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Serum urea concentration as a predictor of dietary lysine requirement in selected lines of pigs¹

Roslin Institute (Edinburgh), Roslin, Midlothian EH25 9PS, U.K.

² Correspondence:
Phone: +44-131-527-4258; fax: +44-131-440-0434; E-mail:
neil.cameron{at}bbsrc.ac.uk.

	Abstract

Top Abstract Introduction Materials and Methods Results Discussion Implications Appendix Literature Cited

 
Serum urea concentrations were measured in Large White pigs from lines divergently selected for components of efficient lean growth rate and performance tested over three 14-d test periods starting at 30, 50, and 75 kg. Two methods of performance testing were used. Phase-fed pigs were fed to appetite isoenergetic diets differing in total lysine:energy ratio (0.58, 0.69, 0.81, 0.91, 1.01, 1.12, and 1.23 g/MJ of digestible energy), whereas diet-choice pigs were offered a choice of the 0.69 and 1.12 lysine:energy diets. Between test periods, all animals were fed one diet: 0.91 g of lysine/MJ of digestible energy. The study consisted of 230 boars and gilts with 150 pigs performance tested on phase-feeding and 80 pigs on diet-choice. The line selected for high lean food conversion had lower urea concentrations on each diet than the line selected for high lean growth rate, despite similar predicted lysine balances. Efficiency of lean growth rather than the rate of lean growth may be a better selection strategy in the context of nitrogen excretion. Urea concentrations at the end of each test period were correlated with lysine intake (0.33, 0.48 and 0.65; standard error, 0.08) and predicted lysine balance (0.39, 0.44, and 0.64), but were uncorrelated with predicted lysine for protein deposition (0.01, 0.08, and 0.08) and maintenance. Urea concentration at the end of a test period was not a useful predictor of protein deposition, even after accounting for pretest variation in urea concentration and food intake during test. The expected response pattern of serum urea concentration to diets differing in total lysine:energy would be nonlinear, with the point of inflection occurring at the required dietary total lysine:energy for each genotype. However, there was no evidence of such an inflection point such that the prediction of lysine requirement from urea concentration was not possible for the selection lines in the study.

Key Words: Growth • Lean • Lysine • Pigs • Selection • Urea

	Introduction

Top Abstract Introduction Materials and Methods Results Discussion Implications Appendix Literature Cited

 
Determination of lysine requirements for maintenance and protein deposition using serial slaughter of animals with chemical analysis of carcass components is both expensive and destructive. An in vivo measure of lysine requirement could predict the consequences of different nutritional inputs at various stages of growth at the within-animal level. A candidate for an in vivo measure of lysine requirement is plasma urea (Coma et al., 1995a) since excess dietary protein is catabolized with the resulting nitrogen excreted as urea. Identification of selection strategies resulting in increased efficiency of nutrient utilization is desirable from an animal breeding and environmental perspective since feed accounts for the majority of production costs and reduced nitrogen excretion is consistent with a policy for sustainable lean meat production. Currently, there is limited information on genetic variation in both nutritional requirements and the allocation of nutrients to protein and lipid deposition. The effects of altering nutrient intake in more than one genetic population have identified between-population differences in urea concentration and growth traits (e.g., Chen et al., 1995). The divergent selection lines, derived from the one base population and of known selection history, from the Edinburgh lean growth selection experiment (Cameron, 1994) provide an experimental resource to estimate the relationship between urea concentration and predicted lysine utilization and also to identify selection strategies that have resulted in more efficient nutrient utilization.

	Materials and Methods

Top Abstract Introduction Materials and Methods Results Discussion Implications Appendix Literature Cited

 
Animals and Performance Test
A total of 230 Large White pigs from the Edinburgh lean growth selection experiment were performance tested using either phase-feeding or diet-choice procedures (Bradford and Gous, 1991). Details on the establishment of the Large White population and seven generations of divergent selection for daily food intake (DFI), lean food conversion (LFC), and lean growth rate (LGA) were given by Cameron (1994). The selection lines were derived from the same base population.

