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Effect of dietary energy concentration and apparent ileal digestible lysine:metabolizable energy ratio on nitrogen balance and growth performance of young pigs¹

W. Urynek^*,2 and L. Buraczewska

^* Degussa AG, 01-231 Warszawa, Poland and and The Kielanowski Institute of Animal Physiology and Nutrition, 05-110 Jabonna, Poland

² Correspondence:
Ulitsa Pocka 15, 01-231, Warszawa (phone: +48-22-862-1711; fax: +48-22-862-1710; E-mail:
waldemar.urynek{at}degussa.com).

	Abstract

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Two experiments were conducted to determine the optimal apparent ileal digestible lysine:ME (Lys:ME) ratio and the effects of lysine and ME levels on N balance (Exp. 1) and growth performance (Exp. 2) in growing pigs. Diets were designed to contain Lys:ME ratios of 0.6, 0.7, 0.8, and 0.9 g/MJ at 13.5 and 14.5 MJ of ME/kg of diet in a 4 x 2 factorial arrangement. In Exp. 1, conventional N balances were determined on 48 crossbred barrows (synthetic line 990, initial BW = 13.1 ± 0.7 kg) at approximately 15, 20, and 25 kg of BW with six pigs per diet. At 15 kg of BW, an energy density x Lys:ME ratio interaction on daily N retention was observed (P < 0.05). At each BW, N retention improved with an increase in N intake associated with increasing ME concentration. In 15-kg BW pigs, increasing the Lys:ME ratio increased daily N retention at the 13.5 (linear, P < 0.001) and 14.5 MJ of ME level (linear, P < 0.01; quadratic, P < 0.05). In 20-kg BW pigs, N retention (g/d) increased (linear, P < 0.001; quadratic, P < 0.01) and N retention (percentage) increased (linear, P < 0.001) as the Lys:ME ratio increased. At 25 kg of BW, N retention (g/d) increased quadratically (P < 0.05) with an increase in Lys:ME ratio. The Lys:ME ratios that maximized daily N retention at 15 kg of BW were 0.88 and 0.85 g/MJ at the 13.5 and 14.5 MJ of ME levels, respectively and 0.81 and 0.77 g/MJ (for both ME levels) at 20 and 25 kg of BW, respectively. Over the 28-d trial, an energy density x Lys:ME ratio interaction on ADG was observed (P < 0.05). Increasing energy density increased growth performance, whereas increasing the Lys:ME ratio in high-energy diets increased ADG (linear, P < 0.05; quadratic, P < 0.01) and gain:feed ratio (G/F) quadratically (P < 0.01). Average daily gain and G/F ratio were greatest in pigs fed the 14.5 MJ of ME diet and the Lys:ME ratio of 0.82 g/MJ. In Exp. 2, 128 individually housed crossbred barrows and gilts (initial BW = 12.8 ± 1.6 kg) were used to determine the effect of diets used in Exp. 1 on growth performance in a 4 x 2 x 2 factorial arrangement. The ME level increased ADG and G/F from d 0 to 14 and from d 0 to 28. Increasing the Lys:ME ratio increased ADG from d 0 to 14, whereas growth performance was maximized in pigs fed Lys:ME ratio of 0.82 g/MJ. These results suggest that pigs from 13 to 20 and from 20 to 30 kg of BW fed diets containing 14.5 MJ of ME/kg had maximum N retention and ADG at 0.85 and 0.77 g of apparent ileal digestible lysine/MJ of ME, respectively.

Key Words: Digestibility • Lysine • Metabolizable Energy • Nitrogen Retention • Performance • Pigs

	Introduction

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Recent improvements in the genetics of modern pigs have resulted in a greater rate of protein deposition and may require modifying the dietary protein (AA):energy ratio. Since protein deposition of growing pigs seems to be limited by feed or energy intake (Bikker et al., 1994), diet formulations should be based on a lysine:energy ratio. However, there is a dearth of information on the optimal apparent ileal digestible lysine:metabolizable energy (Lys:ME) ratio for young pigs (from 10 to 19 kg of BW). Ratios ranging from 0.69 to 0.82 g/MJ have been reported for pigs from 10 to 20 kg of BW (NRC, 1998; Rademacher, 1999). There are several factors that could be responsible for this wide variation in Lys:ME ratios, but in young rapidly growing pigs, a major variable may be the interactive effects between energy density and lysine intake (Campbell and Dunkin, 1983; Zhang et al., 1986).

