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An Evaluation of the Nitrogen Correction in the True Metabolizable Energy Assay
An Evaluation of the Nitrogen Correction in the True Metabolizable Energy Assay A. JABBAR MUZTAR and S. J. SLINGER Department of Nutrition, College of Biological Science, University of Guelpb, Guelph, Ontario, Canada NIG 2W1 (Received for publication June 30, 1980)
1981 Poultry Science 60:835-839 INTRODUCTION
It has been customary to correct the conventional apparent metabolizable (AME) values for nitrogen (N) retention or N loss. The correction is considered to represent the energy content of N-containing excretory materials temporarily retained in the body. In applying N correction, either one of the two factors 8.22 (Hill and Anderson, 1958) or 8.73 (Titus et al, 1959) have been used. In the former case, uric acid was considered to be the sole N excretory product in the chicken while Titus et al. (1959) considered 8.73 to more truly represent the energy of the excretory products of protein metabolism. Sibbald and Slinger (1962) observed a close relationship between the classical AME and N-corrected AME (AME n ) values based on rapidly growing chickens. These workers also found that the source and amount of dietary protein had little effect on the magnitude of the N-correction. Most data for mature roosters in our laboratory have shown the AME n values to be lower than the corresponding classical AME values by about .01 to .03 kcal/g (Muztar et al, 1978a,b; Muztar and Slinger, 1981). Therefore, application of N correction does not seem to be of major consequence when one considers the amount of time and labor in-
volved in determining the N content of feed and excreta samples. Nevertheless, it does have value as an indicator of the N balance of birds in the AME assay. It is important to have birds close to N equilibrium in AME experiments in order to ensure generation of reliable metabolizable energy data. Muztar et al. (1978b) found mature birds to be in negative N balance on 25 and 50%rapeseedmeal (RSM) or rapeseed (RS) diets, although the experiment involved no fast and the birds on an average consumed about 50 to 60 g of each diet daily for 4 days. The true metabolizable energy (TME) method of Sibbald (1976) involves force-feeding of fasted (24 hr) adult roosters a relatively small quantity of feed. Excreta are collected quantitatively for a period of 24 hr. The correction for metabolic fecal energy ( F E m ) plus endogenous urinary energy (UE e ) losses in the TME assay are based on the losses of unfed birds of similar body weight to the fed birds. The TME method involves a rather abnormal situation in that the birds are probably in negative N balance from the beginning to the end of the experiment. The question thus arises as to whether or not birds are able to retain enough N from the relatively small amount of feed being used in the TME assay (20 to 35 g) to offset their body N loss due to fasting. 835
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ABSTRACT This paper examined the effect of N-correction on the true metabolizable energy (TME) values of mixed diets and single ingredients using mature White Leghorn roosters. The N-corrected TME (TME n ) values were consistently higher than the corresponding TME values, the results being significant (P<.05) in about one-half of the samples tested. This indicated a net loss in body N in the birds during the experiments. The correlations between N input and N correction were highly (P<.01) significant. This, however, explained only 46 to 48% of the variation in the TME n data as being due to differences in the amount of N input. In view of the present results and the abnormal condition (fasted birds) in the TME assay, it is doubtful that birds would retain enough N from a single feeding to revert to a positive body N equilibrium. Therefore, positive N retention in such an assay would be indicative of incomplete feed passage through the birds. This appears to be the only valid reason for applying N correction to TME values. However, the variations associated with N correction of TME data raise some doubt concerning the validity of this assay as compared with the conventional apparent metabolizable energy assay. (Key words: TME, N-corrected TME, AME, AME n )
836
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MATERIALS AND METHODS Experiment I. In this experiment the effect of several mixed diets and single ingredients of varying protein content on the N retention in the TME assay were tested. A corn-soy basal diet was also included. Adult male Single Comb White Leghorn (SCWL) roosters of Shaver strain were fasted for 24 hr to empty their digestive tracts of feed residues. Groups of 5 birds were force-fed each material in pellet form in amounts equivalent to .8% of body weight, which were between 20 and 22 g, air-dry weight. Excreta were collected quantitatively for the next 24 hr, freeze dried, allowed to equilibrate with atmospheric moisture, and weighed. A group of 10 birds was fasted for 24 hr to establish the F E m + UE e losses for the fed birds. Experiment 2. A second experiment was carried out to test the effect of a N-free diet and some isonitrogenous diets, varying in crude fiber content, on N retention or loss in mature roosters in the TME assay. Three isonitrogenous (12% crude protein) corn-soy diets adequate in minerals and vitamins (Muztar et al., 1978a) were formulated. One contained no external source of fiber while the other two included 5 and 10% alfafloc, respectively, at the expense of corn starch in the diet. Another diet, containing no N, and with the composition 90% cerelose, 5% pure ground corn cob, and 5% animal-vegetable fat blend was also included.