Animals in the LFC and LGA selection groups, with a group being a pair of high- and low-selection lines, were selected for breeding using the criteria -20.28 FCR_S -18.42 BFAT_S and 23.75 ADG_S -20.86 BFAT_S, respectively, where FCR_S, ADG_S, and BFAT_S were the standardized deviations from the mean for food conversion ratio, ADG, and backfat depth at the end of performance test. Divergent selection was practiced for seven generations, with each line consisting of 10 sires and 20 dams, and with six animals per litter performance tested to identify animals of high or low genetic merit in the high- or low-selection lines. A series of genotype x nutrition interaction studies were performed from generation eight, and animals were selected at random within litter for mating to maintain the selection lines.

Phase-Feeding.
There were 150 pigs performance tested on phase-feeding with five litters of five full-sibs (three boars and two gilts or two boars and three gilts) from each of the six selection lines with one litter per sire. Within each line, the five sires and the litter per sire were chosen at random. Within each litter, each full-sib was given ad libitum access to one of five isoenergetic (14.0 MJ of DE/kg) diets differing in total lysine:energy ratio(Table 1), with diets R, S, T, U, or X; Q, R, S, T, or U; and finally, P, Q, R, S, or T offered in three test periods. For example, the first sequence of diets fed during the three test periods would be R, followed by Q, and then P, so that five diets were fed each test period. The intended start weights for each test period, which lasted 14 d, were 27 to 33, 46 to 54, and 70 to 80 kg with realized mean (SD) weights of 29.3 ± 1.6, 49.5 ± 2.1, and 73.5 ± 2.1 kg. Pigs were penned individually, and prior to the start of test and between test periods, pigs were fed diet S. There were no diet effects or between-selection line, within-group effects on the time taken to reach the required start weight for second and third test periods, which averaged 13.9 ± 4.1 and 15.5 ± 4.6 d, respectively. Although, the interval between test periods two and three was longer for the low-LFC line than for other selection lines (18.2 vs 15.0; SEM = 1.2 d).

Serum Urea Assay.
Prior to each performance-test measurement, 10-mL blood samples were taken from the vena cava, kept at 4°C for 24 h, and then centrifuged for 30 min at 4°C. The red blood cell-free serum was extracted and frozen at -20°C until required for assay. Serum urea concentrations, expressed as mg/dL, were determined in duplicate using a commercially available assay (Randox, Co. Antrim, U.K.). The repeatability of the assay calculated from the between- and within-animal variance components averaged 0.88 (SE = 0.01) for the six sampling times.

Predicted Lysine Utilization.
Prediction of lysine for protein deposition and maintenance (see Appendix) was based on estimated protein weight, with the equation derived from a chemical composition study of carcasses in an earlier generation (Cameron, 2000). Briefly, six litters of five full-sibs in each selection line were slaughtered at 30, 45, 60, 75, or 90 kg, with a full-sib in each litter allocated at random to each slaughter weight for subsequent chemical analysis of carcass and noncarcass components. The regression equation (see Appendix) proportionately accounted for 0.98 of the variation in protein weight.

Statistical Analysis
All traits were analyzed using the REML algorithm of Genstat Committee (1997). The model for phase-fed animals included selection line, diet, and sex as fixed effects, with the selection line with diet interaction with litter and laboratory assay fitted as random effects. There were 14 laboratory assays, each with a maximum of 92 samples measured in duplicate, and a total of 1,220 blood samples. Estimates of the laboratory assay variance component for urea measured at the start (29, 19, 7; SE = 15 [mg/dL]²) and end (16, 25, 23; SE = 18 [mg/dL]²) of the three test periods were not significantly different from zero, but the effect was included in the model to account for interlaboratory assay variation. The laboratory assay variance proportionately accounted, on average, for 0.27 of the variance in urea concentration due to the random effects of litter, laboratory assay, and residual. For diet-choice pigs, the model included selection line and sex as fixed effects with litter and laboratory assay as random effects.

