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Effects of dietary iron supplementation on growth performance, hematological status, and whole-body mineral concentrations of nursery pigs¹

Department of Animal Science, Michigan State University, East Lansing 48824

	Abstract

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An experiment was conducted to evaluate the effects of supplementing increasing concentrations of Fe to the diet of nursery pigs on growth performance and indices of hematological and mineral status. Pigs (n = 225; 6.5 kg; 19 ± 3 d) were allotted randomly by BW, litter, and gender to one of five dietary treatments (five pigs per pen; nine pens per treatment). Basal diets for each phase (Phase 1: d 0 to 7; Phase 2: d 7 to 21; Phase 3: d 21 to 35) were formulated to contain minimal Fe concentration and then supplemented with 0, 25, 50, 100, and 150 mg Fe/kg of diet (as-fed basis) from ferrous sulfate. Three pigs per pen (n = 135) were chosen and bled throughout (d 0, 7, 21, and 35) to determine hemoglobin (Hb), hematocrit (Hct), transferrin (Tf), and plasma Fe (PFe). In addition, pigs (n = 5; 5.9 kg; 19 ± 3 d) from the contemporary group were killed at d 0 to establish baseline (BL), and 30 pigs (six pigs/treatment) were killed at d 35 to determine whole-body and liver mineral concentrations. The improvements in growth performance during Phase 2 (ADG = linear, P = 0.04; ADFI = linear, P = 0.10; G:F = quadratic, P = 0.07) were of sufficient magnitude that dietary treatments tended to increase ADG (linear, P = 0.08), ADFI (quadratic, P = 0.09), and G:F (quadratic, P = 0.10) for the 35-d experiment. Hematological variables were not affected until d 21, at which time dietary Fe supplementation resulted in a linear increase (P = 0.03) in Hb, Hct, and PFe. This linear increase (P = 0.001) was maintained until d 35 of the experiment; however, dietary treatments resulted in a linear decrease (P = 0.01) in Tf on d 35. Whole-body Fe concentration increased (linear, P = 0.01) in pigs due to increasing dietary Fe concentrations. Moreover, pigs fed for 35 d had greater (P = 0.02) whole-body Fe, Zn, Mg, Mn, Ca, and P concentrations and lower (P = 0.001) whole-body Cu concentration than BL. Hepatic Fe concentration increased (linear, P = 0.001) in pigs due to dietary treatments; however, the hepatic Fe concentration of all pigs killed on d 35 was lower (P = 0.001) than the BL. Results suggest that Fe contributed by feed ingredients was not sufficient to maintain indices of Fe status. The decrease in Fe stores of the pigs was not severe enough to reduce growth performance. Even so, the lessening of a pig’s Fe stores during this rapid growth period may result in the occurrence of anemia during the subsequent grower and finisher periods.

Key Words: Growth • Iron • Nursery pig • Transferrin • Whole Body

	Introduction

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The most common potential mineral deficiency in swine is Fe deficiency. Several factors contribute to this deficiency in nursing pigs. The hepatic Fe stores of a newborn pig combined with the low Fe concentration in sow’s milk are not sufficient to meet the pig’s rapid growth and increase in blood volume. Consequently, the use of an exogenous source of Fe to prevent Fe deficiency in neonatal pigs has been well documented (Ullrey et al., 1959; Kernkamp et al., 1962). Similar to the suckling phase, the nursery phase is also characterized by increased blood volume and rapid growth. This is especially true in the swine industry today, with genetics that have been selected for increased growth performance and carcass leanness. The NRC (1998) postweaning dietary Fe requirement, 80 mg/kg, is based on early experiments conducted by Pickett et al. (1960), who reported decreased growth in pigs weaned at 10 to 14 d of age and fed diets containing 60 mg of added Fe/kg diet, and decreased hemoglobin (Hb) and hematocrit (Hct) in pigs fed diets containing 80 mg of added Fe/kg diet. The Fe provided by common feed ingredients may meet most of the postweaning Fe requirement. However, the bioavailability of Fe from different sources varies greatly (Kornegay, 1972; Deming and Czarnecki-Maulden, 1989) and is influenced by such factors as Fe status of the animal, dietary Fe concentration, and various nutritional and nonnutritional elements within the diet.

The experimental objective was to evaluate the effects of increasing concentrations of supplemental ferrous sulfate in diets using ingredients commonly included in commercial diets, but which provide minimal Fe to the basal diet on nursery pig growth performance, hematological status, and whole-body mineral concentrations and chemical composition.

