HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Lopez, G. Articles by Leeson, S. Search for Related Content PubMed PubMed Citation Articles by Lopez, G. Articles by Leeson, S.

Assessment of the Nitrogen Correction Factor in Evaluating Metabolizable Energy of Corn and Soybean Meal in Diets for Broilers

Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada, N1G 2W1

¹ Corresponding author: sleeson{at}uoguelph.ca

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Three experiments were carried out to determine AME and AME_n of corn and soybean meal (SMB) in diets for growing broilers. In experiments 1 and 2, ingredient-specific basal diets or a combination of these basal diets with corn and SBM were prepared. For corn, the substitution was 25, 50, or 75% of the total diet, whereas SBM substitution was at 10, 20, or 30%. In experiment 1, birds were fed the experimental diets continuously from 0 to 33 d, and AME and AME_n were determined during 9 to 12 d and 30 to 33 d of age. In experiment 2, birds were fed the experimental diets only around the time of the collection period. The AME_n of corn was 95 to 97% of corresponding AME, whereas for SBM, AME_n was 93 to 88% of AME. Linear regression was used as an alternative method of calculating ingredient energy values resulting in a significant regression of diet AME and AME_n content on inclusion level, for each period of time and for each ingredient (corn and SBM). Based on varying inclusion levels of test ingredients in the diet, the extrapolated AME and AME_n of corn were estimated more precisely (R² = 0.90 to 0.95) than those of SBM (R² = 0.57 to 0.85), suggesting that the variability of AME and AME_n is better explained by a linear regression of AME or AME_n on percentage of inclusion. For corn, AME and AME_n were little affected by age, and the effect of N correction was consistent at around 3%. Determined energy values of SBM were more variable. Experiment 3 was conducted to assess the effect of formulating diets based on either AME or AME_n on broiler performance. A 2-sample t-test was implemented examining AME vs. AME_n formulation. The analyses for numerous production and carcass traits were nonsignificant except for the case of less abdominal fat in birds fed diets formulated to AME rather than AME_n (P < 0.01). These results showed that the use of the N correction imposed a penalty to corn of 3 to 5% and SBM of 7 to 12%.

Key Words: metabolizable energy • ingredient • broiler

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Energy systems are used to predict values of feeds relative to energy requirements for maintenance and production. In poultry, AME has been commonly accepted and extensively used to define energy values of feedstuffs and diets. Several procedures have been used to measure the ME content of individual ingredients (Hill and Anderson, 1958; Potter et al., 1960; Sibbald and Slinger, 1963; Lockhart et al., 1966; Miller, 1974; Farrell, 1978; Bourdillon et al., 1990a; Farrell et al., 1991; McNab, 2000). In these studies, both nutrient balance and palatability determine the maximum inclusion of an ingredient within assay diets. Differences in the AME values assigned to different ingredients are caused by differences in their chemical composition as well as factors such as bird age and strain. The AME values of ingredients (or diets) are commonly corrected for N retention (AME_n) to convert all data to a basis of N equilibrium for comparative purposes. In the case of growing broilers, correction for N retention accommodates the effect of differential growth rate inherent across birds in any assay. Correction for N retention is expected to greatly influence the ME of ingredients such as soybean meal (SBM) compared with corn (Dale and Fuller, 1984) because of associated higher protein accretion. A similar effect is expected from diets high in CP, such as broiler starter vs. finisher diets. The effect of the N correction has also been used to decrease the variability of estimates of ME of ingredients varying in protein content (Leeson et al., 1977). However, the argument against the N correction is that it is not a biological norm, especially for modern broilers with large feed intakes, fast growth, and the associated accretion of protein and fat. On the other hand, N correction allows strain comparison and accommodates any age-related effects. Because broiler nutrition is now very specialized, the need for cross-strain comparison is less relevant, although within poultry, age and strain may still be influential on AME (Mollah et al., 1983; Härtel, 1986; Bourdillon et al., 1990b; Carré et al., 1995; Farrell et al., 1997).