To determine if variation in urea concentration was lower at the start than at the end of the previous test period due to feeding diet S between-test periods, selection line x diet subclass was fitted as a random effect in a model to estimate the between- and within-selection line x diet subclass variance components.

The diets differed in several aspects other than in the total lysine:energy ratio, but the diets were formulated to ensure that lysine was always the first-limiting amino acid. The data were analyzed from the perspective of dietary differences in total lysine:energy ratio; an approach that has been used in previous studies (e.g., Rao and McCracken, 1990; Van Lunen and Cole, 1996).

Power transformations of urea concentration were examined using the procedure of Solomon (1985) to identify a transformation that maximized the log likelihood under the joint hypotheses of independence of mean and variance and normality and additivity of effects. For a trait (x), the transformed trait (y) was equal to

for 0 and y = log(x) for = 0. An approximate log likelihood

was calculated from the estimated residual (ME) and between full-sib litter (ML), mean squares with N animals, and d full-sib litters. The value that maximized the log likelihood was estimated with differentiation of the cubic function describing the log likelihood in terms of . The range of values resulting in log likelihoods not significantly different from the maximum log likelihood (logL) was determined by solving the cubic equation with log L - 3.86/2 subtracted from the constant since twice the difference between two log likelihoods has a ² distribution with one degree of freedom.

Phenotypic Correlations Between Traits.
Correlations between urea concentrations and predicted lysine utilization were estimated separately for phase-fed and diet-choice animals. The statistical significance of the difference between the two correlation matrices was determined using the difference between the log likelihoods of the correlation matrices (Thompson, 1976), equal to

Twice the difference between log likelihoods has a ²distribution with degrees of freedom (DF) associated with the P_phase-fed matrix, with P_phase-fed and P_diet-choice equal to the correlation matrices for phase-fed and diet-choice animals, respectively. The two correlation matrices were significantly different (² = 205 on 94 DF). The estimated phenotypic correlation matrices for phase-fed and for diet-choice animals were examined using principal component analysis to determine linear functions of traits that describe the overall pattern of correlations between traits (Searle, 1982). In a plot of coefficients of the two eigenvectors, with the largest eigenvalues weighted by the square root of their eigenvalue, the cosine of the angle between two points represents the estimated correlation using only information from the two eigenvectors and eigenvalues. For example, points clustering together are highly correlated, points at right angles to each other are uncorrelated, and points diametrically opposite are highly negatively correlated. The distance of a point from the origin is a measure of the information of a trait provided by the first and second eigenvectors and eigenvalues.

	Results

Top Abstract Introduction Materials and Methods Results Discussion Implications Appendix Literature Cited

 
On determining appropriate transformations of urea concentrations, the values that maximized the log likelihoods were not different from unity (P > 0.05) for the six sampling times except at the end of the first test period when a log transformation was required (Table 2). The log transformation reduced the skewness coefficient for urea concentration from 0.61 (SE = 0.20) to -0.10. Solomon (1985) suggested that the use of simple values (e.g., 0, 1/2, 1) would be practical. No transformation was applied to urea concentrations except for urea at the end of the first test period which was log transformed.

View this table: [in this window] [in a new window]   Table 2. Estimated values that maximized the log likelihood and the range of values (lower, upper) resulting in log likelihoods not significantly different from the maximal log likelihood, the simple power transformation used in the analysis and twice the difference (2D) between the maximal log likelihood and that of the simple transformation

Urea concentration at the start of each test period increased with test period (25, 29, and 35; SEM = 3.6 mg/dL). There was no diet effect (P > 0.05) on urea concentration (Figure 1a), as a result of feeding diet S between-test periods.