The birds were fasted for 30 hr and then force-fed 32 g of the diets in pellet form. Excreta from individual birds were quantitatively collected for 30 hr, freeze dried, allowed to equilibrate with atmospheric moisture, and weighed. Feed and excreta from both experiments were ground to pass a 1-mm screen. Samples of feed and excreta were analyzed for moisture by the AOAC (1975) procedure and for gross energy using an adiabatic oxygen bomb calorimeter. Nitrogen was determined by the Kjeldahl method. The TME values were calculated and adjusted for N retained or lost by the birds using the following equation: TME n = TME (
Total N consumed — Total N excreted ) X 8.22 Feed input
where 8.22 is the energy in kcal/g of N retained or lost by the bird (Hill and Anderson, 1958). The test of significance of TME and TME n results was carried out using the analysis of variance technique and treatment means were compared using Duncan's multiple range test (Steel and Torrie, 1960). Regression analysis was used to measure the effect of the level of N input on N correction of the TME data. RESULTS AND DISCUSSION
The results of Experiment 1 are shown in Table 1. These data are based on 24-hr fast and 24-hr collection periods and feed inputs of 20 to 22 g, air-dry weight. For almost half the number of samples, the TME n values were significantly (P<.05) higher than the corresponding TME counterparts. The TME n values for the remaining samples were also numerically higher than the TME values, but the differences were not significant (P>.05). This indicated that in all cases there was some net N loss by the birds during the experiment. The N loss was greatest and very similar for the birds force-fed the Candle RS, Tower RSM, and the 40% Candle RSM diet. The N input, however, was much greater in the case of Tower RSM compared to either of the Candle RS or the 40% Candle RSM diet (1.11 g vs. .65 and .75 g, respectively). Regression analysis of the TMEn (y) values versus the N (x) input levels showed a highly significant correlation between the two para-
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Shires et al. (1980) reported the N-corrected TME (TME n ) values of corn, soybean meal (SBM), and RSM to be substantially lower than the corresponding TME values. This work suggested that both chicks and mature roosters had retained N in excess of their loss in the 24-hr postfeeding period after being fasted for 21 hr. The decrease in TME by application of the N correction varied between 3 and 12% depending upon the feedstuff, with the high protein feeds such as SBM and RSM showing the highest N retention and corn the least. Thus, N retention in the TME assay appeared to be dependent on the protein content of the feedstuff. In view of the apparent anomaly in the results of Sibbald and Slinger (1962) and those of Shires et al. (1980), it was considered important to investigate this question further. A range of protein-containing single ingredients and mixed diets were included in this study.
NITROGEN CORRECTION IN TME ASSAY
837
TABLE I. True metabolizable energy (TME) and N-corrected TME (TMEn) of some mixed diets and single ingredients*,2 TME
Corn-soy basal (B) B + SBM: 60 + 40 B + Tower RSM: 6 0 + 4 0 B + Candle RSM: 6 0 + 4 0 B + Candle R S : 60 + 40 SBM Tower RSM Candle RSM Candle RS
a
TMEn
N loss
N input
(mg/g feed)
(g)
3 . 8 1 ± .03
b
3 . 9 2 ± .05
13.43 ± 2.66
.49 ± .01
a
3 . 5 6 ± .05
a
3 . 6 4 ± .04
10.62 +
.87
.82 + .01
a
3 . 2 9 + .03
b
3 . 4 1 ± .05
15.73 ±
.09
.73 ± .01
3 . 3 5 + .06
17.84 ± 4 . 1 0
.75 ± .01
4 . 5 0 ± .06
5.72 ± 1.91
.62 ± .01
a
3 . 2 1 ± .04
b
a
4 . 4 5 ± .05
a
a
.09 .06 .08 .34
3.29± 2.60 ± a 2.57 ± a 4.57 ± a
a
3.34 2.74 2.60 a 4.72
b a
± ± ± ±
.11 .06 .09 .30
5.87 17.38 3.26 18.38
± 2.09 ± .23 ± .44 ± 2.38
1.29 1.11 1.10 .65
± .01 ± .01 + .01 ± .05
•*' The TME and TME n values for the same sample with a different superscript are significantly (P<.05) different. 1
Results are given as means ± standard error on a dry matter basis.
