J. Anim. Sci. 2002. 80:2442-2451
© 2002 American Society of Animal Science
Protein and fat utilization in lactating sows: I. Effects on milk production and body composition1
J. P. McNamara*,2 and
J. E. Pettigrew
* Department of Animal Sciences, Washington State University, Pullman 99164-6351 and
and
Department of Animal Sciences, University of Illinois, Urbana 61801
2 Correspondence:
233 Clark Hall, P.O. Box 646351, Washington State University, Pullman WA 99164-6351. Phone: 509 335-4113; Fax: 509-335-4246; E-mail:
mcnamara{at}wsu.edu.
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Abstract
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In order to provide data with which to challenge a model of metabolism of lactating sows, we conducted a study to determine milk production and body and mammary composition in sows consuming a range of energy and amino acid intakes and nursing 11 to 12 pigs. Sows (2nd through 4th parity) consumed the same ration during gestation and consumed 6.1 kg/d (as-fed) for a 20 d lactation. Litter size was standardized at 12 pigs within 3 d of farrowing. Diets were formulated to provide three different amounts of protein intake and two different amounts of fat intake. Protein intakes of sows in high (HP), medium (MP), and low protein (LP) treatment groups were 863, 767, and 678 g/d with 59, 53, and 47 g/d lysine at two levels of fat intake, 117 (LF) and 410 g/d (HF). Number of pigs weaned per litter was 11.4 ± 0.5 and milk production and litter weight gain was less (P < 0.01) in the last week of lactation for sows consuming the least protein. Medium and low protein intakes increased (P < 0.05) loss of body lean and protein. Change in carcass protein during lactation was -1.4, -3.0, -2.2, -1.2, -1.9 and -2.1 kg (SD 2.6) for sows fed HPLF, MPLF, LPLF, HPHF, MPHF, and LPHF. Body fat (carcass and visceral) change was 0.4, -3.7, -4.1, -0.3, 3.4, and -1.3 kg (SD 6.6) in HPLF, MPLF, LPLF, HPHF, MPHF, and LPHF groups. Total amount of mammary parenchyma increased more (P < 0.05) in sows fed a higher fat diet. These data are consistent with general knowledge of changes in body composition in lactation of sows. However, changes in body protein and fat were correlated across treatments and different from that reported for sows nursing smaller litters. These data help our quantitative understanding of nutrient flux in sows nursing large litters and allow a severe challenge of existing models of metabolism in sows.
Key Words: Amino Acids • Fats • Lactation • Metabolism • Simulation Models • Sows
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Introduction
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Ability to predict performance of lactating sows requires quantitative knowledge of the chemical interconversions of amino acids, fatty acids, and glucose in body tissues (Black et al., 1986; Pettigrew et al., 1992a, b; Baldwin, 1995). This knowledge is limited for many biochemical pathways (McNamara and Boyd, 1998). Body fat and body protein are mobilized to meet the needs of lactation, especially when intake of energy and amino acids is limited (Pettigrew et al., 1992a,b; Parmley and McNamara, 1996). Information on the partitioning of amino acids between milk production and body protein is limited, though more recent reports are addressing this lack (Trottier et al., 1997; Clowes et al., 1998; Dourmad et al., 1998).
Rates of body fat synthesis and mobilization are sensitively related to energy intake and milk energy output (Parmley and McNamara, 1996). The recent National Research Council Requirements of Swine (NRC, 1998) suggested an increased requirement for protein and lysine in sows producing large amounts of milk. Since this study was conducted (preliminary results in McNamara, 1998) others have also probed the effects of nutrient intake on body tissue use (Clowes et al., 1998; Dourmad et al., 1998; Jones and Stahly, 1999), mammary growth (Kim et al., 1999, 2001a), and mammary amino acid use (Cooper et al., 2001a,b; Kim et al., 2001b). Such data are useful but often not sufficient to set specific parameters for the next generation of mechanistic models (Pettigrew et al., 1992a,b); also, obviously, they were not available when the model of Pettigrew et al. (1992a,b) was constructed nor when this trial was begun. Objectives were to 1) determine effects of a range of protein and fat intakes on body composition and milk production in sows nursing 11 to 12 piglets and 2) to collect data sufficient for challenging behavior and sensitivity of an existing mechanistic, dynamic model of metabolism in the lactating sow (Pettigrew et al., 1992a,b).
