesting that pregnant gilts may require different quantities of dietary protein during gestation. Based on the maintenance requirement, maternal tissue gain, and conceptus gain, pregnant gilts require 6.8 and 15.3 g/d of true ileal-digestible lysine (or 147 and 330 g/d of true ileal-digestible protein) before and after d 70 of gestation, respectively, to support their true biological needs.

Key Words: Body Composition • Fetus • Gestation • Gilt • Protein

	Introduction

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Most gilts are not physiologically mature at the first mating and thus need the proper quantities of nutrients to support the growth of maternal tissues and fetuses during gestation. Insufficient maternal gain during gestation results in low BW after lactation and delayed return to estrus (Trottier and Johnston, 2001); however, excessive maternal fat gain during gestation should be avoided because it decreases voluntary feed intake during lactation (Baker et al., 1969; Weldon et al., 1994; Revell et al., 1998) and decreases sow longevity (Dourmad et al., 1994). Thus, gestational feeding is important and closely related to lactation performance (Kim and Easter, 2003).

The target of feeding gilts during gestation is to obtain optimal fetal growth and proper maternal protein and fat gain. Current feeding programs for gestating sows are based on a single dietary formulation, with adjustments made in the feeding level (Whittemore, 1998; Trottier and Johnston, 2001). These programs are simple and easy to apply; however, an optimal feeding strategy for pregnant gilts should be sufficiently flexible to adjust for the nutrient allowance of gilts according to their nutrient needs for both maternal and fetal growth. Consideration of actual nutrient accretion in various tissues can allow for the estimation of nutrient needs by gilts during gestation. Changes in weights and composition in maternal and fetal tissues during gestation are essential to this estimation, but information on nutrient deposition in both maternal and fetal tissues of high-lean-type pregnant gilts is lacking. Thus, the objectives of this study were to characterize the quantitative changes in various maternal tissues and fetuses of high-lean-type gilts during gestation and to determine their protein needs based on these changes.

	Materials and Methods

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Animals, Design, and Diets
Thirty-five gilts (158.2 ± 8.3 kg, Camborough-22, PIC) were housed in individual gestation crates at Texas Tech University Swine Research Farm. The animal use and care protocol was approved by Animal Care and Use Committee of Texas Tech University. Gilts were allotted randomly to one of seven slaughter groups within each group representing d 0 (six gilts), 45 (six gilts), 60 (four gilts), 75 (five gilts), 90 (four gilts), 102 (five gilts), and 112 (five gilts) of gestation. Gilts in the d-0 group were slaughtered at the average of 10 d after the first gilt was bred. The remaining gilts were bred in subsequent estrus. The BW of gilts were measured at slaughter. Backfat thickness was measured by real-time ultrasound (LS-1000, Tokimec Inc., Tokyo, Japan) at the P2 position (left side of the 10th rib and 6 cm from the spine). Gestation diet contained 3.1 Mcal/kg of ME, 12.2% CP, and 0.56% lysine (Table 1). Pregnant gilts were fed a single meal of 2.0 kg/d (as-fed basis) of gestation diet, which provided 10.2 and 221.4 g/d of true ileal-digestible lysine and protein, respectively.

Carcass
Hot carcasses were split longitudinally through the midline following removal of mammary glands and internal organs. Both sides were weighed and chilled to 4°C in 24 h. Left sides were weighed, and carcass soft tissue of the left side was separated from the bone. Longissimus muscle area was measured at the 10th rib. Both carcass soft tissues and bones were weighed for individual gilts. Weights of the right side, right carcass soft tissue, and right bone were calculated from the corresponding values for the left half. A Biro-346 mill (The Biro Mfg. Co., Marblehead, OH) with a 4-mm plate opening was used to grind samples. Carcass soft tissues were ground four times and sampled. Bones were cut into pieces of 3 cm³ and sampled. All samples were stored at –20°C for further analyses.

