J. Anim Sci. 2006. 84:1734-1741. doi:10.2527/jas.2005-334
© 2006 American Society of Animal Science
Evaluation of the order of limitation of lysine, threonine, and valine, as determined by plasma urea nitrogen, in corn-soybean meal diets of lactating sows with high body weight loss1,2,3
K. T. Soltwedel*,4,
R. A. Easter
and
J. E. Pettigrew*
* Department of Animal Sciences, and
and
College of Agricultural, Consumer, and Environmental Sciences, University of Illinois, Urbana 61801
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Abstract
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Three experiments were conducted to evaluate the order of limitation for Lys, Thr, and Val in corn-soybean meal diets of lactating sows experiencing high BW loss during lactation. Plasma urea N (PUN) was used as the response criterion. Experimental diets used in all 3 experiments were derived from the basal diet (BSL) formulated to 0.51% Lys using corn and soybean meal as the only sources of AA. The BSL diet was formulated by diluting a reference corn-soybean meal diet (17.2% CP, 0.90% Lys, 0.65% Thr, 0.82% Val, and 0.28% Met) with cornstarch, sucrose, and soybean oil to maintain the same ratio of corn to soybean meal. Experiment 1 was conducted to determine whether PUN is sensitive to dietary Thr and Val levels and whether a 4-d feeding period was adequate to observe a PUN response. The results demonstrated that PUN could be used as the response criterion and that a 4-d feeding period was adequate. Experiment 2 was conducted to determine whether Thr or Val was most limiting in the BSL diet. Dietary treatments used in Exp. 2 consisted of 1) BSL supplemented with 0.09% L-Lys and 0.02% DL-Met, which served as the negative control (NC2); 2) NC2 plus 0.14% L-Thr (NC2 + T); 3) NC2 plus 0.17% L-Val (NC2 + V); and 4) NC2 plus 0.14% L-Thr and 0.17% L-Val (NC2 + T + V). The value for PUN was lower for the NC2 + T and NC + T + V treatments compared with the NC2 and NC2 + V treatments (5.18 and 5.33 vs. 6.43 and 6.62; P < 0.01), indicating that Thr was most limiting in the NC2 diet. Experiment 3 was conducted to determine whether Thr or Lys was most limiting in the BSL diet. Dietary treatments used in Exp. 3 consisted of 1) the BSL diet supplemented with 0.02% DL-Met only, which served as the negative control diet (NC3); 2) the NC3 diet supplemented with 0.20% L-Lys (NC3 + L); 3) the NC3 diet supplemented with 0.14% L-Thr (NC3 + T); and 4) the NC3 diet supplemented with 0.20% L-Lys and 0.14% L-Thr (NC3 + L + T). The value for PUN was lower for NC3 + L compared with NC3 and NC3 + T (7.45 vs. 9.11 and 8.45 mg/dL; P < 0.01), and PUN for NC3 + L + T was lower than the PUN measured for NC3 + L (5.94 vs. 7.45; P < 0.01). The results of Exp. 2 and 3 indicate that Lys was first-limiting and Thr second-limiting in the BSL diet.
Key Words: lactation • lysine • plasma urea nitrogen • sow • threonine • valine
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INTRODUCTION
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The most recent AA requirement estimates for lactating sows include those of NRC (1998)
and Kim et al. (2001). The order of limitation calculated for the 3 most limiting AA in a corn-soybean meal (C-SBM) diet for lactating sows from NRC (1998) requirement estimates is Lys (first), Val (second), and Thr (third), and this order of limitation remains fixed regardless of the rate of sow body tissue mobilization. The results of Kim et al. (2001) suggest that the ideal AA profile changes in relation to the rate of sow body tissue mobilization during lactation. Specifically, the ideal protein proposed by Kim et al. (2001) suggests that as tissue mobilization increases, the sow’s dietary requirement for Thr (relative to Lys) increases. At extreme rates of body tissue loss (i.e., 100 to 90% of maximum), the AA order of limitation in a C-SBM diet (formulated to 0.90% Lys) was predicted by Kim et al. (2001) to be Thr (first), Lys (second), and Val (third). At high rates of body tissue loss (i.e., 80 to 20% of maximum), the order of limitation was predicted to be Lys (first), Thr (second), and Val (third). As the rate of body tissue loss decreased to zero, the order of limitation converged with that of NRC (1998). In essence, AA requirement estimates from NRC (1998) and Kim et al. (2001) for lactating sows experiencing high rates of body tissue mobilization are not in agreement. Therefore, 2 experiments were conducted to determine the order of limitation for Lys, Val, and Thr in C-SBM diets of lactating sows with high rates of body tissue loss. Plasma urea N (PUN) was used as the short-term response criterion. Although it is documented that PUN can be used as a rapid indicator of dietary AA requirements for lactating sows (Coma et al., 1996; Wilkinson et al., 1982, 1984), a preliminary experiment was conducted to confirm that PUN could be used as the response criterion.