	Materials and Methods

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Animal Use and Care
This experiment was conducted at the Michigan State University Swine Teaching and Research Facility. Use of animals in this experiment was approved by the All-University Committee on Animal Use and Care at Michigan State University (Animal Use Form No. 12/02-164-00).

Animals, Diets, and Housing
Two hundred twenty-five pigs (Duroc x Landrace-Yorkshire, Landrace x Yorkshire, and Yorkshire) were weaned at 19 ± 3 d and used during a 35-d feeding experiment. Before initiating the experiment, pigs were managed according to facility standard operating procedures and received 200 mg of Fe via i.m. injection of Fe dextran (Phoenix Pharmaceutical, Inc., Saint Joseph, MO) at 1 to 2 d of age. At the start of the experiment, pigs were blocked on the basis of initial BW (mean = 6.5 kg), while equalizing ancestry and gender across treatments, to five dietary treatments in a randomized complete block design. There were nine replicate pens per treatment with five pigs per pen.

Basal diets (Table 1) for each dietary phase (Phase 1: d 0 to 7; Phase 2: d 7 to 21; Phase 3: d 21 to 35) were formulated based on previous analysis of similar dietary ingredients and, when necessary, on values published in the NRC (1998) feed composition tables. Feed ingredients commonly included in commercial nursery diets, yet containing a low concentration of Fe, were used. To minimize the Fe contribution from dietary Ca and P sources, experimental diets were formulated using calcium sulfate and sodium phosphate, respectively, because they are known to contain minimal Fe (NRC, 1998). To limit the Fe concentration in the basal diets, the base mineral mix used in this experiment contained minimal Fe (729 mg/kg; as-fed basis) in comparison with typical commercial base mineral mixes, which contain 15,000 to 20,000 mg Fe/kg. Dietary treatments were obtained by supplementing the basal diets with 0, 25, 50, 100, and 150 mg of Fe/kg of diet (as-fed basis) from ferrous sulfate monohydrate (FeSO₄• H₂O), a highly available Fe source (Harmon et al., 1967; Miller, 1978). The complexity of the diet changed with phases to meet or exceed NRC (1998) nutrient recommendations, excluding Fe, and to satisfy changes in digestive capabilities of the weanling pig. Phase 1 and 2 diets were fed in pelleted form and consisted of highly digestible protein and carbohydrate sources, whereas Phase 3 diets were typical corn-soybean meal-based diets that were fed in meal form.

Performance, Blood, Tissue, and Whole-Body Collection
Pig weights and pen feed disappearance were determined for each dietary phase and utilized in the determination of ADG, ADFI, and G:F.

Initially (d 0), three pigs were randomly chosen from each pen (n = 135) and blood samples were collected. The same 135 pigs were bled again on d 7, 21, and 35. Blood samples were drawn by jugular venipuncture into 10-mL heparinized (143 USP units of sodium heparin per tube) Vacutainer (Becton Dickinson, Franklin Lakes, NJ) tubes with 21-gauge, 3.81-cm needles. An aliquot of whole blood was transferred into a polypropylene tube and stored on ice until Hb and Hct analysis could be performed later that day. The remaining blood was centrifuged at 2,000 x g, 4°C, for 10 min (Beckman GS-6KR, Palo Alto, CA). Plasma was collected into polypropylene tubes and stored at –80°C until mineral and transferrin (Tf) analysis could be performed.

At the conclusion of the experiment, one pig per pen was randomly chosen from 30 pens (six pigs per treatment; mean BW = 18.4 kg; 54 ± 3 d) and killed via cardiac injection of sodium pentobarbital (87 mg/kg of BW). Liver samples were excised and stored in Whirl-Pak bags (Nasco, Fort Atkinson, WI) at –80°C until mineral analysis could be performed. The remaining whole-body of each pig was frozen and ground (Autio 801GH, Autio Co., Inc., Astoria, OR) twice through a 3.2-mm aperture plate, mixed, and subsampled. Sub-samples were freeze-dried (TriPhilizer MP, FTS Systems Inc., Stone Ridge, NY) and further processed to reduce particle size by submersion in liquid nitrogen and blending in a 1.5-L stainless steel blender (Waring Products Co., New Hartford, CT). Subsamples were then stored in Whirl-Pak bags until analysis for minerals and chemical composition. Additionally, five pigs from the original contemporary group (mean BW = 5.9 kg; 19 ± 3 d) were killed at the initiation of the study. Liver and whole-body samples were collected in a similar manner as that previously described to establish baseline (BL) response criteria for mineral concentrations and chemical composition.