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Materials and Methods
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Experimental Procedures
Sows were Landrace x Yorkshire crossbreds and blocked into two parity groups: parity 2 or parity (3 or 4). They were bred by artificial insemination and fed 1.84 kg as fed of the same diet (corn/soybean; 13.5% CP, 3.3 Mcal ME/kg as fed), balanced to NRC requirements for restricted gestation feeding (NRC, 1988). The lactation feeding experiment was designed as a randomized complete block, with two levels of dietary fat and three levels of dietary protein and parity as block. Primiparous animals were not used, since their metabolism and production are significantly different from older animals, and animals of parity greater than 4 were not used since we had a limited number in our herd and we wished to minimize extraneous variation. From farrowing onward, sows were fed twice daily (between 0700 and 0900 and 1600 and 1800) the following diets (all percentages as fed): Low Protein, Low Fat, (LPLF): 2% added fat, 11.6% CP, 0.8% lysine; Medium Protein, Low Fat (MPLF): 2% added fat, 13.1% CP, 0.9% lysine; High Protein, Low Fat (HPLF): 2% added fat, 14. 7% CP, 1.0% lysine; Low Protein, High Fat (HPHF): 7% added fat, 11.6% CP, 0.8% lysine; Medium Protein, High Fat (MPHF): 7% added fat, 13.1% CP, 0.9% lysine; and High Protein, High Fat (HPHF): 7% added fat, 14.7% CP, 1.0% lysine (Table 1
). These diets were formulated to either meet (LP) or exceed (MP, HP) the amounts recommended as required by the NRC in 1988. There were 18 animals assigned to each treatment group, resulting in 54 in each fat intake group and 36 in each protein intake group. Diets were offered up to 5.5 kg/d (for a given individual animal) as fed during wk 1 and 6.8 kg/d for wk 2 and 3. Feed offerings were adjusted downwards for individual animals if they did not consume all the feed offered; however, animals usually consumed all that was fed. In those few instances when all food offered was not consumed, orts were collected, weighed, and actual feed intake determined. There were several reasons for setting treatments this way. By early 1995, when this experiment was designed, it was already recognized that the recommendations in the 1988 guide were most likely too low for more modern sow genotypes nursing larger litters. We wanted to specifically test effects of total protein and fat (not specific amino acids or glucose (from starch)); we wanted to feed protein above the recommended level to sows nursing large litters. Our herd records indicated our sows voluntarily consumed more feed than the averages given in the 1988 NRC, and we wanted to provide data for a challenge of a metabolic model (Pettigrew et al., 1992a) that was constructed using data from sows, which were in general consuming less feed and nursing smaller litters than the present sows. Day 1 of lactation is first full 24-h period after farrowing, starting at 0700 each calendar day.
Sampling Regimen
Litters were adjusted to 12 pigs within 3 d of parturition. There was no minimal or maximal cut-off for litter sizes for use of sows, and in practice the smallest litter size was three, and the largest was 16. Sow body weights were taken 5 d prepartum and weekly postpartum. Ultrasound measurements of subcutaneous fat over the 10th and last ribs (65 mm lateral to the dorsal midline) were conducted using real-time ultrasound [Aloka 210 B-mode, Corometrics Medical Systems, Wallingford, CT; Animal Ultrasound Services, Inc., Ithaca, NY; and Superflab (cat # 8117) Mick Radio-Nuclear Instrumentation, New York, NY; with a wide view field transducer (U5T-5021-3)]. Litters were weighed twice weekly (Tuesdays and Fridays). Milk samples were taken on d 7 and 20 of lactation. After removal of the litter for 1 h, an injection of 0.5 units of oxytocin was given, followed by reintroduction of half of the litter to induce milk letdown and allow sampling of glands. Every attempt was made to sample from three or more mammary glands; this was not possible for every sow at every milking, yet this was the overwhelming norm. Milk production was calculated from litter weights and growth rates following equations in (Noblet and Etienne, 1989 [milk (kg) = (2.50 x ADG) + (80.2 x piglet BW) + 7]). Milk DM and ash were determined by standard methodologies (AOAC, 1990), fat was extracted in chloroform: methanol (Folch et al., 1957), and milk protein was determined by the dyebinding method of Bradford (1976).