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MATERIALS AND METHODS
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General Procedures
All animal care and use procedures were approved by the University of Illinois Institutional Animal Care and Use Committee. The study consisted of 3 experiments. Experiment 1 was a preliminary experiment conducted to determine whether PUN was sensitive to dietary Thr and Val levels in diets of lactating sows with high rates of BW loss and to determine whether a 4-d feeding period was adequate to allow the PUN concentration to change. Experiment 2 tested whether Thr or Val was more limiting in C-SBM diets of lactating sows with high rates of body protein loss. Experiment 3 tested whether Lys or the AA found most limiting in Exp. 2 was more limiting under the same circumstances. None of the experiments evaluated dietary effects over a full lactation; all experimental treatments were imposed over periods of only 4 to 6 d.
In all 3 experiments, a total of 32 maternal Line C22 (PIC, Franklin, KY) sows were used. Eight sows were used in Exp. 1, and 12 sows were used in Exp. 2 and 3. All 3 experiments were conducted at the University of Illinois Swine Research Center.
Sows were moved into the farrowing facility on approximately d 109 of gestation. Experimental diets were initiated on d 3, 4, or 5 of lactation to allow time for appetite recovery by sows postpartum. In Exp. 1, each of 2 diets was fed to sows over 2 separate 6-d periods in a 24-d study. In Exp. 2 and 3, each of 4 diets was fed to each sow over a 4-d period, covering a total of 16 d. In all 3 experiments, experimental diets were fed to each sow at a level of 4.8 kg/d on an as-fed basis. This feeding level was calculated to provide adequate energy intake (18.0 Mcal of ME/d) and deficient protein intake (480 g/d; NRC, 1998) in order to induce a high rate of body protein loss during lactation. Sows were fed twice daily at approximately 0800 and 1700 in all 3 experiments.
Sow BW and litter weights were recorded on d 0, 6, 12, 18, and 24 in Exp. 1 and on d 0, 4, 8, 12, and 16 in Exp. 2 and 3, where d 0 was the day on which the experimental diets were introduced. In Exp. 1, blood sampling was done on the fourth, fifth, and sixth day of each 6-d feeding period. In Exp. 2 and 3, blood sampling was done before the morning feeding on d 4, 8, 12, and 16 of the experiments. Diets were then switched for the beginning of the next 4-d period.
Blood samples were collected via jugular venipuncture using a 21-gauge needle 5.1 cm in length (Beckton Dickinson, Franklin Lakes, NJ) connected to a 10-mL syringe. Blood samples were transferred to a 7-mL evacuated tube containing the anticoagulant lithium heparin (Becton Dickinson). Blood samples were then centrifuged at 4,000 x g for 20 min at 4°C, after which time plasma from each sample was transferred to a separate vial and stored at –20°C until analysis of urea N concentration. The PUN was analyzed using the Roche Diagnostics Urea/blood urea N assay (Roche Diagnostics, Branchburg, NJ) based on the coupled urease/glutamate dehydrogenase enzyme system.
Experimental Diets
Experimental diets used in all 3 experiments were derived from a basal diet (BSL) formulated to contain 0.51% Lys using corn and soybean meal as the only sources of AA. The BSL diet (Table 1) was formulated to have the same AA profile as a typical C-SBM lactation diet by diluting a reference C-SBM diet (17.2% CP, 0.90% Lys) with cornstarch, sucrose, and soybean oil, thus maintaining the same ratio of corn to soybean meal. Experimental diets were created by supplementing the basal diet with crystalline AA at the expense of cornstarch. Crystalline L-Glu (Exp. 1) and Gly (Exp. 2 and 3) were used to make the experimental diets isonitrogenous.
Feed samples from all experimental diets were collected after mixing, and stored at 4°C until AA analysis (Table 2, Exp. 1; Table 3, Exp. 2; Table 4, Exp. 3) was performed by the University of Missouri Experiment Station Chemical Laboratory. Samples for AA analysis were prepared using a 24-h hydrolysis in 6 N HCl at 110°C under N atmosphere. For Met and cysteine, performic acid oxidation occurred before acid hydrolysis. Samples for Trp analysis were hydrolyzed using barium hydroxide. Amino acids in hydrolysate were determined by HPLC after postcolumn derivatization [982.30 E (a, b, c); AOAC, 2000]. Amino acid concentrations were not corrected for incomplete recovery resulting from hydrolysis. In addition, standardized ileal digestible (SID) AA concentrations were calculated more precisely using the analyzed AA values (Tables 2 to 4) and ileal digestibility coefficients for corn and soybean meal (NRC, 1998). For each AA, the SID AA content (calculated from diet formulation) was multiplied by the ratio of the analyzed AA content to the formulated AA content.