Most studies have been conducted on a wide range of ratios of total lysine:energy (Nam and Aherne, 1994; Van Lunen and Cole, 1998; Roth et al. 1999; Smith et al., 1999) and pigs of higher BW range (Bikker et al., 1994; Roth et al., 2000). Furthermore, lysine requirements have been determined at only one dietary energy level (Martinez and Knabe, 1990).

The aim of the present study was to investigate the effect of four Lys:ME ratios on N balance and growth performance of pigs from 13 to 30 kg of BW at two dietary energy densities. The objective of this study was also to determine the optimal Lys:ME ratios, at a relatively constant CP level, that would maximize N retention in pigs of 15, 20, and 25 kg of BW.

	Materials and Methods

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General
Pigs with a high lean gain potential (390 g/d from 25 to 100 kg of BW, synthetic line 990, Pawowice, Poland) were fed diets based on four protein sources (soybean meal, full fat soybean, soybean protein concentrate, and fishmeal). Dietary treatments were given at two energy levels (13.5 or 14.5 MJ of ME/kg) and four Lys:ME ratios (0.6, 0.7, 0.8, or 0.9 g/MJ) and are presented in Table 1. Soybean oil (0.30 or 4.04%) was added to experimental diets to achieve a ME density of 13.5 or 14.5 MJ/kg. Based on the results of Lewis et al. (1980), who found no interaction between energy intake and lysine requirements in pigs fed diets containing 0 and 5% fat, and the results of Zhang et al. (1986), who found that dietary energy was equally well utilized in young pigs regardless of fat percentage (up to 16.3%), it was assumed that the results of this trial were not affected by this energy source. The ME was calculated based on the chemical composition of the dietary ingredients (Table 2) with equations and digestibility coefficients as given in DLG (1991). Lysine, methionine + cysteine, threonine, and tryptophan were provided at ideal protein profile (Rademacher et al., 1999). Each basal diet of low- and high-energy density containing 0.6 g of apparent ileal digestible lysine/MJ of ME was investigated in a preliminary trial with cannulated pigs (Urynek and Buraczewska, 2001) for apparent digestibility of protein and AA. The determined digestibility coefficients of AA (lower than tabular ones) were used to formulate the experimental diets. The digestibility of supplemented lysine in Biolys 60 (Degussa AG, Hanau, Germany), and of methionine, threonine, and tryptophan was assumed to be 100% (Haydon et al., 1989). Additional calculated Lys:ME ratios of 0.7, 0.8, and 0.9 g/MJ were attained by the substitution of crystalline AA for cornstarch. Supplemented L-lysine provided from 4 to 34% of the total lysine content in the diets. Amino acid supplementation resulted in an increase of dietary CP from 20.8 to 21.6% and from 21.2 to 22.4% in low- and high-energy diets, respectively. The small variation in protein level presumably compensated for the confounding effect of protein concentration on voluntary feed intake, as reported by Kyriazakis (1989). The diets described above were used both in Exp. 1 and 2.

Experiment 1
Animals, Housing, and Feeding.
Forty-eight barrows (initially 13.1 ± 0.7 kg of BW) were used in a 4 x 2 factorial arrangement (four Lys:ME ratios of 0.6, 0.7, 0.8, or 0.9 g/MJ and two ME levels of 13.5 or 14.5 MJ/kg). The eight dietary treatments were replicated six times. The pigs were housed individually in metabolism cages (0.7 x 1.5 m) with a slatted floor in an environmentally controlled room (23°C) and were given ad libitum access to water through a nipple waterer. Barrows were fed twice daily (0800 and 1600) and allowed a 6-d adjustment to the experimental diets. The diet adjustment period was followed by three consecutive N balance periods at approximately 15, 20, and 25 kg of BW. The daily dietary allowance was provided at a rate of 5% (wt/wt) of the individual BW but was kept constant during the N balance periods. Each 9-d N balance period (10-d period at approximately 20 kg of BW) included a 5-d collection of feces and urine to determine N output.