Body Composition
Fifteen sows were killed at d 1 of lactation for a comparative slaughter group (initial slaughter (IS)). They were allotted to this group randomly, and contemporarily with sows going on feeding treatments. At end of treatment, six animals from each treatment were killed at d 21 for body composition. There were no restrictions on selection of IS group or slaughter sows; it was a random allotment. Sows were killed under USDA guidelines by captive bolt stunning followed by exsanguination. Blood and organs were collected and weighed (gastrointestinal contents were not removed because animals were fasted overnight and we were not concerned with variation due to remaining intestinal contents). The right half of each carcass dissected (after an overnight chilling at 4°C) into trimmable fat, lean, and bone. Separation of fat from lean was such that upon visual appraisal, in the fat there were only very minor and small flecks of lean; bone was trimmed of lean and fat and any bits of lean or fat remaining on bone after first cutting were trimmed and placed in the appropriate sample. Lean was ground thorough a 0.95 cm opening plate, mixed, and sampled. Chemical dry matter and ash were determined by AOAC techniques (AOAC, 1990). Fat content of the lean was extracted in chloroform: methanol (Folch et al., 1957). Muscle protein content was calculated as fat-free lean multiplied by 0.207 (Pettigrew et al., 1992a,b). To calculate the change in body protein and fat during lactation, the average percentage of body fat and protein of BW from the IS group was multiplied by the actual individual BW of the sows used for chemical analysis at the end of lactation. Change in body fat or protein was then determined as the difference of the individual sow ending composition and the individual sow initial composition.
Mammary glands were collected in total and weighed. The same mammary gland (4th from first inguinal gland, right side; if this gland was not functional, the next gland caudally was used) was excised, weighed, and dissected into skin, fat, lean, and parenchyma. Parenchyma only was extracted for analysis of DNA (Kensinger et al., 1982).
Experimental Design and Analysis
The production experiment was designed as a randomized complete block with repeated measures in time, with parity (2 or 3 and 4) as blocks. The composition aspect was a randomized complete block with a split-plot in time of different experimental units at each time point (comparative slaughter). Factorial treatments of three protein levels and two fat levels were applied to groups of 18 sows. For production and composition data, main and interaction effects of parity, protein intake, fat intake, and their interaction (protein x fat; parity x protein; parity x fat; parity x protein x fat) were determined against the error mean square for sow within (parity x fat x protein) using the appropriate F statistic (Steel and Torrie, 1980). If the parity x protein x fat interaction was not significant (it almost always was not), it was pooled into the error term. Therefore, the model without that effect was usually the final model for all variables, unless stated otherwise. For measures where an initial value was available (e.g., sow body weight, composition, litter live birth weight), the initial value was tested as a covariable, and if this effect was significant, that analysis was used. Effects were considered significant if P < 0.05, although in a few instances we have reported effects with P value between 0.05 and 0.10 and discussed them separately.
In an additional set of analyses with the objective of describing some empirical relationships between observed or simulated uses of body nutrients, variables of body composition (body fat, lean, protein, change in lean, fat, and protein) were regressed on each other (Steel and Torrie, 1980). Linear, quadratic, and logarithmic regressions were tested; however, for all variables the best fit of data were provided by the linear regression only. Calculations were done using PROC GLM and PROC REG of SAS (SAS Inst. Inc., Cary, NC).
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Results and Discussion
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Intake of diets was similar among treatments, as planned, with no difference (P > 0.10) in feed intake during lactation (Table 2). Covariate adjusted (d 1 value as covariate) sow body weights at 20 d were lower (P < 0.03) and body weight loss was greater (P < 0.03) for sows consuming low fat rations (Table 2). There was a main effect of protein intake (P < 0.02) for BW at d 7, 14, and 21 and for change in BW, with the sows consuming the highest protein losing less BW. There was no effect (P > 0.10) of energy or protein intake on sow backfat thickness nor changes in backfat thickness (Table 3).
Litters gained 50.7 kg (SE = 0.73 kg, n = 108) on average over the 20 d (20.3 d ± 0.09, n = 108; range 17 to 22 d) lactation. Those sows consuming the lowest protein tended to produce a lower litter gain (P < 0.08) than the other groups (Table 4). The reduction in litter growth came primarily in the last week of lactation (Table 4). This is still one of the highest rates of litter gain for sows reported in the scientific literature (refer to any of the several references listed), demonstrating the milking capacity of these animals and providing a severe challenge for model behavior. The decrease in piglet growth due to low protein intake of the sow compared to the average of medium and high protein was 2 kg per litter for the last 3 d of lactation. There was neither a main effect of fat nor an interaction of fat and protein (P > 0.10).