2
Factor of 8.22 used to bring the TME values to a zero N equilibrium. SBM, = soybean meal; RSM, = rapeseed meal; RS, = rapeseed (whole, full-fat).
meters (r = —.691, P<.01, df = 43). This, however, accounted for only 48% of the variation in the TME n values as being due to changes in the N input level while the remaining 52% was unexplained. The regression equation was y = 5.495 — 2.212 x. Table 2 depicts the results of Experiment 2. The TME n values of the equinitrogenous diets,
although not significantly (P>.05) different from the corresponding TME values, were slightly higher. The TME n value of the N-free diet was significantly (P<.05) higher than its TME value. Thus, from these data and also the N loss figures, it is evident that the birds cannot restore their body N equilibrium in the TME assay when fed 32 g of either of the three
TABLE 2. True metabolizable energy (TME) and N-corrected TME (TMEn) values of some isonitrogenous and a N-free diet* Sample
TME
Corn-soy diet A 2 Corn-soy diet B J Corn-soy diet C 2 N-free diet 3
a
TME n 4 - (kcal/g)-
3.91 ± .04 a 3.68± .03 a 3.50± .04 a 3.89± .01
N loss
N input
(mg/g feed) a
3.93 a 3.71 a 3.55 b 4.02
±.04 ±.02 ±.04 ± .01
2.14 3.81 5.92 15.09
± .82 ± 1.24 ± 1.52 ± .79
(g) .66 .66 .66
ab ' The TME and TME n values for the same sample with a different superscript are significantly (P<.05) different. 1
All values are on a dry matter basis with means ± standard error.
2
These diets were equinitrogenous with 12% crude protein: Diets A, B, and C, respectively, contained 0, 5, and 10% alfafloc (purified wood cellulose). 3
This diet was composed of 90% cerelose, 5% ground corn cob, and 5% animal, vegetable blend fat.
4
The N-correction factor of 8.22 was used.
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Sample
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MUZTAR AND SLINGER
However, results shown in Table 1 were derived using a 24-hr collection period and feed input levels ranged between 20 and 22 g which
was much lower than those used by Shires et al. (1980). The values presented in Table 2 were obtained with a feed input of 32 g but using a 30-hr collection period. The above observations suggest that the lower TME n values compared to the corresponding TME data as reported by Shires et al. (1980) were confounded by feed retention and did not necessarily represent true N retention by the birds. The TME n values in this study were subject to 52 to 54% of variation not related to changes in the N input level. The remaining 46 to 48% of the variation, which was explained as being due to differences in the N input level, suggested that the protein level, in addition to some other factors, had a substantial effect on the magnitude of N correction in the TME assay. This observation is contrary to that of the conventional AME assay, where source and amount of protein had little or no effect on the magnitude of the N correction (Sibbald and Slinger, 1962). Also, an effect of the source of protein on N correction of the TME values was apparent from the data contained in this paper. For instance, N correction did not affect the TME values of SBM significantly, while those of the RSM and RS diets and diets containing these products were generally affected significantly. However, the data of the N-free diet (Table 2) seemed at variance with the idea of an effect of the source of protein, since the difference between TME and TME n of this diet was very similar to those of the RSM, RS diets or diets containing any of these feedstuffs. This latter observation is in agreement with previous findings of Muztar et al. (1978b) indicating the need for considerable time for roosters to acclimate to the utilization of RSM; this could explain why the rapeseed meal diets acted like the N-free diet. The question arises as to whether the TME values should be corrected for N retention or N loss and how such a correction would affect the practical value of the TME assay. It is difficult to provide a definitive answer to this question from data presently available. However, certain points are evident from the results of the present study. First, the birds suffer a net N loss in TME assays so that the TME values are smaller than the TME n values. Second, while the N correction appears to be affected in part by type of protein (SBM vs. RSM) and the level of protein fed, up to 50% of the variation in TME n data is due to unknown
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protein-containing diets or the N-free diet. The N loss was considerably greater in birds fed the N-free diet than in those fed the isonitrogenous diets. As in Experiment 1, there was a highly significant correlation between the TME n values and the N input levels (r = —.679, P<.01, df = 24). Again, however, a substantial portion (54%) of the variation in the TME n data was not associated with changes in the N input level. The regression equation in this case was y = 4.017 - .435 x, y being the TME n value and x the N input level. The results presented in this paper are in contrast to those of Shires et al. (1980). These workers reported an overall gain in body N from feeding of various amounts of corn, SBM, RSM, wheat shorts, and alfalfa to fasted chicks and mature roosters. It is hard to rationalize N being retained in the usual TME assay in view of the fact that the birds are carried through almost a constant 48-hr fast except for a short interruption at about the half-way point when they are force-fed a small meal (20 to 35 g). Thus, the birds are likely to remain in negative N balance throughout the entire experiment. The discrepancy between the present work and that of Shires et al. (1980) probably can be explained in terms of the amounts of feed input and the length of the collection period used. These workers used much higher levels of SBM (up to 59 g/bird), RSM and alfalfa meal (> 36 g/bird), and wheat shorts (> 25 g/bird) and a collection period of only 24 hr. There is substantial evidence that such intake levels of these feedstuffs would not be completely voided in 24 hr (Muztar and Slinger, 1979, 1980; Sibbald, 1979). In one case, for example, the TME value of wheat shorts found by Shires et al. (1980) was 3.07 kcal/g; while Muztar and Slinger (1980) found a value of 2.92 kcal/g based on a 24-hr collection period. This latter value was reduced to 2.88 kcal/g with the extension of the collection period to 36 hr at which time the birds were killed and found to have still retained about 1.5 g of dried residue. The retention was mainly in the gizzard, and only 33 g of the air-dry material was fed. Assuming the residual material to be totally indigestible would give a TME value of 2.76 kcal/g for wheat shorts.