Experiment 1.
Dietary treatments used in Exp. 1 consisted of 1) the BSL diet supplemented with L-Lys, DL-Met, and L-Glu at 0.09, 0.02, and 0.38% of the diet, respectively, which served as the negative control (NC1); and 2) the NC1 diet supplemented with 0.14% L-Thr and 0.17% L-Val, and 0.07% cornstarch (NC1 + T + V) at the expense of L-Glu (Table 1
). Supplemental Lys (as L-Lys·HCl) and Met were included in the experimental diets to ensure that these AA were not limiting. Crystalline L-Glu was added to the NC1 diet as a source of supplemental N to compensate for the additional N resulting from supplemental L-Thr and L-Val in the NC1 + T + V diet.
Supplemental L-Thr and L-Val were added to the NC1 + T + V diet at a level calculated to provide a 45% increase in SID Thr and Val intake, respectively, compared with the NC1 diet.
Experiment 2.
Dietary treatments used in Exp. 2 consisted of 1) the BSL diet supplemented with L-Lys, DL-Met, and L-Gly at 0.09, 0.02, and 0.20% of the diet, respectively, which served as the negative control (NC2; Table 1); 2) the NC2 diet supplemented with 0.14% L-Thr (NC1 + T); 3) the NC2 diet supplemented with 0.17% L-Val (NC2 + V); and 4) the NC2 diet supplemented with 0.14% L-Thr and 0.17% L-Val (NC2 + T + V). Supplemental Lys (as L-Lys·HCl) and Met were included in the experimental diets to ensure that these AA were not limiting. L-Threonine and L-Val were added to the NC2 diet at levels calculated to provide a 45% increase in SID Thr and Val intake compared with the NC2 diet. The only difference between the NC2 diet and the NC1 diet was that crystalline Gly was used in place of L-Glu as a source of N to compensate for the additional N resulting from supplemental L-Thr or L-Val, or both, in the NC2 + T, NC2 + V, and NC2 + T + V diets.
Experiment 3.
Dietary treatments used in Exp. 3 consisted of 1) the BSL diet supplemented with DL-Met and L-Gly at 0.02 and 0.30% of the diet, respectively, which served as the negative control diet (NC3; Table 1); 2) the NC3 diet supplemented with 0.20% L-Lys (NC3 + L); 3) the NC3 diet supplemented with 0.14% L-Thr (NC3 + T); and 4) the NC3 diet supplemented with 0.20% L-Lys and 0.14% L-Thr (NC3 + L + T). L-Lys (as L-Lys
HCl) and L-Thr were added to the NC3 diet at levels calculated to provide a 45% increase in SID Lys and Thr intake compared with the NC2 diet.
Experimental Design
Experiment 1.
The preliminary experiment was designed as a replicated 2 x 2 crossover design to test for effects of dietary Thr and Val level, feeding period, and diet carryover effects on PUN concentration. Eight sows were allotted on the basis of parity to 1 of 4 replicate pairs. Within each pair, sows were randomly assigned to each of 2 dietary sequences as follows: 1) NC1, NC1 + T + V, NC1, NC1 + T + V; and 2) NC1 + T + V, NC1, NC1 + T + V, NC1. Each dietary sequence consisted of four 6-d periods, and each diet was fed over 2 separate 6-d periods in the crossover design.
Experiment 2 and 3.
Dietary treatments in Exp. 2 and 3 were arranged in a replicated Latin Square design. In each experiment, sows were allotted on the basis of parity to each of 3 replicate squares: 1 replicate of first-parity sows and 2 replicates of multiparous sows. Each experimental diet was fed to each sow for a 4-d period.
Statitistical Analysis
Treatment effects on sow and litter performance were not analyzed because of the short period for which each experimental diet was-fed.
Experiment 1.
Effects of day of sampling and dietary treatment on PUN concentration were analyzed using the PROC MIXED procedure of SAS (SAS Inst. Inc., Cary, NC). Each individual sow was the experimental unit. The statistical model used to evaluate the effect of day of sampling and treatment on PUN concentration included fixed effects for diet sequence, experimental period, day of sampling, diet, and the diet x day of period interaction. Sow within sequence was included in the model as a random effect.