Mineral Analyses
Feed, liver, and whole-body samples were prepared for mineral analysis via microwave digestion (model MARS-5, CEM, Matthews, NC) as described by Shaw et al. (2002). Calcium, Cu, Fe, Mn, Mg, and Zn analyses were conducted by flame atomic absorption spectrophotometry (Unicam 989, Thermo Elemental Corp., Franklin, MA), and P concentration was determined (Gomori, 1942) using a DU 7400 spectrophotometer (Beckman, Palo Alto, CA). All analyses were performed in duplicate, and feed, liver, and whole-body mineral concentrations were reported on an as-fed, fresh, and DM basis, respectively.

Plasma samples were deproteinized by the addition of trichloroacetic acid (TCA) for Cu (PCu), Zn (PZn), and Fe (PFe) analysis. For PCu and PZn analysis, plasma was diluted 1:4 with 12.5% TCA, mixed, incubated at room temperature for 10 min, and centrifuged (2,000 x g, 4°C, and 15 min). Plasma Fe was diluted 1:3 with 20.0% TCA, mixed, incubated at 90°C for 15 min, and centrifuged at 2,000 x g, 4°C, for 10 min (Olson and Hamlin, 1969). The supernatant fraction was transferred to a clean polypropylene tube and subsequently evaluated using a flame atomic absorption spectrophotometer.

Instrument accuracy for all mineral analyses was established using bovine liver standard (1577b; NIST, Gaithersburg, MD). All glassware used in the mineral analyses was soaked in 30% nitric acid for at least 12 h and rinsed five times with double-deionized water.

Hemoglobin and Hematocrit Analyses
Hemoglobin and its derivatives in whole blood were converted to cyanmethemoglobin and read at 540 nm using a spectrophotometer. Hematocrit was determined by the microhematocrit method (INACG, 1985).

Transferrin Analysis
Porcine plasma Tf concentration was determined by an ELISA at room temperature. Each well of a 96-well Nunc Maxisorp "C"-bottomed microtiter plate (Nalge Nunc Int., Rochester, NY) was coated with 0.1 mL of a 1:100 dilution of Affinity Purified Pig Tf Antibody (Bethyl Laboratories Inc., Montgomery, TX) and allowed to incubate for 60 min at room temperature. Wells were then rinsed three times with wash solution (50 mM Tris buffered saline, 0.05% Tween 20, pH 8.0, Sigma-Aldrich Co., St. Louis, MO) and blotted dry. Next, 0.2 mL of postcoat solution (50 mM Tris-buffered saline, 1% BSA, pH 8.0, Sigma-Aldrich Co.) was applied to each well and incubated for 30 min. Wells were again washed three times with wash solution and blotted dry. Pig reference plasma (Tf = 50 g/L, Bethyl Laboratories Inc.) was diluted to make the Tf standard increments (19.5 to 1,250.0 ng/mL) and plasma samples, standards, and a control plasma sample were applied in duplicate using 0.1 mL per well. Microtiter plates were then incubated for 60 min, rinsed five times with wash solution, and blotted dry. Then, 0.1 mL of horseradish peroxidase conjugated pig Tf antibody (Bethyl Laboratories Inc.) was added to all wells and incubated for 60 min, rinsed five times with wash solution, and blotted dry. Next, 0.1 mL of enzyme substrate, tetramethylbenzidine peroxidase substrate, and peroxidase solution B (vol/vol) (Kirkegaard & Perry Laboratory, Gaithersburg, MD) was added to all wells and incubated until the color developed sufficiently for the 1,250 ng/mL standard to have an optical density reading greater than 1.6 (approximately 20 min). Finally, 0.1 mL of 2 M sulfuric acid was added to each well to stop the reaction. Results were read at 450 nm using a SpectraMax 340 microtiter reading spectrophotometer (Molecular Devices Corp., Sunnyvale, CA). A four-parameter curve was used for the fit of the slope of the standards. Plasma unknowns were plotted against the standard curve and the concentration (ng/mL) was reported.