Milk dry matter percent was unaffected (P > 0.10) by dietary protein intake (Table 5). There was an interaction of protein and fat intake on milk protein percentage, such that milk protein percentage decreased with decreasing protein intake on the low-fat diet (P < 0.02), but not on the high-fat diet. Milk protein percentage and milk DM in this study were lower than is usually reported for sow milk (King et al., 1993; Dourmad et al., 1998; Pluske et al., 1998). There may be a methodological difference in that we used a soluble protein dyebinding assay to measure true protein, whereas many researchers analyze milk total N and use a multiplier for protein. Total N usually overestimates true protein in milk approximately 0.2 percentage units. This alone would not account for the low protein values, which were measured about 2.9 to 3.7%, while other measures have been above 5%. Another potential hypothesis is that because these animals were nursing almost 12 pigs for 20 d and producing a large volume of milk (one of the largest reported) is that the milk of these sows was indeed lower in protein. However, when Dourmad et al. (1998) reported data on sows nursing 11 to 12 pigs, the protein percentage in milk from those sows was close to 5% (they measured total milk N, and whether using a multiplier of 6.38, 6.25, or 5.83 (porcine muscle), the milk protein would be still be about 5%). However, their reported milk production was 1.5 to 2.5 kg per d less than the sows reported here and they determined milk yield with the same method as we used. Renaudeau and Noblet (2001) reported similar milk yields to the present ones but a greater protein percentage. Thus, although there may be a genetic difference in this herd or a production level difference, the low protein values were most likely due to the technique used. For this reason we are not making any inferences of treatment effects on milk protein in this study (the available literature would suggest that at these protein intakes, a large change in protein composition might not be expected), and the effect on errors in model challenges is discussed in that section.
Sow body composition during lactation changed as expected from previous knowledge on use of dietary protein and fat (Tables 6 and 7) with a few exceptions. Most internal organ weights did not change appreciably over the course of lactation or due to treatment. Total body fat was gained in groups consuming low fat with high protein and high fat with either medium or high protein and lost in other treatment groups; however, these trends had only a P = 0.11 for protein main effect and 0.13 for fat main effect. The old sorry story of "not enough animals to test the hypothesis" probably applies here, but the data also illustrate the larger problem of the number of animals it now takes to measure significant treatment differences in important traits.
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Table 6. Body composition of sows entering lactation and of sows fed varying amounts of protein and fat during lactation and used in comparative slaughter analysis
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Trimmed carcass lean and protein decreased during lactation in all groups and the loss was increased (P < 0.05) if the sows consumed the LP rations. The amount of dietary protein intake (approx. 800 g/d) below which sows lost body protein is greater in relation to suggested requirements in the 1988 NRC guidelines; however, it is in good agreement with the newer revision (NRC, 1998). Sows consuming the medium dietary protein amount (740 to 780 g/d) did lose more loin eye area but not more protein than those consuming the HP diet. Recently, Yang et al. (2000) reported that sows consuming 20 g/d of lysine and 11.9% CP had an increase in the fractional breakdown rate of muscle protein, which would be consistent with the intake and body composition data in this experiment. The loss of muscle protein in the LP fed group was noted in sows consuming either amount of fat. There were no interactions of protein by fat in any body composition variables measured except bone, and we have no explanation for this effect.
Several regressions were conducted to explore the relationship between changes in body components. Conducting this type of experiment over a wide range of protein intakes allows exploration of some relationships, which are relatively still undefined, especially in high producing sows. Such statistical comparisons do not answer questions on cause and effect; however, they do point out biological processes that may be more or less important for further experimentation on control mechanisms. In addition, these relationships can help set parameters in empirical and mechanistic models much better than can simple treatment means. The amount of lean in the carcasses of sows over the entire data set was closely related to the body weight (Lean Weight = 0.440 x BW - 14.78 kg; r2 = 0.84). The change in BW and change in lean during lactation were also fairly well correlated (Figure 1), as has been noted previously (Pettigrew et al., 1992a; Tokach et al., 1992), suggesting that a large percentage of the change in BW during lactation is due to gain or loss of carcass protein and fat.