Organs
Liver, pancreas, kidney, spleen, gastrointestinal tract (GIT; composed of the empty stomach, empty small intestine, and empty large intestine, including cecum and rectum), and reproductive tissues were separated. The GIT was separated by removing mesenteric membranes and visceral fats, manually stripped of intestinal luminal contents, and flushed thoroughly with water to remove the digesta. Pancreas, spleen, and kidneys were separated after removing the surrounding visceral fat. All other remaining organs (including lung and heart) and visceral fat were combined as "remaining viscera." Separated individual liver, GIT, pancreas, spleen, kidney, reproductive tract, and remaining viscera were weighed on a fresh basis. Skin and extraneous fat pads were removed from mammary glands to obtain mammary tissues, which were named "the entire mammary glands." The dissected mammary tissues were further separated into individual glands and named "individual mammary glands." Both the entire mammary glands and individual mammary glands were weighed. Fetuses were obtained from the uterine horn by cutting the placenta at the base of the amniotic sac and then at the base of the umbilicus, and fetuses were weighed individually. Remaining uterus including placenta were weighed. When carcass soft tissue, liver, GIT, remaining viscera, uterus, entire mammary glands, and fetal litter were combined for the calculation, it was named "total." Head, feet, blood, kidney, spleen, and hair were not included in "total."

After a 24-h cooling period (4°C), all samples were ground four times individually using a Biro-346 mill (The Biro Mfg. Co.) with a 4-mm plate opening, sampled, and stored at –20°C for further analysis.

Chemical Analyses
Frozen samples were freeze-dried (DURA-Top, FTS systems, Chatswood, Australia) after weighing. Freeze-dried samples were re-weighed and ground with a commercial Waring blender (Waring Products, New Hartford, CT) to produce a homogenous mixture. Dry matter content was calculated by placing the tissue at 105°C for 8 h in a forced-air oven. Crude protein content (N x6.25) was determined using Leco FP-2000 (Leco Corp., St. Joseph, MI). Ether extract (EE) was determined from dried tissue according to AOAC (1995). Ash was measured by the combustion of dried tissue at 500 °C for 8 h.

Statistical Analyses
Data were analyzed as a complete randomized design. Statistical analysis was performed with the GLM procedure in the SAS/STAT software (SAS Inst., Inc., Cary, NC). Pooled standard deviations were used to evaluate the variability of a data set. Data were further analyzed using the REG procedure of SAS with the forward option to obtain regressions between the variables and day of gestation. The model was:

where y is a variable, x is a day of gestation, ß₀ is an intercept, ß₁ is a coefficient for linear regression, ß₂ is a coefficient for quadratic regression, ß₃ is a coefficient for cubic regression, and e is experimental error. Data were subjected to reanalysis to obtain regressions excluding data from d-0 unbred gilts because of qualitative differences between bred and unbred gilts that may have affected the response.

Variables for which variation could be explained by quadratic or cubic regressions were further analyzed to find the breakpoint (day of gestation) where the rate of accretion changed at = 0.05. The NLREG software (Sherrod, 1992) was used to obtain those multiphasic regressions and breakpoints.

	Results

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Gilts and Carcasses
Maternal BW of gilts increased quadratically (P < 0.001) and backfat thickness increased linearly (P < 0.001) as gestation progressed (Tables 2 and 3). Hot carcass weight, cold carcass weight, and carcass soft tissue weight also increased linearly (P < 0.001) with advancing gestation. The weights of bone and LM area did not change (P = 0.577 and 0.565, respectively) during gestation (Table 2).

Reproductive Tract and Mammary Gland of Gilts
Weights of the fetuses plus fetal fluids increased quadratically (P < 0.001), whereas weights of the uterus including placenta increased linearly (P < 0.001) during gestation (Tables 2 and 3). Weights of individual fetuses, fetal litter, entire mammary glands, and individual mammary glands increased cubically (P < 0.001) as gestation progressed.