For modern broilers, the energy deposited as protein is a biological norm, and so the N correction system heavily penalizes dietary protein from ingredients such as SBM, because the system assumes zero protein retention, inferring that all protein is used as a source of energy. The question therefore arises as to whether AME is more relevant than AME_n within the confines of just commercial broiler nutrition and that AME values more equitably describe energy available from protein concentrates vs. cereals. The objective of these studies was to determinate AME and AME_n of corn and SBM for growing broilers and then to assess the effectiveness of these values in formulating broiler diets.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Two experiments were designed to determine AME and AME_n of corn and SBM in growing broilers, fed continuously from 0 to 33 d (experiment 1) or fed at 2 different periods of time (9 to 12 d and 30 to 33 d, experiment 2), whereas experiment 3 utilized these results to study broiler response to diets with corn and SBM formulated to AME or AME_n.

Experiment 1

Two hundred eighty-eight 1-d-old male broiler chicks of a commercial cross (Hubbard x Hubbard) were randomly allocated to 1 of 8 diets in a completely randomized design. Birds were placed in groups of 6 in 48 growing cages (50 x 60 cm) at the University of Guelph Arkell Research Center facilities, which are managed and cared for according to guidelines established by the University of Guelph Animal Care Committee. The 8 treatments (4 for each of corn and SBM) were each replicated 6 times, with each replication consisting of a cage (50 x 60 cm) group of from 1 to 6 broilers. There were 6 broilers per cage during the 9 to 12-d excreta collection, and numbers reduced to 1 bird per cage during collection from 30 to 33 d. Room temperature was maintained at 32°C from 0 to 5 d and gradually reduced according to normal brooding practices to 22°C with 24 h/d of lighting. Water and feed were available ad libitum. Treatments were a corn or SBM basal diet (CB or SB, respectively, Table1) or a combination of these basals with corn or SBM. For corn, the substitution was at 25, 50 or 75% using the C1 basal (Table 1), whereas for SBM, substitution was at 10, 20 or 30% using the S1 basal (Table 1). In this experiment, birds were continuously fed the experimental diets from 0 to 33 d.

The AME_n values were calculated by subtracting GE excreted (adjusted to zero N balance) from GE intake and dividing this value by DM feed intake. For correction to zero N retention, a value of 8.22 kcal/g of N retained was used (Hill and Anderson, 1958).

Experiment 2

Growth was below normal in experiment 1, likely due to the fact that the higher levels of corn and SBM substitution resulted in imbalanced diets relative to requirements over a prolonged period of time. For this reason, experiment 2 was modified such that the experimental diets were only fed around the times of the balance periods.

Three hundred thirty-six 1-d-old male broiler chicks of a commercial cross (Hubbard x Hubbard) were randomly allocated to 1 of the 48 growing cages (50 x 60 cm) and fed a standard broiler starter diet (Table 1). Birds were housed and managed as described in experiment 1. At 7 and 28 d, birds were randomly allocated to 1 of the 4 diets (Table 1) in a completely randomized design, allowing birds at least 2 d adaptation to each diet before any excreta collection for energy balance studies. The 4 treatments were each replicated 6 times, with each replication consisting of a cage (50 x 60 cm) group of from 2 to 5 broilers. There were 5 broilers per cage during the excreta collection from 9 to 12 d and 2 birds per cage during collection from 30 to 33 d. Birds were also weighed at the beginning and end of each collection period. Treatments were a corn or SBM basal diet (Table 1) or a combination of these basal diets with corn or SBM. Corn and SBM substitution levels and diets were as described for experiment 1. Bird management and measurements and chemical analyses were performed as described in experiment 1.

Experiment 3

This experiment was designed to evaluate broiler performance when fed diets formulated using either AME or AME_n values for corn and SBM as determined in the previous experiments. Four hundred twenty broiler males (Ross x Ross) were weighed and randomly allocated to 1 of the 2 diet treatment groups based on the system of energy evaluation, namely AME and AME_n. Each treatment group was replicated 6 times, with each replication consisting of a floor pen (2.44 x 1.83 m) containing 35 birds. Room temperature was maintained at 32°C from 0 to 5 d and was gradually reduced according to normal brooding practices to 22°C with 23 h/d of lighting.