View larger version (12K): [in this window] [in a new window]   Figure 1. Urea concentrations at the (a) start of the test period and (b) change in concentration within test period for the five diets, averaged over selection lines, with the dotted line indicating a zero change. SE of the between-diet differences within test period were 1.8 mg/dL and 2.2 mg/dL for urea concentration at start of test period and change in concentration, respectively.

Urea concentrations were higher (P < 0.05) in the high-DFI line than in the low-DFI line at the end of the third test period (38 vs 31, SEM = 2.2 mg/dL) and in the low-LFC line at the end of each test period (28 vs 21, SEM = 1.9; 30 vs 24, SEM = 1.6; and 29 vs 22, SEM = 2.5 mg/dL) when averaged over diets. There were no differences (P > 0.05) in urea concentration between the LGA lines, except for higher concentrations in high-LGA line animals fed diets R and S in the second and third test periods (40.2 vs 30.2, SEM = 3.4; 52.0 vs 38.9, SEM = 3.8 mg/dL).

Within-selection line linear and quadratic regression coefficients were estimated for urea concentration at the end of the third test period on diet class to determine the sensitivity of selection lines to changes in dietary lysine:energy ratio. There were no between-selection line differences (P > 0.05) in the linear term (range of 4.6 to 6.4, SE = 1.1 mg/dL per diet class). A nonzero quadratic term was estimated in the high-LGA line (-2.69, SE = 0.97 mg/dL per diet class) indicating a maximal urea concentration with diet S. Quadratic terms for the other selection lines were negative but not different from zero (P > 0.05). The model including diet class as a covariate within-selection line had the same residual mean square as the model fitting the selection line x diet interaction (64 and 61, SE = 8.9 [mg/dL]²). Parameters from the covariance analysis provided a sufficient summary of the selection line responses in urea concentration to changes in dietary lysine:energy in the third test period.

Diet-Choice.
At each fixed weight, there was no selection line effect (P > 0.05) on urea concentration of diet-choice pigs, except for higher urea concentrations at 50 and 75 kg in the high-DFI line (see Figure 3a). Averaged over the DFI and LGA selection lines, urea concentrations of diet-choice animals increased with time on test (18, 22, 26, and 29, SEM = 1.5 mg/dL) in line with phase-fed pigs at the start of each test period. For diet-choice animals, the average proportional intakes of diet Q were 0.80 in the DFI lines and 0.95 in the LGA lines (Figure 3b) with resultant dietary lysine:energy contents of 0.79 and 0.71 g/kg (SEM = 0.03), respectively. Only in the first test period, were there differences (P < 0.05) between the DFI and LGA lines in the proportion of food Q consumed.

View larger version (10K): [in this window] [in a new window]   Figure 3. Urea concentrations and proportion of diet Q consumed during each test period for diet-choice pigs (within-test period SEM were 3.1 mg/dL and 0.06, respectively).

The nonlinear response of predicted lysine balance to increasing dietary total lysine:energy ratio in the first test period changed to a linear response in the third test period (Figure 2). There were no within-group, between-selection line differences for predicted lysine balance in the LFC and LGA selection groups. However, predicted lysine balance was higher (P < 0.05) in high-DFI animals fed diet X in the first test period (line difference: 4.1, SEM = 1.4 g/d), in those fed diets S to U in the second test period (line difference: 6.9, SEM = 1.6 g/d), and in animals fed diets S and T in the third test period (line difference: 6.6, SEM = 1.5 g/d).

View larger version (35K): [in this window] [in a new window]   Figure 2. Prediced lysine balance () and urea concentrations at the start () and end (•) of the three test periods by diet and selection line, with filled and open symbols indicating the high and low selection lines.

View larger version (11K): [in this window] [in a new window]   Figure 4. The eigenvector with the largest eigenvalue (x-axis) plotted against the eigenvector with the second-largest eigenvalue (y-axis), weighted by the square root of their eigenvalues, from the correlation matrix for (a) phase-fed and (b) diet-choice animals (ureai: urea concentrations at the start (i = 1,3,5) and end (i = 2,4,6) of each test period; lini, lproti, lmaini, lbali: lysine intake, predicted lysine for protein deposition, predicted lysine for maintenance, and predicted lysine balance in test period i).