Tissue Composition of Gilts
The percentages of DM, CP, EE, and crude ash in carcass soft tissue did not change (P = 0.912, 0.996, 0.995, and 0.928, respectively) during gestation (Table 4). The percentage of EE in liver increased cubically (P< 0.001) as gestation progressed. Percentages of crude ash in GIT and the remaining viscera decreased linearly (P < 0.001) as gestation progressed, whereas the percentages of DM and EE in the remaining viscera increased cubically (P < 0.001) as gestation progressed. The percentage of CP in the remaining viscera decreased quadratically (P < 0.001) with advancing gestation, and the percentages of DM and CP in the uterus decreased linearly (P < 0.001). The percentage of EE in the uterus increased cubically (P < 0.001) as gestation progressed. The percentage of crude ash in uterus increased quadratically (P < 0.05) (Table 4); however, when data from the d-0 group were excluded from the regression analysis, the percentages of DM and crude ash in uterus did not change (P = 0.966 and 0.949, respectively), the percentage of CP in uterus decreased quadratically (P < 0.05), and the percentage of EE in uterus increased quadratically (P < 0.001).

View larger version (11K): [in this window] [in a new window]   Figure 1. Dry matter changes of pregnant gilts during gestation (includes carcass soft tissue, liver, remaining viscera, gastrointestinal tract, entire mammary glands, uterus, and fetal litter). Dry matter, kg = 43.64 + 0.0012 x (day of gestation)2; P < 0.001, R2 = 0.61.

View larger version (10K): [in this window] [in a new window]   Figure 2. Crude protein changes of pregnant gilts during gestation (includes carcass soft tissue, liver, remaining viscera, gastrointestinal tract, entire mammary glands, uterus, and fetal litter). Crude protein, kg = 16.97 + 0.000568 x (day of gestation)2; P < 0.001, R2 = 0.80.

View larger version (12K): [in this window] [in a new window]   Figure 3. Ether extract changes of pregnant gilts during gestation (includes carcass soft tissue, liver, remaining viscera, gastrointestinal tract, entire mammary glands, uterus, and fetal litter). Ether extract, kg = 25.09 + 0.000665 x(day of gestation)2; P < 0.001, R2 = 0.37.

	Discussion

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Weight and compositional changes in maternal tissues and the conceptus occurred at various rates during gestation, implying that maternal nutrient needs for supporting these dynamic changes vary during gestation. This finding has important implications for establishing a flexible feeding strategy for pregnant gilts based on their true biological needs for nutrients.

Pregnant gilts gained 57.7 kg during gestation (Table 2). The gestational weight gain of gilts can be affected by several factors, including dietary nutrients (Frobish, 1970; Willis and Maxwell, 1984; Lee and Mitchell, 1989), ambient temperature (Verstegen et al., 1987; Noblet et al., 1990), as well as age and size at breeding (Trottier and Johnston, 2001). Dietary nutrient intake of pregnant gilts from the current study followed the recommendation of NRC (1998), and their actual weight gain (57.7 kg) was close to the NRC (1998) estimate of 55 kg. Thus, the weight gain by pregnant gilts used in the current study represents the typical pattern of maternal and fetal growth in swine.

The BW of pregnant gilts increased quadratically as gestation progressed, which is consistent with previous findings (Salmon-Legagneur and Rerat, 1962). However, Elsley et al. (1966) and Jones and Maxwell (1982) observed that sow BW increased linearly as gestation progressed. The reported different patterns for maternal BW changes (linear or quadratic) of pregnant gilts can have different implications for developing optimal feeding programs. Results of the present study indicate that a single-phase feeding strategy may not be suitable to support the nutrient needs of pregnant gilts because their BW gain occurs at different rates during gestation. Moreover, consideration of detailed compositional changes would enhance the accuracy of determining maternal and fetal nutrient needs during different stages of gestation.

Approximately 30% of gestational BW gain of gilts was contributed to by maternal carcass gain. Hot carcass weight, cold carcass weight, and carcass soft tissue weight linearly increased 18.2, 17.7, and 15.6 kg during gestation, respectively. However, the accretion rate of CP in maternal carcass soft tissue was 36.1 g/d during the early gestation and 33.9 g/d during the late gestation. This finding suggests that quantities of dietary protein used for carcass soft tissue accretion in gilts were decreased during the late gestation in order to support the accelerated fetal growth. This observation is consistent with the finding of Shields, Jr., et al. (1985) that maternal tissue accretion was higher before d 70 than after; however, EE in carcass soft tissue was found to increase linearly during gestation in the current study, which differs from the report of Shields, Jr., et al. (1985).

