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Effects of dietary arginine supplementation during gestation and lactation on the performance of lactating primiparous sows and nursing piglets¹

R. D. Mateo^*, G. Wu^*,, H. K. Moon, J. A. Carroll^*, and S. W. Kim^,,#,2

^* Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409; and Department of Animal Science, Texas A&M University, College Station 77843; and National Institute of Animal Science-RDA, Cheonan, Korea 330-801; and USDA-ARS-Livestock Issues Research Unit, Lubbock, TX 79403; and and ^# Department of Animal Science, North Carolina State University, Raleigh 27695

	Abstract

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A 2 x 2 factorial arrangement of treatments in a randomized block design was used to determine the effects of dietary Arg supplementation during gestation and lactation on the lactation performance of 38 first-parity sows. At 30 d of gestation, pregnant gilts were allotted based on BW to 1 of 2 diets supplemented with 1% L-Arg·HCl or 1.7% L -Ala (isonitrogenous control). After farrowing, sows were further allotted based on BW within previous gestation treatment groups to 1 of 2 lactation diets supplemented with 1% L-Arg·HCl or 1.7% L-Ala (isonitrogenous control). All gestation diets contained 3.1 Mcal/kg and 12.2% CP (as is) and were fed 2 kg/d in 2 equally sized meals, whereas all lactation diets contained 3.2 Mcal/kg and 18.6% CP (as is) and were fed ad libitum. Litter size was standardized to 10 piglets by cross-fostering within 24 h postfarrowing. On a weekly basis, BW and backfat (BF) thickness of sows, as well as piglet BW were measured, and blood and milk samples were obtained from the sows. Number of days from weaning to estrus and ADFI were also recorded. There were no differences in BW, BF thickness, ADFI, or days until return to estrus among treatment groups. There was no effect of the gestation diet or a gestation x lactation diet interaction on any parameter measured. On d 7 of lactation, plasma concentrations of Arg and insulin in sows, as well as concentrations of most AA in milk, were greater (P < 0.05) in response to Arg supplementation during lactation compared with the control. Weight gain of piglets from sows fed the Arg-supplemented diet during lactation was greater between d 0 and 7 (P < 0.01) and between d 0 and 21 (P < 0.05) of lactation compared with piglets from sows fed the control diet. Collectively, results from this study indicate the potential beneficial effects of dietary Arg supplementation in improving the lactation performance of first-parity sows.

Key Words: L-arginine • lactation performance • litter weight gain • sow

	INTRODUCTION

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Young animals have a high requirement for Arg (Southern and Baker, 1983; Fickler et al., 1994) because of the utilization of Arg by multiple metabolic pathways (Wu and Morris, 1998; Li et al., 2007). However, Arg intake from sow’s milk is low relative to the need for protein deposition in piglets (Davis et al., 1994; Wu and Knabe, 1994; Kim et al., 2007). Estimates based on the supply of Arg from sow’s milk and the Arg requirement of piglets revealed that sow’s milk provides less than 40% of the daily requirement in 7-d-old suckling pigs (Wu et al., 2004). Both metabolic and growth data indicate that an Arg deficiency is a major factor limiting maximal weight gain of milk-fed piglets (Kim and Wu, 2004; Wu et al., 2004; Frank et al., 2007).

Increasing milk Arg intake by suckling piglets could be an effective means of enhancing their growth. In addition to the feed intake of sows and the suckling intensity of piglets, milk production is also influenced by the angiogenesis of mammary tissue and blood flow to mammary glands, which enhance nutrient delivery to the mammary gland for milk synthesis (Trottier et al., 1997). Mammary blood flow and angiogenesis are regulated by Arg-derived nitric oxide (Meininger and Wu, 2002; Lacasse and Prosser, 2003). Furthermore, milk production is highly correlated with mammary gland growth (Ceriani, 1974; Kim et al., 2000) and Arg is required for optimal mammary gland growth (Pau and Milner, 1982). At a high dosage, Arg stimulates the secretion of prolactin and growth hormone, which are necessary for mammary development (Knopf et al., 1968; Davis, 1972).

We hypothesized that supplementing Arg to the diets of first-parity sows during gestation and lactation might stimulate the weight gain of sow-reared piglets, possibly by increasing nutrient utilization, and therefore increasing milk production and altering nutrient composition in milk.

	MATERIALS AND METHODS

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Animals, Experimental Diets, and Design
The animal care and use protocol was approved by the Animal Care and Use Committee of Texas Tech University. A 2 x 2 factorial study was conducted to determine the effects of L-Arg supplementation in gestation in combination with lactation diets on the lactation performance of 38 first-parity sows (Camborough 22, Pig Improvement Co., Franklyn, KY).