From 0 to 21 d, birds were fed 1 of 2 starter diets (AME or AME_n, Table 2). Using AME and AME_n values for corn and SBM (experiment 2), both diets were formulated to be isoenergetic and contain similar levels of protein, amino acids, and other major nutrients (3,100 kcal of ME/kg and 21% CP, Table 2). As shown in Table 2, formulation to AME, rather that AME_n, results in less supplemented fat, because the energy value of SBM is increased with this system of evaluation. On d 21, birds were offered corresponding AME and AME_n grower diets providing 3,250 kcal of ME/kg and 19% CP to 42 d (Table 2). All birds were weighed individually at 21 and 42 d of age, and feed intake was measured over these times. Birds were allowed free access to water and feed. Mortality was recorded daily throughout the experiment. At 42 d of age, 10 birds were randomly selected from each pen for processing at the University’s plant. After processing, the abdominal fat pad was removed and weighed (Griffiths et al., 1977). Remaining carcasses were chilled in water at 4°C for 1 h, then weighed, and the breast meat on both left and right side of the carcass, consisting of pectoralis and supracoracoideus muscles, was removed and weighed.

Experiment 1 and 2. Diet AME values were uncorrected, whereas diet AME_n values were corrected to zero N retention using a value of 8.22 kcal/g of N retained, with ingredient values determined according to the equation:

Treatments means were tested by ANOVA, and response variables were further analyzed using Tukey test. The statistical model used was:

where Y_ij = the average of AME or AME_n, obtained from cage j under treatment i, i = 1,...6, j = 1,...6; µ = general mean; _i = effect of the ith treatment, i = 1,...6, on the response variable; and _ij = random variable with mean 0, variance ² independent and normally distributed. To look for significant differences between a pair of means, the Tukey test was used at the 5% level of significance.

Because the treatment factor is quantitative, to study the effect of the percentage of inclusion of corn and SBM on AME or AME_n, a linear regression analysis was performed. For each combination of period of time and ingredient (corn or SBM), a separate regression analysis was conducted. The statistical model used was:

where Y_i = AME (or AME_n) measured in kilocalories per kilogram on the ith sample; β₀, β₁ = regression coefficients; X_i = percentage of ingredient (corn or soybean); _i = random variable assumed to be normally distributed with mean 0 and variance ².

The SAS GLM procedure (SAS Institute, 2000) was used to fit the regression models and PROC.

To obtain a 95% confidence interval for the expected mean of the response variable AME or AME_n given a fixed level of ingredient substitution, the following equation was used.

where and = the point estimates; x = the value of the ingredient substitution; = the quantile of the tn – 2 distribution; and = the SE of the mean of y given x, which is obtained as:

where S² = the residual MS error from the ANOVA table; n = number of observations; x = percentage of inclusion level under study; = mean percentage of inclusion level; and S_xx = corrected sum of squares of the test ingredient level (CB, C25, C50, C75, SB, S10, S20, and S30).

Experiment 3. To test whether the means of groups 1 and 2 can be considered statistically different, a t-test was performed. The hypothesis of interest is: Ho: µ₁ = µ₂ vs. Ha: µ₁ µ₂, where the observations from group 1 are Y₁₁,...Y_1n and those from group 2 are Y₂₁,...Y_2n. The test statistic is:

where and = the sample average, and the pooled SD, Sp, is defined as

where n₁ and n₂ = the sample sizes of groups 1 and 2, respectively. The test was applied to each of the variables of interest (e.g., BW, feed intake, feed conversion, BW gain, abdominal fat, carcass weight, and breast weight).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Energy values determined by the substitution method are given in Tables 3 and 4 for experiment 1 and 2, respectively. The AME and AME_n of corn were very consistent and not affected by the level of substitution or bird age. Method of feeding the diets (experiment 1 vs. 2) also resulted in similar overall values. The AME_n of corn was around 95 to 99% of AME, implying a N correction penalty of 1 to 5%. The energy values for SBM were much more variable. The energy values for SBM were more consistent in experiment 2 (Table 4, P > 0.05) where the imbalanced diets were not fed for the duration of the trial, rather being introduced only at time of collection. For SBM, the N correction caused a 7 to 12% decline in available energy.

View larger version (12K): [in this window] [in a new window]   Figure 1. Experiment 1: 95% confidence interval for the expected mean of AME and AMEn at different levels of corn substitution in broiler diets (period 9 to 12 d).