Whole-Body Chemical Composition
Whole-body samples were analyzed for protein, lipid, and ash composition by standard AOAC (1998) methods utilizing a combustion instrument (FP-2000, Leco Corp., St. Joseph, MI), soxhlet extraction, and a muffle furnace (Type 30400, Thermolyne, Subsidiary of Sybron, Rochester, NY), respectively.

Statistical Analyses
Performance (ADG, ADFI, G:F) data were analyzed as a randomized complete block design using the MIXED procedures of SAS (SAS Inst., Inc., Cary, NC). The model included the effects of block (replication), treatment, and block x treatment (error), with block considered a random effect. The effects of increasing dietary concentrations of supplemental Fe were partitioned into linear and curvilinear components using orthogonal polynomial contrasts. Due to unequally spaced dietary concentrations of supplemental Fe, coefficients were derived using the integrative matrix language (PROC IML) procedures of SAS. Pen was the experimental unit for analysis of performance data. For tissue and whole-body data, analyses were performed using the MIXED procedures of SAS with individual pig as the experimental unit. Orthogonal polynomial contrasts were used to test for linear and quadratic effects of dietary Fe supplementation. One planned nonorthogonal comparison was made: BL pigs vs. all Fe treatments. In addition, data for blood variables were analyzed using the MIXED model methodology of SAS for analysis of repeated measure data. The subject for the repeated measures on d 7, 21, and 35 was individual pig within pen x treatment, and d-0 Hb, Hct, Tf, and PFe values were used as covariates for their respective individual analyses. Differences were considered significant at the level of P < 0.05 and highly significant at the level of P < 0.01.

	Results

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The basal diets contained 189.0, 223.8, and 97.8 mg of Fe/kg (as-fed basis) for Phase 1, 2, and 3, respectively. The analyzed concentrations of supplemental Fe for the five dietary treatments in each phase were within 15% of their targeted values (data not shown).

The effects of increasing concentrations of supplemental Fe on pig performance are shown in Table 2. Dietary treatments did not affect (P > 0.10) growth performance during Phases 1 and 3; however, increasing dietary Fe concentrations resulted in a linear increase (P = 0.04) in ADG during Phase 2. Also during Phase 2, dietary Fe supplementation tended to increase ADFI (linear, P = 0.10) and improve G:F (quadratic, P = 0.07). Overall, increasing concentrations of supplemental Fe tended to increase ADG (linear, P = 0.08), ADFI (quadratic, P = 0.09), and G:F (quadratic, P = 0.10).

The effects of dietary Fe supplementation on liver mineral concentrations of nursery pigs are presented in Table 4. Mean baseline liver Fe concentration on d 0 was greater (P = 0.001) than pigs fed any of the dietary treatments for 35 d. A dietary effect was also observed as pigs fed diets containing increasing concentrations of supplemental Fe had a linear increase (P = 0.001) in liver Fe concentration. Furthermore, pigs fed for 35 d had lower hepatic Cu, Zn, and Mg (P = 0.001) concentrations in contrast to BL. Finally, no differences were observed in hepatic Mn, Ca, and P concentrations.

	Discussion

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Underwood and Suttle (1999) described the physiological events induced by Fe deprivation and resultant subnormal bodily Fe status in four stages. The first stage is depletion, during which liver, kidney, and spleen stores (i.e., ferritin and hemosiderin) are decreased. Blood variables related to Fe status are not altered during depletion. The onset and duration of the depletion period is determined by the abundance of the initial storage pools, primarily in the liver. For the present experiment, mean BL liver Fe concentration (232 mg/kg) was within the normal range for the nursery pig reported by Underwood and Suttle (1999). Following completion of the 35-d feeding experiment, liver Fe stores were decreased in pigs fed all dietary treatments. The hepatic Fe concentration of pigs fed diets with 0 or 25 mg added Fe/kg decreased to within the marginal range separating deficient from adequate hepatic Fe concentrations reported by Underwood and Suttle (1999). Recent work by Yu et al. (2000) noted a linear decrease in liver ferritin, nonheme Fe, and total Fe as dietary Fe supplementation decreased from 120 to 0 mg Fe/kg. A linear decrease in liver ferritin, nonheme, hemosiderin, and total Fe was also reported by Furugouri (1972) when dietary Fe decreased.