At d 30 of gestation, pregnant gilts with an average BW of 166.3 ± 1.8 kg and BF thickness of 13.3 ± 0.2 mm were housed in individual gestation crates (2.1 x 0.6 m), and gilts with similar BW were paired and then randomly allotted to 2 dietary treatments, which consisted of corn- and soybean meal-based diets supplemented with either 1% L-Arg·HCl or 1.7% L-Ala (isonitrogenous control, Ajinomoto, Tokyo, Japan; Table 1). At 110 d of gestation, pregnant gilts were transferred to individual farrowing crates (1.5 x 2.2 m). Within 24 h postfarrowing, sows in each treatment group were assigned randomly to corn- and soybean meal-based lactation diets supplemented with either 1% L-Arg·HCl or 1.7% L-Ala (isonitrogenous control; Table 1). Litter size was standardized to 10 piglets, depending on their availability, by cross-fostering within 24 h postfar-rowing. The number of piglets per sow ranged from 9 to 13 piglets but was equalized within BW groups (blocks). Alanine was chosen for the isonitrogenous control because Ala is not toxic and is not a substrate for Arg synthesis, but is extensively catabolized by pigs (Kim and Wu, 2004; Kohli et al., 2004). Furthermore, previous studies have shown no differences in reproductive performance between first-parity sows provided either conventional diets or diets supplemented with 1% Ala (Ji, 2004; Mateo et al., 2007). The supplemental level of 1% L-Arg·HCl was chosen because it was shown in our previous study to increase plasma concentration of Arg in pregnant pigs by 65% at 2 h after feeding. This indicated that Arg provided through dietary supplementation can be delivered to the body successfully for further metabolism (Wu et al., 2007).

Blood samples were collected from the sows at 1000 h, 2 h after feeding, via jugular venipuncture by using heparinized tubes (Becton-Dickinson Vacutainer Systems, Rutherford, NJ) on d 7 and 21 of lactation. Blood samples were centrifuged at 2,000 x g for 15 min. Plasma was separated by transfer pipettes into 1.5-mL microcentrifuge tubes (National Scientific, San Rafael, CA) and stored at –20°C until further AA, insulin, and urea analyses. All litters were weaned and the sows were returned to individual gestation crates at d 21 of lactation. The days until return to estrus were also recorded.

Chemical Analyses
Plasma samples (1 mL) were deproteinized with an equal volume of 1.5 M HClO₄ and neutralized with 0.5 mL of 2 M K₂CO₃. The extracts were analyzed for urea concentrations by using a colorimetric method that involved a reaction with phenol and hypochlorite (Wu and Knabe, 1994). For analysis of all milk AA except for Trp, 0.2 mL of whole milk was hydrolyzed in 6 mL of 6 N HCl at 110°C for 24 h under N₂ (Wu and Knabe, 1994). For analysis of Trp, milk samples (0.2 mL) were hydrolyzed in 6 mL of 4.2 M NaOH plus 0.1 mL of thiodiglycol (25% aqueous solution, an antioxidant) as described by Wu et al. (1999). Amino acids in plasma and milk hydrolysates were analyzed by HPLC methods involving precolumn derivatization with o-phthal-dialdehyde (Wu et al., 1997). Amino acid standards and other chemicals were obtained from Sigma Chemical Company (St. Louis, MO). An enzyme immunoassay was used for the quantification of plasma insulin concentrations according to the manufacturer’s instructions (Porcine insulin ELISA kit, Mercodia Inc., Winston Salem, NC). The detection limit was 0.01 µg/L, and the coefficient of variation was 3.1% within assay and 1.5% between assay.

Statistical Analysis
Data were analyzed by using the MIXED procedure (SAS Inst. Inc., Cary, NC) for a factorial arrangement with a randomized complete block design. Sow was considered as the experimental unit. Separation of means was done by using the PDIFF option of SAS. Probability values less than 0.05 were considered statistically significant, and values between 0.05 and 0.07 were considered as trends.