NITROGEN CORRECTION IN TME ASSAY
ACKNOWLEDGMENTS The financial assistance for this work was kindly provided by Rapeseed Utilization and Assistance Program of Canada, the Natural Sciences and Engineering Research Council Canada, the Ontario Ministry of Agriculture and Food and Agriculture Canada. REFERENCES Association of Official Analytical Chemists, 1975. Methods of analysis. AOAC, Washington, DC. Hill, F. W., and D. L. Anderson, 1958. Comparison of
metabolizable energy and productive energy determinations with growing chicks. J. Nutr. 64:587-603. Muztar, A. J., H. J. Likuski, and S. J. Slinger, 1978a. Metabolizable energy content of Tower and Candle rapeseeds and rapeseed meals determined in two laboratories. Can. J. Anim. Sci. 58:485-492. Muztar, A. J., M. Sadiq, and S. J. Slinger, 1978b. Effect of duration of experiment and substitution level on the apparent metabolizable energy content of Brassica napus rapeseed and rapeseed meals. Nutr. Rep. Int. 18:639-646. Muztar, A. J., and S. J. Slinger, 1979. Effect of length of excreta collection period and feed input level on the true metabolizable energy value of rapeseed meal. Nutr. Rep. Int. 19:689-694. Muztar, A. J., and S. J. Slinger, 1981. A comparison of the true and apparent metabolizable energy measures using corn and soybean meal samples. Poultry Sci. (In press). Muztar, A. J., and S. J. Slinger, 1980. Rate of passage of feedstuffs through mature roosters and effect on true metabolizable energy. Nutr. Rep. Int. 22:361-367. Shires, A., A. R. Robblee, R. T. Hardin, and D. R. Clandinin, 1980. Effect of the age of chickens on the true metabolizable energy values of feed ingredients. Poultry Sci. 59:396—403. Sibbald, I. R., 1976. A bioassay for true metabolizable energy in feedingstuffs. Poultry Sci. 55:303—308. Sibbald, I. R., 1979. The effect of the duration of the excreta collection period on the true metabolizable energy values of feedingstuffs with slow rates of passage. Poultry Sci. 58:896-899. Sibbald, I. R., and S. J. Slinger, 1962. The relationship between classical and corrected metabolizable energy values. Poultry Sci. 41:1007-1009. Steel, R.G.D., and J. H. Torrie, 1960. Principles and procedures of statistics. McGraw-Hill Book Co., Inc., New York. Titus, H. W., A. L. Mehring, Jr., D. Johnson, Jr., L. L. Nesbitt, and T. Tomas, 1959. An evaluation of M.C.F. (Micro-Cel-Fat), a new type of fat product. Poultry Sci. 38:1114-1119.
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factors. This is in contrast with the results of conventional AME assays where source and quantity of the dietary protein had no effect on N correction and the variation in AME n data was almost entirely attributable to differences in the classical AME values (Sibbald and Slinger, 1962). Furthermore, the results reported herein suggest that the N retentions obtained by Shires et al. (1980) in the TME assay were the result of feed retention in the gastrointestinal tract and not to N retention in the birds per se. In this regard, it may well be worthwhile to correct TME values to N equilibrium as a means of detecting feed residue retention in the assay. However, it would not appear logical to ascribe additional energy to feeds that result in greatest N losses, which would in fact be the case if TME values were replaced by TME n values in this assay. The variations associated with N correction of TME values raises some doubt as to the validity of the assay as compared with the conventional AME assay.