Another major portion of weight gain in pregnant gilts was the reproductive tract. The weight and DM of uterus including placenta increased linearly during gestation, which was consistent with the findings of Salmon-Legagneur and Rerat (1962). In early gestation, the accretion rate of fetus was low and the most gain was from allantoic and amniotic fluids in the conceptus. Fluid contents represented 81.9, 77.6, 52.7, 39.4, 22.4, and 24.1% of the conceptus weight at d 45, 60, 75, 90, 102, and 112 of gestation, respectively. Similarly, Noblet et al. (1985) found that fluid content (%) in the conceptus was maximal in the early gestation, and the quantity of fluids in the conceptus had the most marked effect on uterine growth in the early gestation. Fetal growth accelerated during the late gestation as shown by Moustgaard (1962), Ullrey et al. (1965), and Wu et al. (1999). The accretion rates of individual fetus and fetal litter were 4.1 and 45.3 g/d during the early gestation, and 29.6 and 310.5 g/d during the late gestation. Based on the composition data of fetal pigs at d 45, 60, 75, 90, 102, and 112 of gestation (McPherson et al., 2004), the DM, CP, EE, and crude ash in both individual fetuses and fetal litter increased cubically as gestation progressed. The accretion rates of DM, CP, and EE in fetal litter were 9.4, 5.6, and 1.2 g/d during the early gestation and 60.1, 34.4, and 7.8 g/d during the late gestation based on the average litter size (10.5 ± 1.0) of the current study. Whittemore (1998) also demonstrated that protein deposition in the conceptus was faster during the late gestation than during the early gestation.

The growth of entire mammary glands as well as individual mammary glands occurred cubically during gestation. The accretion rates of individual mammary glands and entire mammary glands were 1.6 and 15.0 g/d during the early gestation and 4.5 and 77.6 g/d during the late gestation. The current results are consistent with the findings reported in the literature. Kensinger et al. (1982) and Weldon et al. (1991) demonstrated that most of the mammary development in gilts took place between d 75 and d 110 of gestation. The average number of mammary glands was 13.9 ± 0.2 in the current study. On the basis of the composition data of the porcine mammary gland at farrowing (Kim et al., 1999), the accretion rates of DM, CP, and EE were 7.5, 2.4, and 4.5 g/d during the early gestation and 20.3, 6.6, and 12.3 g/d during the late gestation. The pattern of protein accretion in the mammary gland obtained from the current study was similar to that reported by Whittemore (1998).

When carcass soft tissue, liver, GIT, remaining viscera, uterus, entire mammary glands, and fetal litter were summed as the total, the weights of DM, CP, and EE increased quadratically during gestation. Derived from the regressions, the weights of DM, CP, and EE in total increased 15.6, 7.1, and 8.3 kg during the entire period of gestation and the accretion rates of DM, CP, and EE in total were 86.8, 39.8, and 46.6 g/d during the early gestation and 225.7, 103.4, and 121.0 g/d during the late gestation.

Maternal nutrition plays a critical role in fetal growth and development as well as postnatal performance and health in animals (Wu et al., 2004). Thus, pregnant gilts should be fed adequately to support both maternal tissue gain and fetal development during gestation. The NRC (1998) suggested that the quantity of true ileal-digestible lysine for maintenance was 36 mg/kg BW^0.75. The average BW of gilts before and after d 70 of gestation were 170.6 and 201.1 kg, respectively, which would give 1.70 and 1.92 g/d of true ileal-digestible lysine for the maintenance requirement of pregnant gilts if the NRC (1998) equation were used. The NRC (1998) also suggested the quantity of true ileal-digestible lysine above maintenance for each gram of accreted protein; this was given as a constant conversion factor of 0.129. Considering that the accretion rates of tissue protein were 39.8 and 103.4 g/d for gilts before and after d 70 of gestation, respectively, the quantities of true ileal-digestible lysine needed for these tissue protein gains were 5.13 and 13.34 g/d. Combining the amount for maintenance and tissue protein accretion indicates that the gilts needed 6.83 and 15.26 g/d of true ileal-digestible lysine before and after d 70 of gestation, respectively. The gestation diet used in this study was designed to provide 244.8 g/d of CP and 11.15 g/d of total lysine to the gilts throughout gestation. Using the true ileal digestibilities of corn, soybean meal (Stein et al., 2001), and alfalfa meal (NRC, 1998), 221.4 and 10.25 g/d were the amounts of true ileal-digestible protein and lysine from the gestation diet, respectively. Thus, the NRC-recommended amounts of true ileal-digestible lysine for gilts before and after d 70 of gestation were greater and lesser than, respectively, the values that we determined from the current study. When the NRC-recommended quantities of true ileal-digestible lysine for gilts at the two stages of gestation are averaged, the resulting value is 10.0 g/d, which is very close to the daily allowance of true ileal-digestible lysine from this study (10.25 g/d). Unfortunately, the equation and the conversion factor provided by the NRC (1998) were intended for gilts during the entire gestation, which may result in differences between the actual quantities of nutrients needed by pregnant gilts and the amounts of nutrients provided from the diet.