Sampling and Measurements.
Feces were collected in bags (Combihesive II S, 45 mm; ConvaTec Limited, Uxbridge, U.K.) attached to the back part of the body (Van Kleef et al., 1994). The bags were emptied three to four times daily and feces was stored at -20°C. At the end of each collection period all feces was thawed, mixed thoroughly, and subsampled for subsequent analyses. Urine was collected continuously over the 5-d collection period in plastic containers to which H₂SO₄ was added daily. Urine volume was measured and recorded daily and a 10% aliquot was stored (4°C) until it was analyzed. During the 28-d N balance experiment, ADG and gain:feed ratio (G/F) were measured.

Chemical Analysis and Calculations.
Dry matter, N, ether extract, crude fiber, total starch, and ash were analyzed using standard methods (AOAC, 1990). The content of NDF and ADF was determined with Fibertec System M (FOSS Tecator AB, Höganäs, Sweden) by methods described by Van Soest and Wine (1967) and Van Soest (1973). The AA contents of the diets were determined by Degussa AG, (Hanau, Germany) after 24-h hydrolysis with 6 N HCl in closed glass vessels according to Commission Directive 98/64/EC (1998). Methionine and cysteine were determined after oxidation with performic acid (Llames and Fontaine, 1994). Supplemented AA were determined after extraction with 0.1 N HCl. Cation-exchange chromatography and post-column derivatization with ninhydrin was used to analyze the hydrolyzates and extracts for all AA except tryptophan. The tryptophan concentrations of the diets were determined by reversed-phase HPLC following alkaline hydrolysis with barium hydroxide according to Commission Directive 2000/45/EC (2000).

On the basis of determined dietary lysine concentrations (protein bound and free form) and its digestibility coefficients (previously determined), actual ratios of Lys:ME were calculated (Table 1). Nitrogen retention (g) was calculated as: N intake (NI) - (fecal N output [NF] + urinary N output [NU]). Nitrogen retention (%) was calculated as: {[NI - (NF + NU)]/NI} x 100, and N digestibility was calculated as: [(NI - NF)/NI] x 100.

Experiment 2
A total of 128 pigs (64 barrows and 64 gilts) with an initial average BW of 12.8 ± 1.6 kg were used to determine the effect of the four Lys:ME ratios at two ME levels used in Exp. 1 (Table 1) and the effect of sex on growth performance in a 4 x 2 x 2 factorial arrangement of treatments. Pigs were housed in individual pens (1.0 x 2.0 m) with half-slatted floors in an insulated building, and the room temperature was maintained at about 23°C. Pigs were randomly allotted within sex to dietary treatments and were given ad libitum access to pelleted feeds after a 7-d adjustment period. During the 28-d growth period, pig weights and feed intakes were recorded weekly to determine ADG, ADFI, and G/F.

Statistical Analysis
Experiment 1.
Data were subjected to statistical analysis using a completely randomized design according to the GLM procedure of Statistica 5.5 (StatSoft, Inc., Tulsa, OK). Individual pig was considered the experimental unit. Two-way ANOVA was used to test the main and interactive effects of Lys:ME ratio and dietary ME level on daily N retention. The results of N balance in 15-kg BW pigs were analyzed with one-way ANOVA. Orthogonal polynomial contrasts were used to examine responses to increasing Lys:ME ratio. A nonlinear regression analysis with means of treatments was conducted to determine the optimal Lys:ME ratio for daily N retention. In 15- and 20-kg BW pigs, exponential response curves were fitted to the experimental data points using the following equation:

where y = daily N retention (g), a = intercept (performance on basal diets with Lys:ME ratios of 0.60 and 0.62 g/MJ), b = maximal daily N retention (g) due to increased Lys:ME ratio, c = curvature steepness, d = Lys:MJ ratios (g/MJ) of the basal diets, and x = Lys:ME ratio (g/MJ) of the treatment groups. The values for the optimal Lys:ME ratio were calculated at 95% of the asymptotic response.