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion Implications Appendix Literature Cited

 
The study examined the use of serum urea concentration as a predictor of lysine utilization in growing pigs by measuring the response in serum urea concentration to changes in dietary total lysine:energy. The underlying assumption was that lysine intake in excess of that required for maintenance and protein deposition is catabolized with the resulting nitrogen excreted as urea. The response pattern of serum urea concentration on diets differing in total lysine:energy was expected to be nonlinear with the point of inflection occurring at the required dietary total lysine:energy for each genotype. If lysine intake were below the requirement for maintenance and protein deposition, then serum urea concentration would not be expected to change, but protein accretion would be constrained. Chen et al. (1995) demonstrated an equivalent relationship between dietary lysine:energy, urea concentration, and protein deposition. Based on plasma urea concentrations, Chen et al. (1995) inferred the protein requirements of two pig populations at different stages of growth with the population of higher genetic merit for lean tissue growth having a higher protein requirement.

In the current study, there was generally no evidence of a point of inflection for urea concentration in response to dietary lysine:energy, such that the prediction of lysine requirement from urea concentration was not possible. There are at least two hypotheses for the lack of an inflection point. In the first and second test periods, urea concentration did not increase with dietary lysine:energy, with the exception of diet X, suggesting that excess amino acids may have been excreted in the urine without being utilized for gluconeogenesis. In the third test period, dietary protein intake may have been insufficient with the reduction in urea concentration due to recycling of amino acids. Further, the different outcomes between Chen et al. (1995) and the current study may have resulted from using circa 1960 and post-1990 genotypes, such that the physiological responses were genetic population-dependent. Chen et al. (1995) stated that "the Gene Pool animals were typical of pigs 30 years ago," and the Hampshire pigs had substantially higher backfat depths than the "fat" lines of the current study (26 vs 18 mm).

Dietary effects on urea concentration have been reported in several studies, with animals phase-fed (Lee et al., 2000) or fed a single diet throughout performance testing (e.g. Cai et al., 1996; Chen et al., 1995). The range of diets in the current study was intended to bracket the required dietary lysine content during each test period and to avoid problems with interpretation of experimental results in situations of severe under- or oversupply of dietary lysine. For example, Van Lunen and Cole (1996) suggested that a reduction in nitrogen deposition might result from oversupply of dietary protein. The nonlinear response of plasma urea concentration and nitrogen retention in the Coma et al. (1995a) study reiterate the issue of wide ranging dietary compositions being confounded with different physiological responses.

The use of urea concentration to predict lysine requirement would gain credibility if lysine requirement predicted from urea concentration and from lysine utilization equations provided similar estimates. Based on urea concentrations, Chen et al. (1995) suggested that Gene Pool pigs required 130g of CP/kg at between 30 and 80 kg of live weight, whereas Hampshire pigs required 190g of CP/kg to 45 kg of live weight and 160g of CP/kg thereafter. However, the required dietary protein content, determined from lysine utilization equations (see Appendix), was proportionally 0.74 of that indicated by urea concentrations. Coma et al. (1995a) suggested that required dietary protein content derived from growth rate will estimate a lower protein requirement than when determined from urea concentrations, which reflects protein metabolism. Dourmad et al. (1999) reported that two-thirds of nitrogen intake is excreted by pigs, with growing pigs being the prime contributor to the problem of nitrogen excretion. In that context, the substantially higher dietary protein content predicted from urea concentrations compared to predictions from lysine utilization equations is of concern. In the current study, the increase in the correlation between urea concentration and predicted lysine balance with test period, from 0.39 to 0.64, indicated that use of urea concentration to differentiate between diets on the basis of protein or lysine requirements should be limited to post-70 kg of live weight. The nonlinear response of predicted lysine balance to increasing dietary total lysine:energy ratio in the first test period changing to a linear response in the third test period suggests that further research is required for prediction of lysine requirement prior to 70 kg of live weight.