839
1981 Poultry Science 60:835-839 INTRODUCTION
It has been customary to correct the conventional apparent metabolizable (AME) values for nitrogen (N) retention or N loss. The correction is considered to represent the energy content of N-containing excretory materials temporarily retained in the body. In applying N correction, either one of the two factors 8.22 (Hill and Anderson, 1958) or 8.73 (Titus et al, 1959) have been used. In the former case, uric acid was considered to be the sole N excretory product in the chicken while Titus et al. (1959) considered 8.73 to more truly represent the energy of the excretory products of protein metabolism. Sibbald and Slinger (1962) observed a close relationship between the classical AME and N-corrected AME (AME n ) values based on rapidly growing chickens. These workers also found that the source and amount of dietary protein had little effect on the magnitude of the N-correction. Most data for mature roosters in our laboratory have shown the AME n values to be lower than the corresponding classical AME values by about .01 to .03 kcal/g (Muztar et al, 1978a,b; Muztar and Slinger, 1981). Therefore, application of N correction does not seem to be of major consequence when one considers the amount of time and labor in-
volved in determining the N content of feed and excreta samples. Nevertheless, it does have value as an indicator of the N balance of birds in the AME assay. It is important to have birds close to N equilibrium in AME experiments in order to ensure generation of reliable metabolizable energy data. Muztar et al. (1978b) found mature birds to be in negative N balance on 25 and 50%rapeseedmeal (RSM) or rapeseed (RS) diets, although the experiment involved no fast and the birds on an average consumed about 50 to 60 g of each diet daily for 4 days. The true metabolizable energy (TME) method of Sibbald (1976) involves force-feeding of fasted (24 hr) adult roosters a relatively small quantity of feed. Excreta are collected quantitatively for a period of 24 hr. The correction for metabolic fecal energy ( F E m ) plus endogenous urinary energy (UE e ) losses in the TME assay are based on the losses of unfed birds of similar body weight to the fed birds. The TME method involves a rather abnormal situation in that the birds are probably in negative N balance from the beginning to the end of the experiment. The question thus arises as to whether or not birds are able to retain enough N from the relatively small amount of feed being used in the TME assay (20 to 35 g) to offset their body N loss due to fasting. 835
Downloaded from http://ps.oxfordjournals.org/ at H.E.C. Bibliotheque Myriam & J. Robert Ouimet on June 11, 2015
ABSTRACT This paper examined the effect of N-correction on the true metabolizable energy (TME) values of mixed diets and single ingredients using mature White Leghorn roosters. The N-corrected TME (TME n ) values were consistently higher than the corresponding TME values, the results being significant (P<.05) in about one-half of the samples tested. This indicated a net loss in body N in the birds during the experiments. The correlations between N input and N correction were highly (P<.01) significant. This, however, explained only 46 to 48% of the variation in the TME n data as being due to differences in the amount of N input. In view of the present results and the abnormal condition (fasted birds) in the TME assay, it is doubtful that birds would retain enough N from a single feeding to revert to a positive body N equilibrium. Therefore, positive N retention in such an assay would be indicative of incomplete feed passage through the birds. This appears to be the only valid reason for applying N correction to TME values. However, the variations associated with N correction of TME data raise some doubt concerning the validity of this assay as compared with the conventional apparent metabolizable energy assay. (Key words: TME, N-corrected TME, AME, AME n )
836
MUZTAR AND SUNGER
MATERIALS AND METHODS Experiment I. In this experiment the effect of several mixed diets and single ingredients of varying protein content on the N retention in the TME assay were tested. A corn-soy basal diet was also included. Adult male Single Comb White Leghorn (SCWL) roosters of Shaver strain were fasted for 24 hr to empty their digestive tracts of feed residues. Groups of 5 birds were force-fed each material in pellet form in amounts equivalent to .8% of body weight, which were between 20 and 22 g, air-dry weight. Excreta were collected quantitatively for the next 24 hr, freeze dried, allowed to equilibrate with atmospheric moisture, and weighed. A group of 10 birds was fasted for 24 hr to establish the F E m + UE e losses for the fed birds. Experiment 2. A second experiment was carried out to test the effect of a N-free diet and some isonitrogenous diets, varying in crude fiber content, on N retention or loss in mature roosters in the TME assay. Three isonitrogenous (12% crude protein) corn-soy diets adequate in minerals and vitamins (Muztar et al., 1978a) were formulated. One contained no external source of fiber while the other two included 5 and 10% alfafloc, respectively, at the expense of corn starch in the diet. Another diet, containing no N, and with the composition 90% cerelose, 5% pure ground corn cob, and 5% animal-vegetable fat blend was also included.