The second physiological stage described by Underwood and Suttle (1999) in Fe disorder is deficiency that is characterized by decreases in PFe, Hb, Hct, and myoglobin concentrations. A continued decrease in Fe stores is also observed during this stage. These changes are accompanied by increases in the concentration of the principal Fe transport protein, Tf, in an effort by the body to distribute nonheme Fe to the Fe-requiring tissues.

A whole blood Hb concentration of 1.00 g/L is considered adequate, whereas 0.80 g/L suggests borderline anemia, and 0.70 g/L or less indicates anemia (Zimmerman, 1980). At the start of this study, pigs in all dietary treatments had a mean Hb concentration of 1.10 g/L or greater. Mean Hb concentrations did not decrease until d 21; however, Hb concentration for all dietary treatments remained above the borderline anemia concentration (0.80 g/L). The linear decrease in Hb concentration noted in this experiment is in agreement with the results of Furugouri (1972) and Hedges and Kornegay (1973), who reported linear decreases in Hb concentration with decreasing dietary Fe concentrations. An increase in mean Hb concentration was observed in all treatments during Phase 3 of the current study, even though the basal diet (corn-soybean meal-based) fed during Phase 3 had a lower Fe concentration by analysis (98 mg/kg) than the Phase 1 (189 mg/kg) or Phase 2 (224 mg/kg) diets. Harmon et al. (1968) reported that the percentage of dietary Fe retained by the body increases as dietary Fe concentration decreases.

Hematocrit is the proportion, by volume, of the blood that consists of red blood cells, and it is expressed as a percentage. Initial mean Hct for pigs in this study were comparable to values reported by Talbot and Swenson (1970) but greater than those reported by Miller et al. (1961) for similar age pigs. Hematocrit is affected by the hydration status of the animal, with dehydration producing a falsely high Hct. Stress at weaning can influence the hydration state of an animal. Hematocrit in pigs, similar to Hb, responded to dietary treatments with a linear increase on d 21. These results agree with those of Hedges and Kornegay (1973) and Dove and Haydon (1991), who reported increased percentages of Hct after 28 d as dietary Fe concentration increased. Yu et al. (2000) reported a linear increase in Hct as dietary Fe supplementation increased up to 120 mg Fe/kg.

Several factors limit the diagnostic usefulness of PFe measurements. Circulating PFe turns over 10 to 20 times daily, so an Fe atom typically spends no longer than 2 h in plasma (Schreiber, 1989). In addition, PFe exhibits a diurnal variation, with a decrease in concentration in the evening (Schreiber, 1989). Initial (d 0) PFe values were lower than values reported by Smith (1989); however, by d 7, they had increased to within the range (0.60 to 1.49 mg/L) reported by Underwood and Suttle (1999) for the nursery pig. The observed linear increase at d 35 in PFe due to increasing concentrations of dietary Fe in this experiment are similar to results reported by Yu et al. (2000).

A key facilitator in the maintenance of Fe homeostasis is the plasma glycoprotein, Tf, which is the primary means of interorgan transport of nonheme Fe. Elevated Tf concentration is associated with an increase in Fe absorption from the gut or mobilization of Fe from tissues stores. The abundance of plasma Tf is inversely related to Fe status in rats (Morton and Tavill, 1977; Zakin, 1992) and chicks (McKnight et al., 1980). To our knowledge, there are no published values for plasma Tf concentration in swine. The high mean plasma Tf concentration on d 0 may be indicative of the erythropoietic demand by the pig at this stage of development. Talbot and Swenson (1970) reported that during the first 6 wk of life, the erythrocyte volume per kilogram of BW was greatest in pigs at 3 wk of age given an Fe supplement. On d 35, the observed linear decrease in plasma Tf concentration suggests a greater need to transport nonheme Fe due to demand by Fe-dependent tissue in pigs fed lower dietary Fe (0 and 25 mg of added Fe/kg). Other researchers have shown that an increase in plasma Tf concentration in chicks (McKnight et al., 1980) and rats (Idzerda et al., 1986) is associated with a simultaneous increase in Tf gene transcription in the liver, the primary site of Tf synthesis.

The third physiological stage described by Underwood and Suttle (1999) is dysfunction. During this stage, Fe-dependent functions, including the incorporation of Fe into the heme proteins Hb, myoglobin, and cytochrome c oxidase, become rate limiting to particular metabolic pathways. The principal use of Fe in the body is incorporation into Hb, accounting for 60 to 80% of body Fe (Miller et al., 1981). Because Hb functions to carry oxygen from lungs to other tissues, it is commonly used as an indicator of the pig’s Fe status. Myoglobin accounts for 10% of body Fe and is predominantly located in muscle cells, where it functions to bind oxygen (Clydesdale and Francis, 1971). Although myoglobin was not determined in this study, Hagler et al. (1981) reported that rats fed an Fe-deficient diet had decreased muscle myoglobin concentration.