Experiment 2 and 3.
Effects of dietary treatment on PUN concentration were analyzed using the PROC MIXED procedure of SAS. Each individual sow was the experimental unit. The statistical model used to evaluate the effect of treatment on PUN concentration on d 4 of each period included square, sow within square, period within square, and treatment. Treatments were arranged in a Latin square to minimize error variance in the model due to individual animal variation.
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RESULTS
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Experiment 1
Three sows refused to eat upon initiation of experimental diets and had to be removed from the study. The 5 sows that completed the experiment consumed an average of 4.8 kg/d throughout the entire 24-d study. Sow weight loss and litter weight gain averaged 1.27 and 2.30 kg/d, respectively (Table 5). The diet x day of period interaction (P = 0.75) was not significant and therefore was removed from the model. The PUN measured in sows for NC1 + T + V was lower (P < 0.01) than the PUN measured in sows for NC1 (Table 6). Additionally, the PUN measured on d 4, 5, and 6 were not different, and the SE did not diminish with additional days. These results demonstrate that a 4-d feeding period was adequate to observe a response in PUN to dietary Thr and Val levels.
Experiment 2
Feeding level did not exceed sow appetite; all of the sows consumed the allotted 4.8 kg/d during each period in which each diet was fed. Feed intake averaged 4.8 kg/d for all sows. Mean overall sow BW loss and litter weight gain averaged 1.22 and 2.19 kg/d, respectively, over the entire 16-d study (Table 5). The PUN was lower (P < 0.01) for the Thr-supplemented treatments and was not affected by supplemental Val intake (Table 7).
Experiment 3
Feeding level exceeded daily appetite of 1 sow consistently throughout the trial, so feed intake averaged 4.7 kg/d for all sows over the entire 16-d experiment. Similar to Exp. 1, sow weight loss and litter weight gain averaged 1.13 and 2.32 kg/d, respectively, over the entire 16-d trial (Table 5). The value for PUN measured at the end of the 4-d feeding period was lower for NC3 + L compared with NC3 and NC3 + T (P < 0.01; Table 8). In addition, PUN for NC3 + L + T was lower than the PUN measured for NC3, NC3 + L and NC3 + T at the end of a 4-d feeding period (P < 0.01; Table 8).
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DISCUSSION
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The diets fed in all 3 experiments were formulated to induce a high rate of body protein loss by restricting protein intake to approximately 50% of the lactating sow’s requirement. Sows lost a considerable amount of BW in each trial (>1 kg/d) and maintained satisfactory litter growth rates (>2.1 kg/d). To maintain a constant N intake among sows within each feeding period, feeding level was standardized to 4.8 kg/d per sow throughout the entire study period, and all diets used within each experiment were formulated to be isonitrogenous. This was to ensure that the observed changes in PUN were due to changes in dietary AA levels and not differences in total N intake.
The results of Exp. 1 demonstrated that PUN could be used as a rapid indicator of Thr or Val needs of lactating sows, or both, and that a 4-d feeding period was adequate for PUN to stabilize after changing dietary Thr and Val levels. These results are in agreement with those of Coma et al. (1996) who determined that PUN could be used as a rapid indicator of Lys requirements for lactating sows.
The results of Exp. 2 provide empirical data supporting the hypothesis that Thr is more limiting than Val in C-SBM diets of lactating sows with high rates of body protein mobilization. These data are in agreement with the order of limiting AA for lactating sows with high rates of body tissue mobilization predicted by the model of Kim et al. (2001) but not the model of NRC (1998).
Kim et al. (2001) suggested that Thr may become more limiting than Lys in C-SBM diets for lactating sows with high rates of body tissue loss. Therefore, Exp. 3 was conducted to test the hypothesis that Lys (and not Thr) is first-limiting in a C-SBM diet when fed to lactating sows experiencing a high rate of sow BW loss. The lower PUN values (P < 0.01) observed for the NC3 + L treatment compared with the NC3 and NC3 + T treatments (Table 8) provide empirical data to support our hypothesis that Lys is first-limiting in C-SBM diets of lactating sows experiencing high rates of body protein mobilization. Furthermore, the NC2 + L + T resulted in a lower PUN compared with the NC2 + L treatment, indicating a further improvement in AA use, thereby providing empirical evidence that Thr is second-limiting in C-SBM diets of lactating sows with high rates of body protein mobilization when PUN is used as an indicator.