At 25 kg of BW, a quadratic response curve was fitted to the experimental data points using the following equation:

where y = daily N retention (g), x = Lys:ME ratio (g/MJ), and a, b, and c represent components of the quadratic equation.

The exponential and quadratic models were fitted to the experimental data using the nonlinear procedure of StatSoft.

Experiment 2.
Twenty pigs were excluded from the analysis as outlined due to extremly low ADG or due to ill health. Data were subjected to statistical analysis using a completely randomized design according to the GLM procedure of StatSoft. Individual pig was considered the experimental unit. Three-way ANOVA was used to test the main and interactive effects of Lys:ME ratio, dietary ME level and sex. Growth performance data were analyzed with initial BW as a covariate. Orthogonal polynomial contrasts were used to evaluate increasing Lys:ME ratio.

	Results

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Experiment 1.
At 15 kg of BW, an energy density x Lys:ME ratio interaction was observed for daily N intake, N retention, and apparent ileal digestible lysine intake (Table 3). Nitrogen digestibility was not affected by ME level or dietary treatments. Increasing energy density and Lys:ME ratio increased daily N intake, daily N retention (g), and apparent digestible lysine intake. Furthermore, increasing Lys:ME ratio also increased daily N retention expressed as a percentage of daily N intake. In pigs fed low-energy density diets, increasing the Lys:ME ratio from 0.62 to 0.91 g/MJ increased (linear, P < 0.001) N retention (g/d) from 11.3 to 13.9 g and N retention (percentage) from 47.1 to 55.9%. In pigs fed high-energy diets, increasing Lys:ME ratio increased (linear, P < 0.01; quadratic, P < 0.05) N retention (g/d and percentage) up to a maximum (14.3 g/d and 56%, respectively), which occurred at 0.82 g of apparent digestible lysine/MJ of ME and no further response was observed.

The Lys:ME ratios required to maximize daily N retention (g) were calculated using exponential and quadratic models. From the exponential response curves, it is clear that in 15-kg BW pigs, different responses existed for each of the dietary ME densities (Figures 1 and 2). Maximal N retention was reached at the Lys:ME ratio of 0.88 and 0.85 g/MJ, estimated at the low- and high-energy levels, respectively. At both 20 and 25 kg of BW, the combined (low- and high-ME diets) exponential and quadratic responses of daily N retention to the Lys:ME ratio were observed (Figures 3 and 4, respectively). Nitrogen retention was maximized at 0.81 and 0.77 g of apparent ileal digestible lysine/MJ of ME for pigs of 20 and 25 kg of BW, respectively.

View larger version (17K): [in this window] [in a new window]   Figure 1. Exponential response curve of daily N retention to apparent ileal digestible lysine:metabolizable energy (Lys:ME) ratio in pigs at a BW of 15 kg for diets of 13.5 MJ of ME/kg. Each point represents mean from six pigs.

View larger version (17K): [in this window] [in a new window]   Figure 2. Exponential response curve of daily N retention to apparent ileal digestible lysine:metabolizable energy (Lys:ME) ratio in pigs at a BW of 15 kg for diets of 14.5 MJ of ME/kg. Each point represents mean from six pigs.

View larger version (17K): [in this window] [in a new window]   Figure 3. Exponential response curve of daily N retention to apparent ileal digestible lysine:metabolizable energy (Lys:ME) ratio in pigs at a BW of 20 kg for diets of 13.5 and 14.5 MJ of ME/kg. Each point represents mean from 12 pigs.

View larger version (16K): [in this window] [in a new window]   Figure 4. Quadratic response curve of daily N retention to apparent ileal digestible lysine:metabolizable energy (Lys:ME) ratio in pigs at a BW of 25 kg for diets of 13.5 and 14.5 MJ of ME/kg. Each point represents mean from 12 pigs.

	Discussion

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The results of this study indicate that although daily N retention, ADG, and G/F were affected by ME concentration, increasing the Lys:ME ratio may further increase daily N retention and growth criteria if sufficient ME is available. However, Lawrence et al. (1994) failed to detect a response to increasing the lysine:DE ratio because energy intake may have been limiting N retention. In Exp. 1, in 15-kg BW pigs, the response to the Lys:ME ratio observed at the 14.5 MJ of ME level indicates that the greatest daily N retention occurs at 0.82 g of apparent digestible lysine/MJ of ME.