	RESULTS

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Piglet Performance
Litter size both after cross-fostering and at d 7, 14, and 21 did not differ among treatment groups. Litter sizes at d 0, 7, 14, and 21 of lactation were 10.9 ± 0.20, 10.6 ± 0.20, and 10.3 ± 0.19 (pooled means ± SEM), respectively. Both gestation and lactation diets did not affect BW, BF thickness, ADFI, or days until return to estrus (Table 2). Main effects of gestation and gestation x lactation interactions were not significant for all piglet performance data during lactation. Piglet BW at the initiation of cross-fostering did not differ among treatment groups. However, BW of piglets from sows fed the Arg-supplemented diets during lactation were greater (P < 0.05) at d 7 (2.62 ± 0.11 vs. 2.44 ± 0.11 kg), 14 (4.18 ± 0.20 vs. 3.86 ± 0.21 kg), and 21 (5.76 ± 0.22 vs. 5.36 ± 0.23 kg) of lactation compared with piglets from control-fed sows. Weight gains of piglets from sows fed the Arg-supplemented diet during lactation were greater between d 0 and 7 (1.26 ± 0.09 vs. 1.00 ± 0.09 kg; P < 0.01) and between d 0 and 21 (4.34 ± 0.21 vs. 3.92 ± 0.22 kg; P < 0.05) compared with piglets from the control-fed sows. However, there was no difference in piglet BW gain during either d 7 to 14 or d 14 to 21 of lactation.

	DISCUSSION

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Litter weight gain is known to be correlated with milk production or nutrient concentrations in milk (Noblet and Etienne, 1987; King et al., 1993). Increased piglet or litter weight gain in Arg-supplemented sows may be indicative of increased milk production or increased nutrient concentrations in milk. Results of the current study indicate that voluntary feed intake and BW changes of sows were not affected by dietary Arg supplementation (Table 2), suggesting that increased concentrations of total AA in milk were not due to alterations in dietary protein intake or whole-body protein mobilization. On the basis of reduced levels of urea in plasma, Arg supplementation appears to enhance the efficiency of dietary protein utilization for milk protein synthesis. By increasing the synthesis of nitric oxide (a major vasodilator) in the endothelial cells of blood vessels (Moncada et al., 1989; Wu and Meininger, 2000), dietary Arg supplementation can enhance blood flow and nutrient supply to the mammary gland for milk protein, resulting in improved BW gain of suckling piglets. The increased concentrations of total AA in milk were associated with increased BW gain of piglets during the first week of lactation, which affected the overall improvement of piglet growth performance during the entire lactation period.

On average, piglets from Arg-supplemented sows gained 20 g more BW per day, or 420 g more during the 21-d lactation period, compared with the piglets from sows in the control groups (Table 2). Considering that body composition in neonatal pigs is approximately 25% DM and 12.5% protein (McPherson et al., 2004), 420 g of BW gain is translated into 50 g of protein gain in 3 wk for a piglet. As shown in Table 5, the increase in concentrations of total AA in the milk of Arg-supplemented sows is approximately 3.4 g/L. Considering that a piglet with 200 g of daily BW gain obtains 0.78 L of milk per day (Wu et al. 2004), the Arg treatment would provide 2.65 g of additional protein to the piglet per day, or 56 g during the 21-d lactation period. Because the digestibility of milk protein is high (95 to 100%) in neonatal pigs (Lin et al., 2006), the intake of an additional 56 g of protein from milk is sufficient to support the gain of an additional 50 g of protein in each piglet during a 21-d lactation period.

Furthermore, mammary blood flow and substrate concentrations in blood are major factors that determine substrate availability for milk synthesis (Davis and Collier, 1985) and therefore nutrient delivery to the neonate. Arginine is the physiological precursor for the synthesis of nitric oxide, the endothelium-derived relaxing factor (Wu and Meininger, 2000) and a key angiogenic factor (Meininger and Wu, 2002). Increasing nitric oxide availability has been reported to increase mammary blood flow rapidly in ruminants (Lacasse et al., 1996; Lacasse and Prosser, 2003). Interestingly, a short-term increase in nitric oxide provision within several hours may not lead to increased milk production (Prosser et al., 1996; Lacasse and Prosser, 2003), possibly because of a lack of increase in the number of secreting cells and the synthesis of proteins, fat, and lactose.

As noted above, rapidly growing piglets have a high requirement for Arg. However, previous studies have clearly demonstrated that limited Arg availability from sow’s milk (Wu and Knabe, 1994; Wu et al., 2004) and the limited capability for endogenous Arg synthesis (Wu and Knabe, 1995) are both major obstacles in realizing the maximum growth potential of sow-reared piglets (Kim and Wu, 2004; Wu et al., 2004; Kim et al., 2007). The marked decrease in the availability of Arg coincides with the period when submaximal growth in piglets occurs (Boyd et al., 1995; Flynn et al., 2000; Kim and Wu, 2004). In support of this view, Kim and Wu (2004) demonstrated that dietary Arg supplementation dose-dependently enhanced the growth performance of artificially reared piglets.