View larger version (12K): [in this window] [in a new window]   Figure 2. Experiment 1: 95% confidence interval for the expected mean of AME and AMEn at different levels of soybean meal (SBM) substitution in broiler diets (period 9 to 12 d).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Several methods have been developed to evaluate ME of individual ingredients (Hill and Anderson, 1958; Potter et al., 1960; Sibbald and Slinger, 1963; Lockhart et al., 1967; Miller, 1974; Sibbald, 1976; Farrell, 1978; Farrell et al., 1991). These methods vary in their procedures, the simplest being feeding only the test ingredient (Lockhart et al., 1967). However, in most cases, procedures are based on substituting ingredients into a basal diet (Miller, 1974). Usually, nutrient balance and palatability are factors dictating maximum inclusion level. The use of such substitution inevitably includes an additional confounding factor in that diets can be potentially imbalanced as occurred in experiment 1. It has been proposed that values derived using this approach may not be a true representation of actual values within balanced diets as used commercially (Miller, 1974; Sibbald, 1982). However, there is little information on the ability of the bird to derive energy from such unbalanced diets, for either short or prolonged periods of time during an assay. For example, in the case of proteins, it is expected that imbalanced amino acids relative to requirements imply an energetic cost related to its excretion (Van Milgen et al., 2001) in addition to a diet-induced protein turnover (Kita et al., 1993), which increases the energy cost to the animal and concomitantly lowers the energy value of the protein source.

The ME values of ingredients and diets are commonly corrected for N retention to convert all data to a basis of N equilibrium for comparative purposes. Because N retention differs with bird age, type of birds, and possibly strain, a correction factor is essential if comparisons of ME values for the same ingredient are to be made (Leeson et al., 1977). Species comparison (and their age-related cofactors) has been the main argument in favor of the use of N correction. This concept is of questionable value for broilers (our unpublished data), because species-age comparison is rarely critical for broiler nutritionists, and broilers are considered to be relatively uniform in protein accretion over time.

Obviously, methodology influences AME and AME_n evaluations. Within the same experiment, the use of the substitution method, which is based on only 6 observations, showed large variability in values (Tables 2 and 3). On the other hand, the results obtained by the regression analysis (extrapolation to 100% inclusion), which used 24 observations, seems a more reliable estimator based on reduction in the SEM. These results show that the SE of the predicted value of AME or AME_n for SBM is noticeably higher than the value for corn. This difference is due to the fact that with SBM, lower inclusion levels necessarily mean a greater degree of extraplation and the fact that SE diverges the further the prediction is away from actual analyses levels. This indicates that the precision of the ME value obtained is dependent on the proportion of the test ingredient substituted. Leeson et al. (1977) explained the need for high proportions of the test ingredients in the test diets and that this phenomenon explains some of the variability associated with determination of ME of fats, for example, where inclusion level is necessarily low (Miller, 1974).

There is evidence that AME and AME_n values vary according to the age of the birds used in the assay. Results from our current data indicate that AME and AME_n are influenced by bird age, and this is especially so for SBM. In experiment 2, the variability in correction for N retention with SBM decreases as birds get older, agreeing with Leeson et al. (1977), who used N correction to decease the variability of estimates of ME in which ingredients vary in protein content.

Nitrogen retention is expected to have a greater influence on the ME of ingredients such as SBM compared with corn because of expected higher protein accretion (Dale and Fuller, 1984). For modern broilers, the use of the N correction will penalize the biological characteristic of depositing ME as protein during growth, again especially with high-protein ingredients such as SBM.

Corn and SBM are considered the 2 major ingredients in formulation for broiler diets. Due to its chemical composition, corn is considered a primary source of energy (Cowieson, 2005), whereas SBM is considered a protein (or amino acid) source, with lower ME contribution (Coon et al., 1990; Dale, 2000). These 2 ingredients together constitute approximately 85 to 90% of the total energy content in a typical broiler diet (Leeson and Summers, 2005). Broilers fed diets formulated using AME or AME_n values for corn and SBM grew at remarkably similar rates. Feed efficiency was also comparable, which is an indication that diets were of comparable energy value or that the sensitivity of the response criteria was insufficient to detect diet differences. On this basis, formulating to AME values within starter and grower diets is more economical. It would be interesting to conduct further studies to measure the NE of these diets. There is an indication of greater available energy in birds fed to AME_n, because there was greater fat accumulation.