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Figure 1. Relation of change in body trimmed lean with change in body weight in sows fed varying protein and energy and nursing large litters. Change in total trimmed lean related to change in total BW during lactation. Carcass composition change was calculated by subtracting that at 20 d of lactation from that in a comparative slaughter group at d 1 to 3 of lactation. The equation describing the linear fit is: Change in body lean, kg = 0.4146 x Change in BW, kg - 1.278, r2 = 0.840, n = 52, SEyx = 4.86 kg.
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The relationship between body lean and loin eye area was (LEA (cm2) = 0.401 x Lean + 33.8 kg, r2 = 0.40). Also, the loss of body lean and the reduction in loin eye area were related but only with a regression coefficient of 0.26 (Figure 2). This would only weakly support the idea that LEA measures may have utility for estimating loss of carcass lean during lactation. Nevertheless, this technique is less costly, obviously less invasive, and may help in practical research studies designed to determine the effects of feeding various protein levels or to determine protein requirements for maintenance of sow muscle during lactation.
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Figure 2. Relation of change in body lean to change in loin eye area in sows fed varying protein and energy and nursing large litters. Change in area of loin eye related to change in mass of trimmed lean during lactation. Change was calculated by subtracting value at d 20 of lactation from the comparative slaughter average measured from d 1 to 3 of lactation. Equation for linear fit of data: loin eye area, cm2 = -8.92 + 0.404 x change in trimmed lean, kg, r2 = 0.26, n = 37, SEyx = 16.4 cm2.
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The amount of body fat (carcass plus trim) was related to body weight in lactating sows (Body Fat = 0.174 x BW x 1.48 kg, r2 = 0.35), but the strength of this regression demonstrates that there are several other contributors to changes in body weight during lactation than body fat. The amount of body fat and body weight loss were also correlated with a regression coefficient of 0.40 (Figure 3). Interpreting the relation of BW loss, body protein (and associated muscle water) loss, and body fat loss, one might infer from this study that loss of body protein was the major contributor to loss of body weight and loss of body fat a lesser, but significant, contributor. The loss of fat in the trimmed lean was correlated with the loss of lean at approximately 40% as well (Figure 4). This has been noted in cows previously (Komaragiri and Erdman, 1997). The point here is not whether or not each regression coefficient is close to 1.0 but the overall pattern of changes in body protein, fat, and weight loss within a range. Certainly one would expect that the intercept, slope, and regression coefficient of this relationship would be a function of the range of treatments applied and the severity of the deficit (or excess) in energy and amino acids. That is, lactating animals, which are deficient in both fat and amino acids, would be more likely to lose both body lean and body fat than those in a deficit only of fat. So, one goal is to conduct sufficient trials over sufficient ranges to put together a metaanalysis of these biological processes. Also, in some previous studies with sows, no such relationship has been noted. In fact, feeding additional protein to sows resulted in an increase in nitrogen balance but a loss in backfat thickness (King et al., 1993; Tokach et al., 1992). Recently, feeding two widely different (8.3 and 20.6%) amounts of protein to sows nursing 13 pigs, Jones and Stahly (1999) reported greater (but not significantly) total body fat loss on the higher protein ration. In that study, the sows on the greater protein diet also consumed more feed and lost less total weight. The interpretation was that the increase in protein supply stimulated more milk production, resulting in a need for more fat.
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Figure 3. Relation of change in total body fat with change in body weight in sows fed varying protein and energy and nursing large litters. Change in total fat (fat in lean and trimmed fat) related to change in BW during lactation. Change was calculated by subtracting value at d 20 of lactation from the comparative slaughter average measured from d 1 to 3 of lactation. Equation for linear fit of data: change in total fat, kg = 1.35 + 0.1466 x BW, kg; r2 = 0.379, n = 48, SEyx = 5.19 kg.
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Figure 4. Relation of change in fat contained in trim lean with change in total trimmed lean in sows fed varying protein and energy and nursing large litters. Change in fat in trimmed lean only related to change in trimmed lean during lactation. Change was calculated by subtracting value at d 20 of lactation from the comparative slaughter average measured from d 1 to 3 of lactation. Equation for linear fit of data: change in fat in trimmed lean, kg = 0.340 + 0.1414 x trimmed lean, kg; r2 = 0.395, n = 34, SEyx = 2.04 kg.