Prior to each test period, animals were fed one diet, such that the response of urea concentration to changes in dietary lysine:energy could be estimated at different stages of growth, independent of residual dietary effect from previous test periods. Coma et al. (1995a) demonstrated that plasma urea responded within 24 h to changes in dietary lysine content, with a new equilibrium urea reached within 3 d. Whang and Easter (2000) reported little diurnal variation in urea concentration in ad libitum fed animals. In the current study, the minimal interval between test periods was 7 d, such that sufficient time should have elapsed for animals to attain equilibrium urea concentrations. Evidence for a sufficient time interval was given by the reduction in the selection line x diet subclass variance component for urea concentration at the start of a test period relative to the end of the previous test period. Coma et al. (1995b) recommended that urea concentration at the start of the test period should be included in the model as a covariate when analyzing urea concentration at the end of the test period from the perspective of using urea concentration to assess protein requirements. In the current study, there was no change in the correlation between urea concentration at the end of test period and predicted lysine for protein deposition when urea at the start of the test period was included (or not) in the model. One reason for the lack of a pretest effect was that correlations between urea concentration at the start and end of each test period were substantially lower in the current study than in the Coma et al. (1995b) study (0.86).

Coma et al. (1995b) suggested that a negative correlation between urea concentration and protein deposition rate would not be apparent if increased protein deposition rate was primarily a result of greater food intake. One of the correlated responses in the selection lines of the Edinburgh lean growth experiment was in food intake, such that the selection lines formed a powerful experimental resource to determine if a trait, such a food intake, had an effect on the trait of interest: the correlation between urea concentration and estimated protein deposition. Inclusion of food intake in the model, for the current study, reduced the correlation between urea concentration at the end of test period and predicted lysine for protein deposition, but only for the first test period, to -0.28. Again, the correlation between urea concentration and estimated protein deposition was smaller in magnitude in the current study than in the Coma et al. (1995b) study (-0.88). A conclusion from the current study is that urea concentration at the end of a test period was not a useful predictor of protein deposition, even after accounting for pretest period variation in urea concentration and food intake during test.

Both phase-feeding and diet-choice procedures were included in the study as part of an evaluation of the two procedures for determining nutritional requirements of different genotypes (Cameron et al., 2002a). In each test period, urea concentrations were generally lower in diet-choice animals than in phase-fed animals for each of the six selection lines. The similar lysine intakes and predicted lysine utilization of diet-choice and phase-fed animals fed diets S, R, and Q in the three test periods coupled with the lower urea concentrations of diet-choice pigs suggested that either the efficiency of lysine utilization was different in diet-choice pigs or that amino acids were being excreted directly. In the Cameron et al. (2002a) study, 578 pigs were performance tested on phase-feeding or diet-choice, with the same experimental protocol as in the current study. Low genetic correlations between diet-choice and phase-fed animals for growth, lysine intake, and lysine conversion ratio in the third test period indicated that the objectives being achieved by diet-choice animals differed from those of phase-fed animals, resulting in different nutritional requirements. Therefore, determining nutritional requirements for each genotype with phase-feeding or with diet-choice procedures will produce different estimates, as will identification of dietary inputs to minimize urea concentration or predicted lysine balance. Despite Coma et al. (1995a) suggesting that dietary protein requirements derived from weight gain will estimate a lower protein requirement than that derived from urea concentrations, there has been no published research providing an explanation for the difference.