Further increases in lysine intake above the optimal level failed to increase daily N retention, which can be explained by the reduction of net energy in the high-protein diet due to excess AA deamination and N elimination (Noblet et al., 1987; Van Lunen and Cole, 1996). Maximal N retention at 20 kg of BW in this study was 18.2 g/d and was achieved at a Lys:ME ratio of 0.82 g/MJ, despite the significantly higher apparent digestible lysine intake at a Lys:ME ratio of 0.91 g/MJ. Because energy intake appears to be the major determinant of maximal protein deposition, the constraint of lean tissue growth in feed-restricted pigs could not be removed by simple increase of the first-limiting AA intake (Coma et al., 1995).

In Exp. 2, ADG and G/F of pigs given the 14.5 MJ of ME/kg diets increased, whereas ADFI was not affected. Similar ADFI responses were reported by Zhang et al. (1984) and Campbell and Taverner (1988), who found no difference in voluntary feed intake in pigs fed isoenergetic diets varying in protein content. In contrast, Nam and Aherne (1994) and Smith et al. (1999) observed decreased ADFI as the energy density of the diets increased. During the 28-d period of our trial, ADFI of feed-restricted pigs and those with ad libitum access to feed were 1.02 and 1.13 kg, respectively, for the combined (low- and high-ME diets) response. Thus, the feeding level of feed-restricted pigs was reduced to 90% of ad libitum intake; however, the actual difference between feed-restricted pigs in Exp. 1 (1.02 kg/d) and those on an ad libitum intake in Exp. 2 (1.13 kg/d) might be somewhat smaller due to greater feed wastage, as seen in similar ADG responses to ME in both experiments.

At higher BW, the dietary lysine concentration required by pigs appears to be similar regardless of feeding level (Coma et al., 1995). This requirement can be expressed as a Lys:ME ratio if the relationship between energy intake and rate of protein deposition is linear, as described with pigs weighing <26 kg (Nam and Aherne, 1994) and <45 kg (SCA, 1987). However, it appears that in highly selected pigs from 9 to 25 kg of BW, there may not be clearly delineated protein- and energy-dependent phases of N retention (Van Lunen and Cole, 1998).

In this study, a Lys:ME x ME interaction was detected at 15 kg of BW for N retention (g/d) and for ADG over the entire 28-d N balance experiment. In contrast, no interaction was observed for growth performance parameters measured in pigs with ad libitum access to feed. This is consistent with the findings of Nam and Aherne (1994), who did not detect any interactions between energy density and the lysine:calorie ratio in pigs fed from 9 to 26 kg of BW. Chiba et al. (1991) also did not observe an interaction between energy density and the lysine:calorie ratio in heavier pigs (from 20 to 50 kg of BW). Therefore, in our trial, for 15-kg BW pigs, the optimal Lys:ME ratio related to the 13.5 and 14.5 MJ of ME levels was proposed. For 20- and 25-kg BW pigs, the optimal Lys:ME ratios for the combined ME concentration were calculated.

Fitting an exponential (quadratic) model to daily N retention data was done to determine the optimal Lys:ME ratio for each N balance. In order to calculate the optimal ratio at 15 and 20 kg of BW, exponential regression equations were fitted to data points. Although the quadratic model is physiologically questionable (Morris, 1983), it was considered adequate to calculate the optimal Lys:ME ratio for 25-kg BW pigs. Based on exponential regression analysis, the optimal Lys:ME ratio for 15-kg BW pigs was 0.88 and 0.85 g/MJ for the low- and high-energy diets, respectively. The difference in optimal Lys:ME ratio between two energy concentrations suggests that some protein may have been deaminated to supply the additional energy required for greater N retention (Van Lunen and Cole, 1998). Daily ME intake was 10.07 and 10.69 MJ, and ileal digestible lysine intake was 7.52 and 8.18 g/d for the low- and high-energy diets, respectively. Therefore, for 15-kg BW pigs the optimal Lys:ME ratio of 0.85 g/MJ at 14.5 MJ of ME/kg is recommended. Regardless of the ME concentration, the optimal Lys:ME ratio at 20 and 25 kg of BW was 0.81 and 0.77 g/MJ based on exponential and quadratic regression analysis, respectively.