Disease is the final stage described in Fe deprivation (Underwood and Suttle, 1999). This stage is characterized by clinical symptoms such as a decrease in growth and feed intake, lethargy, and labored breathing or "thumps." These signs are preceded by and largely caused by the development of hypochromic microcytic anemia (Underwood and Suttle, 1999).

In this study, dietary Fe concentration did not decrease growth performance. Amine et al. (1972) suggested that BW gain is not a sensitive indicator of Fe deficiency because a decrease in growth is part of the final stage of Fe disorder. Studies reported by Hedges and Kornegay (1973), Dove and Haydon (1991), and Yu et al. (2000) also showed no effect on growth performance due to reduced dietary Fe concentrations. The life of the red blood cell in pigs is approximately 72 d (Withrow and Bell, 1969). Because this study lasted 35 d, the animals did not have complete red blood cell turnover. If the study had been extended another 35 d, pigs fed low-Fe diets might have developed hypochromic microcytic anemia because of inadequate erythrocyte generation, leading to a decrease in growth performance. Talbot and Swenson reported (1970) a decrease in BW and erythrocyte volume per kilogram of BW after 6 wk in pigs not receiving an Fe supplement.

Pharmacological concentrations of Zn are routinely supplemented to nursery pig diets because of the beneficial responses in growth performance (Smith et al., 1997; Hill et al., 2000). Phase 1 and 2 basal diets were formulated to contain pharmacological Zn concentrations at 1,755 and 1,947 mg Zn/kg, respectively. The basal diet for Phase 3 contained 108 mg of Zn/kg. The liver Zn concentration (87.8 mg/kg) of BL pigs in this experiment was similar to hepatic Zn concentration (74 and 105 mg/kg) of pigs weaned at 21 d reported by Hill et al. (1983), but higher than the hepatic Zn concentration (28 mg/kg) of pigs weaned at 24 d reported by Carlson et al. (1999). Carlson et al. (1999) also reported that liver Zn concentration (218 mg/kg or greater) was reflective of the duration (28 d) that pharmacological Zn (3,000 mg/kg) was fed. Following completion of Phase 2 in this experiment, pigs that had consumed diets containing pharmacological Zn concentrations may have had a liver Zn concentration comparable to that reported by Carlson et al. (1999). However, after feeding diets containing 108 mg of Zn/kg (adequate) during Phase 3 (14 d), pigs had a hepatic Zn concentration ranging from 42.8 to 56.1 mg/kg. Perhaps during Phase 3, homeostatic mechanisms decreased the hepatic Zn stores accumulated during pharmacological Zn supplementation in Phases 1 and 2.

The roles of Cu in Fe metabolism are many and varied. Ceruloplasmin, a key Cu-containing enzyme, is required for the binding of Fe to Tf via its ferroxidase activity (Osaki and Johnson, 1969). The accumulation of Cu during fetal development results in the neonate having high hepatic Cu stores (Underwood and Suttle, 1999); even so, liver Cu concentration decreases as a result of normal growth and development of the young pig. In this experiment, pigs fed basal diets containing the recommended NRC (1998) Cu concentration for 35 d had decreased liver Cu stores compared with BL pigs. Hepatic Cu concentrations for BL pigs and pigs fed dietary treatments for 35 d were within the normal range for hepatic Cu reported by Underwood and Suttle (1999) for pigs of that age.

Whole-body mineral concentration data are limited, especially for current genetics in the swine industry. Mineral concentrations (Fe, Zn, Cu, Mg, Ca, and P) of BL pigs in this experiment were greater than the mineral concentrations of pigs weaned at 21 d reported by Spray and Widdowson (1950). Also, pigs fed for 35 d in our study had greater Ca, P, and Mg concentrations compared with mineral concentrations reported by Rymarz et al. (1982) and Hendriks and Moughan (1993) for 28.3- and 25.7-kg pigs, respectively. Calcium and P are essential for proper development and maintenance of the skeletal system. Pigs fed for 35 d in the current study had a Ca:P ratio in the body of 1.67, which is slightly higher than the ratios of 1.55, 1.60, and 1.52 reported by Spray and Widdowson (1950), Nielsen (1972), and Hendriks and Moughan (1993), respectively. Spray and Widdowson (1950) also compared nursing pigs receiving a daily dose of supplemental Fe (11 mg/kg BW) during the first 3-wk of life with pigs receiving no supplemental Fe and noted that supplemental Fe greatly increased the amount of Fe in the body. These results are in agreement with the increases in whole-body Fe concentration due to increases in dietary Fe concentration reported in the current study.