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However, a different result obtained in the present study, in that even at protein intakes greater than what was the requirement in 1988, decreasing protein intake while maintaining total feed and energy intake increased the treatment means for protein loss and fat loss (admittedly the greater loss of fat on the LP rations was not significantly different). This result was also supported by the empirical relationship shown in Figure 3 and 4
: the greater the loss in body weight or lean, the greater the loss of body fat or of fat in the lean. There was also on average (not significant; P > 0.10) a loss of body fat on LP treatments and no change or gain on MP and HP treatments. The explanation for this relationship is certainly neither simple nor clear. One may be random chance, which can only be ruled out by further repetition. On a strictly anatomical basis, it may be that as muscle protein is broken down, fiber shapes change and associated fat is also mobilized, and not necessarily in direct response to a deficit of energy or fat. Also, it may be that some of the fat lost from the intermuscular areas was recycled as fatty acids to the subcutaneous fat depots. Both lipogenesis and lipolysis in subcutaneous adipose tissue proceed at significant rates in lactating sows fed adequate energy (Parmley and McNamara, 1996). This is consistent with the lack of change in backfat thickness and little or no change in trimmed fat. Also, this study was conducted on sows producing a large amount of milk every day to nurse 11 to 12 pigs. Thus, some of the fat mobilized from the lean may have been used to support oxidative processes while sparing glucose and gluconeogenic amino acids. All of these speculations may only be tested by further experiments, which continue to be needed to improve our models of nutrient metabolism.
The total weight of a representative mammary was increased (P < 0.05) during lactation in sows fed diets with a greater fat content, but dietary protein content had no effect (P > 0.10) on total mammary gland weight (Table 8). Of all the measurements taken, only for trimmed parenchyma was there an increase due to higher fat inclusion in the diet (P < 0.05). There was a consistent (but not significant (P > 0.10)) trend that the glands of sows fed the low protein, low-fat ration had the least mammary parenchyma and least total protein (Table 8), which is also consistent with this group of animals having the smallest starting body weights and the smallest mammary glands overall in the carcass dissection (Table 6). Thus, any real treatment effect here is unlikely. These data are not in complete agreement with the few data sets that do exist on mammary composition, but the basic weight ranges are consistent. Dourmad et al. (1998) reported a 1 kg increase in udder weight in sows fed 15.5% CP and 0.66% lysine and changes of + 0.6 to + 0.8 kg for those fed more lysine or more CP. The present data agree with mammary growth in general but would suggest that lower protein intakes would limit mammary growth. More recently, Kim et al. (1999) measured an increase of about 200 g per gland in wet weight from 5 to 21 d of lactation in glands that were known to have been suckled. Our measurement of total trimmed mammary gland weights would have included nonsuckled glands, and we measured about 690 g of total weight increase. However, for the measurements on a single representative, suckled gland we measured a range of 50 to 180 g of increase per gland on animals fed diets considered nutritionally adequate (LFHP; and all HF diets). The DNA content (mg/g) of the mammary gland was not changed during lactation or by dietary treatment (Table 8). Our mean estimates of DNA (on a percentage basis) were quite close to those of Kim et al. (1999): they reported an increase in DNA concentration in wet tissue from 5 to 21 d of lactation from 0.21 to 0.26 percentage units. On a mass basis, they did report an increase of about 30% in mg of DNA in dry tissue and almost 100% in total suckled glands. We determined an increase in mg of DNA from 1,558 mg at IS to 2,119 mg in the HFLP treatment at d 20, or a 36% increase (calculated from total wet gland wt and mg/d DNA in Table 8), so the results are generally consistent given the differences in the trials.
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Implications
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The modeling approach has been used with success in swine research and production. Adequacy of a model toward its objective is limited by data available to set equation forms and parameter values. The present data were compiled to challenge the behavior of an existing model of metabolism. Findings corroborate that decreases in protein or fat intake limit milk production and grossly alter body composition of sows nursing larger litters. Protein and fat intake interact such that increased energy can partially relieve the effects of decreased protein intake on milk production. Recommendations in the recent Nutrient Requirements of Swine are close to what we found optimal in this study. Sows mobilize amino acids from muscle to support mammary growth and milk production, and this is limiting only under a severe deficit of amino acids. The major impacts of this study are explored in the next paper in which the metabolic model is challenged with this data set.
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Footnotes
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1 College of Agriculture and Home Economics Research Center, Washington State University, Pullman. Project Numbers 0107, 0249, and 3407. Supported in part by the National Institutes of Health (RD24529) and the National Pork Producers Council (1598). 
Received for publication June 1, 2001.
Accepted for publication May 14, 2002.
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Abstract
Introduction