Inclusion of the divergent selection lines in the study provided the experimental resource to determine if particular selection strategies have resulted in more efficient nutrient utilization, as reflected by urea concentration. The high-LFC line had lower urea concentrations than the high-LGA line despite similar predicted lysine balances, which implies reduced nitrogen excretion. Therefore, efficiency, and not rate, of lean growth may be a better selection strategy in the context of nitrogen excretion. Unfortunately, inclusion of efficiency in the LFC selection objective was associated with reduced food intake during performance testing (Cameron and Curran, 1994) and with deleterious effects on reproductive performance (Cameron et al., 2002b). Therefore, indirect inclusion of efficiency in the selection strategy may be preferable, such that efficiency is increased by faster growth rather than by reduced food intake.

	Implications

Top Abstract Introduction Materials and Methods Results Discussion Implications Appendix Literature Cited

 
The expected response pattern of serum urea concentration to diets differing in dietary total lysine:energy ratio was nonlinear, with the point of inflection occurring at the lysine requirement for each genotype. However, there was no evidence of such an inflection point, which prevented prediction of lysine requirement from urea concentration. The low correlation between urea concentration and predicted protein deposition implied that the former was not a useful physiological indicator of the latter. Selection for efficiency of lean growth resulted in lower urea concentration than selection for rate of lean growth, indicating that efficiency of lean growth may be a better selection strategy in the context of nitrogen excretion.

	Appendix

Top Abstract Introduction Materials and Methods Results Discussion Implications Appendix Literature Cited

 
Prediction Equations for Daily Lysine Utilization
Lysine dietary intake (L_IN, g/d) = F x total lysine x 0.85 where F is the daily food intake (kg/d), total lysine is lysine in diet (g/kg), and an ileal digestibility of 0.85 is assumed.

Lysine for maintenance (L_MAIN, g/d) = 0.091 x 0.5 x (Pwt_start + Pwt_end)/0.65 where each kilogram of protein has a daily maintenance cost of 91 g (Close, 1994), assuming a protein turnover rate of 4 g/kg with a lysine content of replacement proteins of 22.7 g/kg. Protein weights (kg) at the start (Pwt_start) and end (Pwt_end) of a test period were predicted by 0.176 W - 0.117 BF, where W and BF refer to live weight (kg) and backfat depth (mm), respectively. A net efficiency of 0.65 is assumed.

Lysine for protein deposition (L_PROT, g/d) = 70 x (Pwt_end - Pwt_start)/(0.65 x 14) where 1 kg of protein is equivalent to 70 g of lysine (ARC, 1981) with a test period of 14 d.

Lysine balance (g/d) = L_IN - L_MAIN - L_PROT

Calculation of Required Dietary Protein Content from Data of Chen et al. (1995)
Protein weight gain (kg/d) = carcass protein gain (g/d) x growth rate (kg/d)/carcass gain (g/d) assuming similar protein contents of carcass and noncarcass components.

Lysine required for protein deposition (PL_PROT, g/d) = 70 x protein weight gain/0.65

Average protein weight (Pwt, g) = 0.5 (end of test weight + start of test weight) x 0.16 assuming protein content of the live animal of 160 (g/kg)

Lysine required for maintenance (PL_MAIN, g/d) = 0.091 x Pwt/0.65

Required lysine intake (PL_IN, g/d) = (PL_PROT + PL_MAIN)/0.85

Required dietary protein content (g/kg) = PL_IN/(0.05 x F) using the average dietary lysine: protein of 0.05 for the Chen et al. (1995) diets. For example, growth rate (0.900 kg/d), carcass weight (658 g/d), and protein (94 g/d) gains of Hampshire pigs fed the 16% CP diet implied the lysine required for maintenance (1.6 g/d) and protein deposition (13.8 g/d), so that given the daily food intake (2.96 kg/d), the required dietary protein content can be determined (123 g/kg).

	Footnotes

 
¹ This project was funded by the Dept. for Environment, Food and Rural Affairs. The diets were designed by P. Boyd and manufactured by Cranswick Mill Ltd.

Received for publication May 9, 2002. Accepted for publication September 10, 2002.

	Literature Cited

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