Results obtained for whole-body chemical composition are comparable to those reported by de Lange et al. (2001) for both BL pigs and pigs at the conclusion of the 35-d feeding study. Dietary change from milk to nursery diets is characterized by a change from a liquid diet that is high in fat, to a complex, nutrient-dense pelleted feed. During the initial transition period, the pig is challenged to consume enough feed to meet its energy requirement. Consequently, pigs will mobilize their body lipid reserves to meet the high energy demands during the initial growth phase of the nursery, explaining the observed decrease in percentage of lipid and increase in percentage of water from 19 to 54 d of age. Whole-body protein (15.8 to 16.1%) of pigs analyzed in this study was greater than the protein content (13.1%) reported by Rymarz et al. (1982) but comparable to the protein content (15.6%) reported by Hendriks and Moughan (1993). Early work by Spray and Widdowson (1950) reported that pigs weaned at 21 d had 13.9% or less protein, in contrast to 15.8% protein for BL pigs (19 d of age) reported in the current study. The difference in the percentage of protein between early experiments and this study reflects changes in current genetic selection with an emphasis on increased muscling and growth in the swine industry.

In conclusion, most of the postweaning Fe requirement is thought to be met by the Fe provided by common feed ingredients. Examples of feed ingredients that have a high Fe concentration include dicalcium phosphate, limestone, and blood meal (NRC, 1998). However, the bioavailability of Fe from different sources varies greatly (Kornegay, 1972; Deming and Czarnecki-Maulden, 1989) and is influenced by such factors as Fe status of the animal, dietary Fe concentration, and various nutritional and nonnutritional elements within the diet. Consequently, the feed ingredients used in this experiment provided minimal Fe to the basal diets. Even though Fe contributed by feed ingredients provided basal dietary Fe concentrations in excess of the NRC (1998) postweaning requirement (80 mg/kg), the dietary Fe was not adequate to sustain Fe stores in pigs fed lower supplemental Fe concentrations. The decrease in liver Fe stores resulted in a decrease in commonly measurable indicators of Fe status (i.e., Hb, Hct, and PFe). Moreover, the increase in plasma Tf concentration due to dietary treatments indicated mobilization of Fe from storage tissue to meet the erythropoietic demands of growing pigs. The increase in whole-body percentage of protein and mineral composition in growing pigs in this experiment compared with pigs in earlier research suggests that erythropoietic demands of growing pigs in this experiment would be greater than those of pigs used in early experiments to derive the postweaning dietary Fe requirement (Pickett et al., 1960). Consequently, the supplementation of 100 mg of Fe/kg of diet, via the highly available ferrous sulfate, was required in addition to the Fe provided by dietary Fe ingredients to alleviate severe decreases in Fe stores.

	Implications

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Iron contributed by feed ingredients commonly included in nursery pig diets provided analyzed dietary iron concentrations in excess of the postweaning requirement (80 mg/kg), but the iron in these feedstuffs was not sufficient to maintain liver stores of growing pigs. This decrease in liver iron stores decreased indices of iron status, which could result in depressed growth. The addition of at least 100 mg of iron/kg of diet via the highly available iron source like ferrous sulfate is necessary to prevent an excessive decline in iron stores. This highly adequate and available iron source is especially required by pigs that have been selected for increased growth and muscling, which is typical of pigs currently used in the swine industry.

	Footnotes

 
¹ Partially funded by Six-State Consortium on Animal Waste Management of the Environmental Protection Agency.

³ Current address: 107 J. B. Francioni Hall, Louisiana State Univ., Baton Rouge 70803.

² Correspondence: 2209 Anthony Hall (phone: 517-355-9676; fax: 517-432-0190; e-mail: hillgre{at}msu.edu).

Received for publication April 20, 2004. Accepted for publication July 13, 2004.

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