Apparent ileal digestible lysine requirements, defined here as the optimal Lys:ME ratio, were higher than the recommendations for dietary energy level and Lys:ME ratio (14.0 MJ and 0.82 g/MJ, respectively) reported by Rademacher et al. (1999) for young pigs from 10 to 19 kg of BW. These values are also somewhat greater than the optimum for ADG found for starter pigs (initial BW = 6.3 kg) fed for 28 d on 20% protein corn–peanut meal-based diets (Martinez and Knabe, 1990). When converted to ME, assuming that ME is 96% of DE (NRC, 1998), the optimal ratio observed by Martinez and Knabe (1990) was approximately 0.75 g of apparent digestible lysine/MJ of ME at 13.7 MJ/kg dietary density. The difference between the three experiments presumably reflects variations in feed intake and genetic capacity. A number of experiments have been published in which lysine requirements were estimated on the basis of total lysine. In a study on weanling pigs (from 9 to 26 kg of BW), Nam and Aherne (1994) observed that the ratio of Lys:DE had a greater effect on ADG than did the DE content of the diets. If converted to ME, the optimal combination of the Lys:ME ratio and ME level for maximal ADG suggested by response surface analysis was 0.99 g of lysine/MJ of ME and 13.6 MJ of ME/kg. However, the reported optimal ratio of Lys:DE could increase if protein quality were improved by switching from conventional to ideal protein. For example, Gatel et al. (1992) reported that maximal ADG was achieved at about 1.13 g of lysine/MJ of ME for pigs from 8 to 25 kg of BW when fed diets based on ideal protein. Van Lunen and Cole (1998) reported that maximal ADG for pigs from 9 to 25 kg of BW occurs at a lysine:energy (converted to ME) ratio of 1.25 g/MJ regardless of dietary energy concentration. It also appears that lysine:energy ratios above the optimal level resulted in growth depression. Williams et al. (1997) suggested that high-health pigs (from 6 to 27 kg of BW) require 1.13 g of lysine/MJ of ME. Immune-challenged pigs with a lowered health status require only 0.90 g of lysine/MJ of ME. The intermediate optimal ratio (1.04 g of lysine/MJ of ME) that maximized ADG and G/F in pigs from 10 to 25 kg of BW was proposed by Smith et al. (1999). These current estimates are considerably higher than the ratio of 0.84 g of lysine/MJ of ME in the NRC requirement for pigs from 10 to 20 kg of BW.

In conclusion, the results from the present experiments indicate that the Lys:ME ratio and ME level have, with a few exceptions, independent effects on N retention and growth performance. Results of Exp. 1 suggest that pigs from 13 to 30 kg of BW fed diets with 14.5 MJ of ME/kg had maximal daily N retention, ADG, and G/F. Based on nonlinear regression analysis, the Lys:ME ratios that maximized daily N retention in pigs at 15, 20, and 25 kg of BW were 0.85, 0.81, and 0.77 g/MJ, respectively. Likewise, from d 0 to 14 of Exp. 2, a Lys:ME ratio of 0.82 g/MJ was required to maximize ADG and G/F; however, there was no Lys:ME ratio effect on growth performance from d 14 to 28 and over the entire experimental period.

	Implications

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Our results suggest that diets fed to pigs from 13 to 30 kg of body weight should contain 14.5 MJ of metabolizable energy per kilogram of feed and should have an apparent ileal digestible lysine at a ratio to metabolizable energy that decreases with age. Our data also show that pigs with high-lean gain potential, from 13 to 20 and from 20 to 30 kg of body weight, can maximize their growth rate at an apparent ileal digestible lysine:metabolizable energy ratio of at least 0.85 and 0.77 g/MJ, respectively.

	Footnotes

 
¹ The authors wish to thank Degussa AG for financial support. Appreciation is also expressed to J. Fickler, M. Rademacher, J. Fontaine, and S. Mack (Degussa AG, Applied Technology) for their assistance with this experiment.

Received for publication June 14, 2002. Accepted for publication January 22, 2003.

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