Information from this experiment indicates that within the confines of commercial broiler nutrition, the uncorrected AME values determined with broilers describe more equitably the energy available from cereals and protein concentrates. From a practical commercial point of view, the study shows that birds fed a diet formulated to similar AME and AME_n contents performed equally. However, because feed price is around $10/ton less with formulation to AME contents, the effect of dietary energy evaluation requires further study. If these results are confirmed, then data suggest that the energy value of SBM is currently being undervalued.

	ACKNOWLEDGMENTS

 
This work was supported by the Ontario Ministry of Agriculture Food and Rural Affairs, Guelph, Ontario, Canada.

Received for publication July 5, 2007. Accepted for publication October 22, 2007.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Bourdillon, A., B. Carré, L. Conan, J. Duperray, G. Huyghebaert, B. Leclercq, M. Lessire, J. McNab, and J. Wiseman. 1990a. European reference method for the in vivo determination of metabolisable energy with adult cockerels: Reproducibility, effect of food intake and comparison with individual laboratory methods. Br. Poult. Sci. 31:557–565.[CrossRef][Web of Science][Medline]

Bourdillon, A., B. Carré, L. Conan, M. Francesch, M. Fuentes, G. Huyghebaert, W. M. M. A. Janssen, B. Leclercq, M. Lessire, J. McNab, M. Rigoni, and J. Wiseman. 1990b. European reference method of in vivo determination of metabolisable energy in poultry: Reproducibility, effect of age, comparison with predicted values. Br. Poult. Sci. 31:567–576.[CrossRef][Web of Science][Medline]

Carré, B., J. Gomez, and A. M. Chagneau. 1995. Contribution of oligosaccharide and polysaccharide digestion, and excreta losses of lactic acid and short-chain fatty acids, to dietary metabolisable energy values in broiler chickens and adult cockerels. Br. Poult. Sci. 36:611–629.[CrossRef][Web of Science][Medline]

Coon, C. N., K. L. Leske, O. Akavanichan, and T. K. Cheng. 1990. Effect of oligosaccharide-free soybean meal on true metabolizable energy and fiber digestion in adult roosters. Poult. Sci. 69:787–793.[Web of Science][Medline]

Cowieson, A. J. 2005. Factors that affect the nutritional value of maize for broilers. Anim. Feed Sci. Technol. 119:293–305.[CrossRef]

Dale, N. 2000. Soy products as energy sources for poultry. Pages 283–288 in Soy in Animal Nutrition. J. K. Drackley, ed. Fed. Anim. Sci. Soc., Savoy, IL.

Dale, N., and H. L. Fuller. 1984. Correlation of protein content of feedstuffs with the magnitude of nitrogen correction in true metabolizable energy determinations. Poult. Sci. 63:1008–1012.[Web of Science][Medline]

Farrell, D. J. 1978. Rapid determination of metabolisable energy of foods using cockerels. Br. Poult. Sci. 19:303–308.[CrossRef][Web of Science]

Farrell, D. J. S., A. Smulders, P. F. Mannion, M. Smith, and J. Priest. 1997. The effective energy of six poultry diets measured in young and adult birds. Pages 371–374 in Proc. 14th Symp. Energy Metab., Newcastle Co. Down, Northern Ireland. K. J. McCraken, E. F. Unsworth, and A. R. G. Wylie, ed. CAB Int., London, UK.

Farrell, D. J., E. Thomson, J. J. Du Preez, and J. P. Hayes. 1991. The estimation of endogenous excreta and the measurement of metabolisable energy in poultry feedstuffs using four feeding systems, four assay methods and four diets. Br. Poult. Sci. 32:483–499.[CrossRef][Web of Science]

Griffiths, L., S. Leeson, and J. D. Summers. 1977. Fat deposition in broilers: Effect of dietary energy to protein balance and early life caloric restriction on productive performance and abdominal fat pad size. Poult. Sci. 56:638–646.[Web of Science]