The results of Exp. 2 and 3 provide empirical data demonstrating that Lys is first-limiting and Thr second-limiting in C-SBM diets of lactating sows with high rates of body tissue mobilization when PUN is used as the response criterion. These results are in agreement with the order of limitation for Lys and Thr in a C-SBM lactation diet predicted from the model of Kim et al. (2001) for sows experiencing a moderate to high rate of tissue mobilization; i.e., 20 to 80% of the maximum rate of tissue mobilization. The maximum rate of tissue mobilization used in the calculations of Kim et al. (2001) was defined as the sum of the estimates of marginal AA mobilization from each body tissue per pig increase in litter size. At 0 and 10% of maximum tissue mobilization, the limiting order of AA predicted was Lys first-limiting, Val second-limiting, and Thr third-limiting. At 90 and 100% of maximum mobilization, the limiting order of AA predicted was Thr first-limiting, Lys second-limiting, and Val third-limiting. Therefore, our results from Exp. 2 and 3 are in agreement with the order of limitation for Lys and Thr from Kim et al. (2001) requirement estimates calculated for moderate to high tissue mobilization rates (i.e., 80 to 20% of maximum tissue mobilization), whereas they are not in agreement with the order of limitation predicted from NRC (1998) requirement estimates.
Comparison of responses to AA supplementation in experiments conducted under varying conditions is tenuous because the adequacy of AA supply for the lactating sow depends not only on the AA concentration in the diet but also on other conditions including levels of other AA, feed intake, amount of milk produced (demand), and the amount of body protein mobilized. For that reason, we focused our discussion on requirement estimates of NRC (1998) and Kim et al. (2001) because they considered those other factors explicitly. However, it is useful to consider other studies also.
Our results suggest that Lys is the first-limiting and Thr the second-limiting AA in a typical C-SBM diet. Further, the results of Exp. 3 suggest that the Thr to Lys (Thr:Lys) ratio required in diets of lactating sows experiencing high rates of sow BW loss during lactation is less than 0.71, the Thr:Lys ratio in the NC3 diet. This is in general agreement with published Thr:Lys ratio requirement estimates. Lewis and Speer (1975) and Cooper et al. (2001) estimated the Thr:Lys requirement ratio for lactating sows to be 0.70 and 0.69, respectively. More recently, Gaines et al. (2004) reported that milk production (litter weight gain) and the number of pigs weaned by modern high-producing sows were optimized at a Thr:Lys ratio of 0.64 in both younger (parity 1 to 2) and older (parity 3 to 6) sows.
Our data suggest that Val was not limiting in our experimental diets, even after correction of the Lys and Thr deficiencies. It is not clear what limited protein use in the diet with both Lys and Thr supplementation in Exp 2, but it was not Val. This result is in agreement with those studies where sow lactation performance was not improved in response to increasing the dietary Val to Lys (Val:Lys) ratio above 0.90 (Carter et al., 2000; Paulicks et al., 2003; Etienne et al., 2004) but is not in agreement with those studies where sow lactation performance was improved in response to Val:Lys ratios in excess of 1.0 (Richert et al., 1996; Richert et al., 1997a,b). Unfortunately, it is not possible to make more specific comparisons because of differences in AA levels of the basal diet, digestibility of AA in the basal diet, sow feed intake, milk production (litter growth), and sow BW loss, all of which influence dietary AA requirement estimates.
It is recognized that caution must be used in the interpretation of the results of Exp. 2 and 3. Data from these experiments are only qualitative in nature and do not provide specific quantitative estimates of AA requirements. Furthermore, experimental diets were fed to sows over short periods of only 4 d each, so we cannot be certain that these results would hold true for measures of sow and litter performance over a full lactation period.
The results of these experiments suggest that Lys is first-limiting and Thr is second-limiting in C-SBM diets of lactating sows experiencing high BW loss during lactation. Kim et al. (2001) predicted this order of limitation for lactating sows with moderate to high rates of sow body tissue loss, whereas NRC (1998) did not. Further research is needed to determine if these results would hold true for sow and litter performance over a full lactation period.
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Footnotes
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1 Funding provided by the Illinois Council on Food and Agricultural Research. 
2 Appreciation is expressed to Ajinomoto Heartland LLC, Chicago, IL, for donation of crystalline L-Val.
3 The authors are grateful to Jennifer Miguel, Jestina Kusina, Maria Palacios, and the farm staff at the University of Illinois Swine Research Center for their assistance in data collection.
4 Corresponding author: ksoltwedel{at}scanutrition-usa.com
Received for publication June 25, 2005.
Accepted for publication February 7, 2006.
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Abstract
INTRODUCTION