The birds were fasted for 30 hr and then force-fed 32 g of the diets in pellet form. Excreta from individual birds were quantitatively collected for 30 hr, freeze dried, allowed to equilibrate with atmospheric moisture, and weighed. Feed and excreta from both experiments were ground to pass a 1-mm screen. Samples of feed and excreta were analyzed for moisture by the AOAC (1975) procedure and for gross energy using an adiabatic oxygen bomb calorimeter. Nitrogen was determined by the Kjeldahl method. The TME values were calculated and adjusted for N retained or lost by the birds using the following equation: TME n = TME (
Total N consumed — Total N excreted ) X 8.22 Feed input
where 8.22 is the energy in kcal/g of N retained or lost by the bird (Hill and Anderson, 1958). The test of significance of TME and TME n results was carried out using the analysis of variance technique and treatment means were compared using Duncan's multiple range test (Steel and Torrie, 1960). Regression analysis was used to measure the effect of the level of N input on N correction of the TME data. RESULTS AND DISCUSSION
The results of Experiment 1 are shown in Table 1. These data are based on 24-hr fast and 24-hr collection periods and feed inputs of 20 to 22 g, air-dry weight. For almost half the number of samples, the TME n values were significantly (P<.05) higher than the corresponding TME counterparts. The TME n values for the remaining samples were also numerically higher than the TME values, but the differences were not significant (P>.05). This indicated that in all cases there was some net N loss by the birds during the experiment. The N loss was greatest and very similar for the birds force-fed the Candle RS, Tower RSM, and the 40% Candle RSM diet. The N input, however, was much greater in the case of Tower RSM compared to either of the Candle RS or the 40% Candle RSM diet (1.11 g vs. .65 and .75 g, respectively). Regression analysis of the TMEn (y) values versus the N (x) input levels showed a highly significant correlation between the two para-
Downloaded from http://ps.oxfordjournals.org/ at H.E.C. Bibliotheque Myriam & J. Robert Ouimet on June 11, 2015
Shires et al. (1980) reported the N-corrected TME (TME n ) values of corn, soybean meal (SBM), and RSM to be substantially lower than the corresponding TME values. This work suggested that both chicks and mature roosters had retained N in excess of their loss in the 24-hr postfeeding period after being fasted for 21 hr. The decrease in TME by application of the N correction varied between 3 and 12% depending upon the feedstuff, with the high protein feeds such as SBM and RSM showing the highest N retention and corn the least. Thus, N retention in the TME assay appeared to be dependent on the protein content of the feedstuff. In view of the apparent anomaly in the results of Sibbald and Slinger (1962) and those of Shires et al. (1980), it was considered important to investigate this question further. A range of protein-containing single ingredients and mixed diets were included in this study.
NITROGEN CORRECTION IN TME ASSAY
837
TABLE I. True metabolizable energy (TME) and N-corrected TME (TMEn) of some mixed diets and single ingredients*,2 TME
Corn-soy basal (B) B + SBM: 60 + 40 B + Tower RSM: 6 0 + 4 0 B + Candle RSM: 6 0 + 4 0 B + Candle R S : 60 + 40 SBM Tower RSM Candle RSM Candle RS
a
TMEn
N loss
N input
(mg/g feed)
(g)
3 . 8 1 ± .03
b
3 . 9 2 ± .05
13.43 ± 2.66
.49 ± .01
a
3 . 5 6 ± .05
a
3 . 6 4 ± .04
10.62 +
.87
.82 + .01
a
3 . 2 9 + .03
b
3 . 4 1 ± .05
15.73 ±
.09
.73 ± .01
3 . 3 5 + .06
17.84 ± 4 . 1 0
.75 ± .01
4 . 5 0 ± .06
5.72 ± 1.91
.62 ± .01
a
3 . 2 1 ± .04
b
a
4 . 4 5 ± .05
a
a
.09 .06 .08 .34
3.29± 2.60 ± a 2.57 ± a 4.57 ± a
a
3.34 2.74 2.60 a 4.72
b a
± ± ± ±
.11 .06 .09 .30
5.87 17.38 3.26 18.38
± 2.09 ± .23 ± .44 ± 2.38
1.29 1.11 1.10 .65
± .01 ± .01 + .01 ± .05
•*' The TME and TME n values for the same sample with a different superscript are significantly (P<.05) different. 1
Results are given as means ± standard error on a dry matter basis.