Härtel, H. 1986. Influence of food input and procedure of determination on metabolisable energy and digestibility of a diet measured with young and adult birds. Br. Poult. Sci. 27:11–39.[CrossRef][Web of Science][Medline]

Hill, F. W., and D. L. Anderson. 1958. Comparison of metabolizable energy and productive energy determinations with growing chicks. J. Nutr. 64:587–603.[Abstract/Free Full Text]

Kita, K., T. Muramatsu, and J. Okumura. 1993. Effect of dietary protein and energy intakes on whole-body protein turnover and its contribution to heat production in chicks. Br. J. Nutr. 69:681–688.[CrossRef][Web of Science][Medline]

Leeson, S. 1974. Metabolizable energy studies with turkeys. PhD Thesis. Univ. Nottingham, Nottingham, UK.

Leeson, S., K. N. Boorman, and D. Lewis. 1977. Metabolizable energy studies with turkeys: Nitrogen correction factor in metabolizable energy determinations. Br. Poult. Sci. 18:373–379.[CrossRef][Web of Science]

Leeson, S., and J. D. Summers. 2005. Feeding programs for broiler chickens. Pages 229–296 in Commercial Poultry Nutrition. S. Leeson and J. D. Summers, ed. Univ. Books, Guelph, Ontario, Canada.

Lessire, M., and B. Leclercq. 1982. Metabolisable energy value of fats in chicks and adult cockerels. Anim. Feed Sci. Technol. 7:365–374.[CrossRef]

Lockhart, W. C., L. Reece, R. L. Bryant, and D. W. Bolin. 1967. A comparison of several methods in determining the metabolizable energy content in durum wheat and wheat cereal by chicks. Poult. Sci. 46:805–810.[Web of Science]

McNab, J. M. 2000. Rapid metabolizable energy assays. Pages 307–315 in Farm Animal Metabolism and Nutrition. J. P. F. D’Mello, ed. CABI Publ., Oxon, UK.

Miller, W. S. 1974. The determination of metabolisable energy. Pages 91–112 in Energy Requirements of Poultry. T. R. Morris and B. M. Freeman, ed. Br. Poult. Sci. Ltd., Edinburgh, UK.

Mollah, Y., W. L. Bryden, I. R. Wallis, D. Balnave, and E. F. Annison. 1983. Studies on low metabolisable energy wheats for poultry using conventional and rapid assay procedures and the effects of processing. Br. Poult. Sci. 24:81–89.[CrossRef][Web of Science]

Potter, L. M., L. D. Matterson, A. W. Arnold, W. J. Pudelkiewickz, and E. P. Singsen. 1960. Studies in evaluating energy content of feeds for the chicks. 1. The evaluation of metabolizable energy and productive energy of cellulose. Poult. Sci. 39:1166–1178.[Web of Science]

SAS Institute. 2000. SAS User Guide. Version 8.1. SAS Inst. Inc., Cary, NC.

Sibbald, I. R. 1976. A bioassay for true metabolizable energy in feeding stuffs. Poult. Sci. 55:303–308.[Web of Science][Medline]

Sibbald, I. R. 1982. Measurement of bioavailable energy in poultry feeding stuff: A review. Can. J. Anim. Sci. 62:983–1048.

Sibbald, I. R. 1989. Metabolizable energy evaluation of poultry diets. Pages 12–26 in Recent Developments in Poultry Nutrition. D. J. A. Cole and W. Haresign, ed. Butterworths, Essex, UK.

Sibbald, I. R., and S. J. Slinger. 1963. A biological assay for metabolisable energy in poultry feed ingredients together with findings which demonstrate some of the problems associated with the evaluation of fats. Poult. Sci. 42:313–325.[Web of Science]

Spratt, R. S., and S. Leeson. 1987. Determination of metabolizable energy of various diets using leghorn, dwarf, and regular broiler breeder hens. Poult. Sci. 66:314–317.[Web of Science][Medline]

Van Milgen, J., J. Noblet, and S. Dubois. 2001. Energetic efficiency of starch, protein and lipid utilization in growing pigs. J. Nutr. 131:1309–1318.[Abstract/Free Full Text]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Lopez, G. Articles by Leeson, S. Search for Related Content PubMed PubMed Citation Articles by Lopez, G. Articles by Leeson, S.