Trottier et al. (1997) also reported that Arg uptake by the mammary gland is much greater than milk Arg output, which reflects the high capacity of the porcine mammary gland to catabolize Arg (O’Quinn et al., 2002). Thus, Arg supplementation did not result in a substantially greater Arg concentration in sow’s milk. However, an increase in the volume of milk consumed by piglets (Kirchgessner et al., 1991) would translate into an increase in the provision of Arg and other nutrients to the neonates to support their growth. This was clearly observed for suckling piglets on d 0 to 7 (Table 2). However, there was a lack of a significant increase in piglet BW gain during wk 2 and 3 in response to Arg supplementation (Table 2). The underlying reasons are not known at present, but may be related to unaltered concentrations of total AA in milk after the first week of lactation (Table 6).

As expected, Ala-supplemented sows had greater concentrations of Ala in plasma compared with Arg-supplemented sows. However, an interesting observation from the current study is that dietary Arg supplementation to lactating sows decreased plasma concentrations of Ser, Glu, His, and Thr at d 7 of lactation. It is possible that there was an increase in the utilization of these AA by the mammary gland for the synthesis of proteins, peptides, and other milk components. In support of this suggestion, we showed that concentrations of total AA (primarily protein) in milk increased at d 7 of lactation in Arg-supplemented sows. We surmise that Arg supplementation to gestating sows may have stimulated mammary growth (including vascular growth), thereby promoting blood flow and AA uptake by the mammary gland to increase milk protein synthesis during the lactation period. This suggestion is consistent with the finding that the majority of growth in these tissues occurs during the later parts of gestation (Kim et al., 1999). Tucker (1966) and Kim et al. (2000) showed that total DNA content is an indicator of the number of mammary cells and is highly correlated with litter weight gain in pigs and rodents. Measuring DNA content in mammary tissue would provide a useful indicator in both control and Arg-supplemented sows; however, mammary DNA was not measured in this study.

The size of suckling piglets is positively correlated with the mass of mammary gland suckled (Nielsen and Sorensen, 1998; Kim et al., 2000). Consistent with this observation, we observed that piglets suckling from Arg-supplemented sows were heavier throughout lactation, with an increase in BW gain. Furthermore, the secretagogue effects of Arg on anabolic hormones, such as insulin (Floyd et al., 1966; Kim and Wu, 2004; Laspiur et al., 2006), may also play a role in the increased uptake of AA by the mammary gland (Laarveld et al., 1981). Previous reports from studies with other species have shown that the mammary gland becomes highly sensitive to insulin during lactation (Burnol et al., 1990). Thus, an increase in concentrations of plasma insulin and its sensitivity in Arg-supplemented lactating sows may stimulate the utilization of AA by the mammary gland to produce proteins. In dairy cows subjected to an insulin clamp, there was an increase in both mammary blood flow and the efficiency of extraction of blood AA by the mammary gland (Mackle et al., 2000). The increase in insulin secretion during lactation may also explain, in part, the decreased plasma concentrations of several AA measured (Fukagawa et al., 1986). These results suggest that supplementing Arg to the diets of lactating sows may increase the uptake of substrates (e.g., AA) by the porcine mammary gland for milk protein synthesis. Although this effect was more apparent during the initial period of lactation, the increased piglet performance during the first week of life translated to an overall improvement in piglet performance.

Urea is the major end product of AA oxidation in mammals (Meijer et al., 1990). Previous studies suggested that plasma urea concentrations in lactating sows may be an indicator of efficiency of whole-body nitrogen utilization (Coma et al., 1995). There were no differences in feed intake among all groups of lactating sows (Table 2). Thus, a reduction in plasma urea levels in Arg-supplemented sows may reflect an increase in the use of dietary AA for tissue or milk protein synthesis, as previously reported for Arg-supplemented gestating gilts (Mateo et al., 2007), lactating sows (Laspiur and Trottier, 2001), and neonatal pigs (Kim and Wu, 2004). Interestingly, no differences in plasma urea concentration among sows were observed at d 21 of lactation, which agrees with the observation that piglet BW gain in wk 2 and 3 was not affected by Arg supplementation of the sow’s diet (Table 2).

In summary, supplementing dietary Arg to lactating sows enhanced the growth performance of suckling piglets. The increased litter weight gain was associated with increased concentrations of total AA in milk at d 7 of lactation. We propose that the Arg treatment may increase mammary blood flow and extraction of AA during lactation. However, further studies are necessary to test this new hypothesis.

	Footnotes

 
¹ Supported by Rural Development Administration (Suwon, Korea), Texas Agricultural Experiment Station (H-8200), and Ajinomoto Co. Inc. (Tokyo, Japan).

² Corresponding author: sungwoo_kim{at}ncsu.edu

Received for publication June 20, 2007. Accepted for publication December 18, 2007.

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