2
Factor of 8.22 used to bring the TME values to a zero N equilibrium. SBM, = soybean meal; RSM, = rapeseed meal; RS, = rapeseed (whole, full-fat).
meters (r = —.691, P<.01, df = 43). This, however, accounted for only 48% of the variation in the TME n values as being due to changes in the N input level while the remaining 52% was unexplained. The regression equation was y = 5.495 — 2.212 x. Table 2 depicts the results of Experiment 2. The TME n values of the equinitrogenous diets,
although not significantly (P>.05) different from the corresponding TME values, were slightly higher. The TME n value of the N-free diet was significantly (P<.05) higher than its TME value. Thus, from these data and also the N loss figures, it is evident that the birds cannot restore their body N equilibrium in the TME assay when fed 32 g of either of the three
TABLE 2. True metabolizable energy (TME) and N-corrected TME (TMEn) values of some isonitrogenous and a N-free diet* Sample
TME
Corn-soy diet A 2 Corn-soy diet B J Corn-soy diet C 2 N-free diet 3
a
TME n 4 - (kcal/g)-
3.91 ± .04 a 3.68± .03 a 3.50± .04 a 3.89± .01
N loss
N input
(mg/g feed) a
3.93 a 3.71 a 3.55 b 4.02
±.04 ±.02 ±.04 ± .01
2.14 3.81 5.92 15.09
± .82 ± 1.24 ± 1.52 ± .79
(g) .66 .66 .66
ab ' The TME and TME n values for the same sample with a different superscript are significantly (P<.05) different. 1
All values are on a dry matter basis with means ± standard error.
2
These diets were equinitrogenous with 12% crude protein: Diets A, B, and C, respectively, contained 0, 5, and 10% alfafloc (purified wood cellulose). 3
This diet was composed of 90% cerelose, 5% ground corn cob, and 5% animal, vegetable blend fat.
4
The N-correction factor of 8.22 was used.
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Sample
838
MUZTAR AND SLINGER
However, results shown in Table 1 were derived using a 24-hr collection period and feed input levels ranged between 20 and 22 g which
was much lower than those used by Shires et al. (1980). The values presented in Table 2 were obtained with a feed input of 32 g but using a 30-hr collection period. The above observations suggest that the lower TME n values compared to the corresponding TME data as reported by Shires et al. (1980) were confounded by feed retention and did not necessarily represent true N retention by the birds. The TME n values in this study were subject to 52 to 54% of variation not related to changes in the N input level. The remaining 46 to 48% of the variation, which was explained as being due to differences in the N input level, suggested that the protein level, in addition to some other factors, had a substantial effect on the magnitude of N correction in the TME assay. This observation is contrary to that of the conventional AME assay, where source and amount of protein had little or no effect on the magnitude of the N correction (Sibbald and Slinger, 1962). Also, an effect of the source of protein on N correction of the TME values was apparent from the data contained in this paper. For instance, N correction did not affect the TME values of SBM significantly, while those of the RSM and RS diets and diets containing these products were generally affected significantly. However, the data of the N-free diet (Table 2) seemed at variance with the idea of an effect of the source of protein, since the difference between TME and TME n of this diet was very similar to those of the RSM, RS diets or diets containing any of these feedstuffs. This latter observation is in agreement with previous findings of Muztar et al. (1978b) indicating the need for considerable time for roosters to acclimate to the utilization of RSM; this could explain why the rapeseed meal diets acted like the N-free diet. The question arises as to whether the TME values should be corrected for N retention or N loss and how such a correction would affect the practical value of the TME assay. It is difficult to provide a definitive answer to this question from data presently available. However, certain points are evident from the results of the present study. First, the birds suffer a net N loss in TME assays so that the TME values are smaller than the TME n values. Second, while the N correction appears to be affected in part by type of protein (SBM vs. RSM) and the level of protein fed, up to 50% of the variation in TME n data is due to unknown
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protein-containing diets or the N-free diet. The N loss was considerably greater in birds fed the N-free diet than in those fed the isonitrogenous diets. As in Experiment 1, there was a highly significant correlation between the TME n values and the N input levels (r = —.679, P<.01, df = 24). Again, however, a substantial portion (54%) of the variation in the TME n data was not associated with changes in the N input level. The regression equation in this case was y = 4.017 - .435 x, y being the TME n value and x the N input level. The results presented in this paper are in contrast to those of Shires et al. (1980). These workers reported an overall gain in body N from feeding of various amounts of corn, SBM, RSM, wheat shorts, and alfalfa to fasted chicks and mature roosters. It is hard to rationalize N being retained in the usual TME assay in view of the fact that the birds are carried through almost a constant 48-hr fast except for a short interruption at about the half-way point when they are force-fed a small meal (20 to 35 g). Thus, the birds are likely to remain in negative N balance throughout the entire experiment. The discrepancy between the present work and that of Shires et al. (1980) probably can be explained in terms of the amounts of feed input and the length of the collection period used. These workers used much higher levels of SBM (up to 59 g/bird), RSM and alfalfa meal (> 36 g/bird), and wheat shorts (> 25 g/bird) and a collection period of only 24 hr. There is substantial evidence that such intake levels of these feedstuffs would not be completely voided in 24 hr (Muztar and Slinger, 1979, 1980; Sibbald, 1979). In one case, for example, the TME value of wheat shorts found by Shires et al. (1980) was 3.07 kcal/g; while Muztar and Slinger (1980) found a value of 2.92 kcal/g based on a 24-hr collection period. This latter value was reduced to 2.88 kcal/g with the extension of the collection period to 36 hr at which time the birds were killed and found to have still retained about 1.5 g of dried residue. The retention was mainly in the gizzard, and only 33 g of the air-dry material was fed. Assuming the residual material to be totally indigestible would give a TME value of 2.76 kcal/g for wheat shorts.