Currently, the most common strategies of feeding sows are to provide a single diet with constant or adjusted feeding levels during gestation (Whittemore, 1998; Trottier and Johnston, 2001). These strategies are based on the previous findings that maternal BW gain during gestation was not affected by the pattern of daily feed allowance (Pike and Boaz, 1969; Baker et al., 1974). Baker et al. (1974) found that reproductive performance of gilts fed a fixed amount of CP (228 g/d) throughout gestation seemed to be equal to those fed 165 g/d between d 0 and 80 and 304 g/d CP between d 80 and farrowing. However, Baker et al. (1974) found that the gilts with two-phase feeding consumed 10% less CP than the controls during gestation, indicating that dietary protein can be spared by the two-phase feeding program without decreasing reproductive performance. Pettigrew and Yang (1997) suggested that the key to the feeding program of pregnant gilts is to control (limit) the amount of body fat accretion and to improve the amount of body protein accretion until farrowing. Current strategies of feeding a common diet with fixed or adjusted amounts may not be optimal in meeting this goal because fixed-level feeding with a single diet cannot satisfy the gilt’s need for protein, whereas multiple-level feeding with a single diet will affect the daily allowance of all nutrients, which will affect both body fat and protein accretion in maternal tissues.

If the target is to enhance both fetal growth and maternal protein gain without excessive fat gain, the results of the current study suggest the need for a two-phase feeding program (d 0 to 70 and d 70 to farrowing) for pregnant gilts, with diets providing different quanties of lysine (or protein) contents. As indicated above, the pregnant gilts need 6.83 g/d of true ileal-digestible lysine (or 147.6 g/d of true ileal-digestible protein) before d 70 of gestation and 15.26 g/d of true ileal-digestible lysine (or 329.6 g/d of true ileal-digestible protein) after d 70 of gestation. If the NRC-recommended value for the lysine requirement of pregnant gilts is adopted in feeding, up to 30% of dietary AA may be unnecessarily oxidized before d 70 of gestation, whereas requirements of AA for optimal fetal growth and maternal gain after d 70 of gestation may not be met. Feeding pregnant gilts at their physiological requirement is expected to enhance the efficiency of dietary protein utilization, thereby decreasing N excretion through manure. In addition, increasing protein allowance after d 70 of gestation may help improve the growth of fetuses and maternal body condition. As noted above, a previous study showed that the application of a two-phase feeding program could save CP consumption by pregnant gilts (Baker et al., 1974). A phase-feeding program has been widely accepted as an efficient way to decrease N excretion from growing and finishing pigs (Chauvel and Granier, 1996; Ferket et al., 2002). Results of the current study suggest that a two-phase feeding program for pregnant gilts might be both effective and efficient in enhancing fetal growth and maternal protein accretion, while decreasing N excretion to the environment.

	Footnotes

 
¹ The authors acknowledge the financial support of CJ Corp. (Seoul, Korea), Texas Tech Univ., and National Research Initiative Competitive Grant 2001-35206-11247.

² Correspondence: Box 42141, 203 Anim. Food Sci. Bldg. (phone: 806-742-2805; fax: 806-742-0169; e-mail: sungwoo.kim{at}ttu.edu).

Received for publication March 8, 2004. Accepted for publication November 15, 2004.

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