NITROGEN CORRECTION IN TME ASSAY
ACKNOWLEDGMENTS The financial assistance for this work was kindly provided by Rapeseed Utilization and Assistance Program of Canada, the Natural Sciences and Engineering Research Council Canada, the Ontario Ministry of Agriculture and Food and Agriculture Canada. REFERENCES Association of Official Analytical Chemists, 1975. Methods of analysis. AOAC, Washington, DC. Hill, F. W., and D. L. Anderson, 1958. Comparison of
metabolizable energy and productive energy determinations with growing chicks. J. Nutr. 64:587-603. Muztar, A. J., H. J. Likuski, and S. J. Slinger, 1978a. Metabolizable energy content of Tower and Candle rapeseeds and rapeseed meals determined in two laboratories. Can. J. Anim. Sci. 58:485-492. Muztar, A. J., M. Sadiq, and S. J. Slinger, 1978b. Effect of duration of experiment and substitution level on the apparent metabolizable energy content of Brassica napus rapeseed and rapeseed meals. Nutr. Rep. Int. 18:639-646. Muztar, A. J., and S. J. Slinger, 1979. Effect of length of excreta collection period and feed input level on the true metabolizable energy value of rapeseed meal. Nutr. Rep. Int. 19:689-694. Muztar, A. J., and S. J. Slinger, 1981. A comparison of the true and apparent metabolizable energy measures using corn and soybean meal samples. Poultry Sci. (In press). Muztar, A. J., and S. J. Slinger, 1980. Rate of passage of feedstuffs through mature roosters and effect on true metabolizable energy. Nutr. Rep. Int. 22:361-367. Shires, A., A. R. Robblee, R. T. Hardin, and D. R. Clandinin, 1980. Effect of the age of chickens on the true metabolizable energy values of feed ingredients. Poultry Sci. 59:396—403. Sibbald, I. R., 1976. A bioassay for true metabolizable energy in feedingstuffs. Poultry Sci. 55:303—308. Sibbald, I. R., 1979. The effect of the duration of the excreta collection period on the true metabolizable energy values of feedingstuffs with slow rates of passage. Poultry Sci. 58:896-899. Sibbald, I. R., and S. J. Slinger, 1962. The relationship between classical and corrected metabolizable energy values. Poultry Sci. 41:1007-1009. Steel, R.G.D., and J. H. Torrie, 1960. Principles and procedures of statistics. McGraw-Hill Book Co., Inc., New York. Titus, H. W., A. L. Mehring, Jr., D. Johnson, Jr., L. L. Nesbitt, and T. Tomas, 1959. An evaluation of M.C.F. (Micro-Cel-Fat), a new type of fat product. Poultry Sci. 38:1114-1119.
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factors. This is in contrast with the results of conventional AME assays where source and quantity of the dietary protein had no effect on N correction and the variation in AME n data was almost entirely attributable to differences in the classical AME values (Sibbald and Slinger, 1962). Furthermore, the results reported herein suggest that the N retentions obtained by Shires et al. (1980) in the TME assay were the result of feed retention in the gastrointestinal tract and not to N retention in the birds per se. In this regard, it may well be worthwhile to correct TME values to N equilibrium as a means of detecting feed residue retention in the assay. However, it would not appear logical to ascribe additional energy to feeds that result in greatest N losses, which would in fact be the case if TME values were replaced by TME n values in this assay. The variations associated with N correction of TME values raises some doubt as to the validity of the assay as compared with the conventional AME assay.
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