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METABOLISM AND NUTRITION |
Department of Animal and Poultry Science, University of Guelph, Ontario, Canada, N1G 2W1
1 Corresponding author: sleeson{at}uoguelph.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: metabolizable energy • energy requirement • broiler • body composition
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Metabolizable energy values are commonly corrected for N retention (MEn) to convert all data to a basis of N equilibrium for comparative purposes (Spratt and Leeson, 1987; Sibbald, 1989; Bourdillon et al., 1990a,b; Farrell et al., 1991, 1997; Emmans, 1994; Fisher, 2000; Leeson and Summers, 2001; MacLeod, 2002). The MEn values reported for feedstuffs are usually determined with adult roosters, and this information is used as the basis to formulate diets for different types of birds, including broilers (Dale and Fuller, 1984; Bourdillon et al., 1990a,b; Farrell et al., 1991). Nitrogen correction is used to account for the variable effects of growth and body protein accretion among birds, N retention as eggs, or both (Hill and Anderson, 1958; Miller, 1974; Sibbald and Wolynetz, 1985; Bourdillon et al., 1990b; McNab, 2000). Nitrogen correction is also important if comparisons are to be made across breeds that inherently retain N at different rates. Many studies have indicated that MEn values for broilers are lower than estimates derived from adult Leghorn birds (Mollah et al., 1983; Härtel, 1986; Bourdillon et al., 1990b; Carré et al., 1995; Farrell et al., 1997).
The correction for N retention is made under the assumption that the oxidation of protein tissue will yield uric acid, which has a GE per gram of N of 8.22 kcal (Hill and Anderson, 1958; 2.74 kcal/g of uric acid at 33.33% N). The correction value is added to the excreta energy for each gram of N retained (e.g., if the N had not been retained, it would have been excreted as uric acid). This removes the effect of differences in growth, inherent across birds in any assay. In his extensive review of ME, Sibbald (1982) reported alternate values (8.73 kcal/g of N) as being more representative of the combustion energy of the total mixture of endogenous nitrogenous constituents of chicken urine. Nitrogen correction has also been used to decrease variability of estimates of ME of ingredients varying in protein content (Leeson et al., 1977).
It is generally accepted that correction to zero N is essential in comparison of ME values across species that inherently have differential rates of growth and N retention. Likewise, N correction seems essential for comparison of the ME values determined with juvenile vs. mature birds, because the former will be growing and the latter usually at zero N retention. However N retention is expected to more greatly influence the ME of ingredients such as soybean meal compared with corn because of associated higher protein accretion. A similar effect is expected from diets higher in CP, such as starter diets. The N correction therefore heavily penalizes high-protein ingredients (or diets) and so their MEn is correspondingly reduced. If birds were at similar rates of N retention, then theoretically the N correction is unwarranted, because it biases certain ingredients and diets. It is proposed that such a situation applies with the modern broiler. Nitrogen retention (NR) in fast-growing broilers is expected to vary little from 0 to 49 d as compared with adult roosters. Therefore, ME may be a more appropriate measure of energy used for a commercial broiler nutritionist, because that comparison with other avian species, ages, or both, is not a major consideration.
The objective of the present study was to compare diet AME vs. AMEn for growing broiler chickens and adult roosters.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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) or a series of starter (0 to 21 d), grower (21 to 35 d), and finisher (35 to 49 d) diets (Table 1). The standard diet was a compromise that met the mean nutrient requirements of broilers from 0 to 42 d. When starter, grower, and finisher diets were used, birds were allowed at least 4 d of adaptation to each diet before any excreta collection for energy balance studies.
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Broilers were randomly distributed to cages in a Petersime cage brooder and maintained in these facilities to 14 d. All birds were then subsequently relocated to growing cages (50 x 60 cm). The cage 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. Rooster cages were 25 x 46 cm.
Measurements and Chemical Analyses
Energy and N balance were determined over 7 periods: 4 to 7, 11 to 14, 18 to 21, 25 to 28, 32 to 35, 39 to 42, and 46 to 49 d of age. For each period, the reported values are the averages of the corresponding response variables based on 3-d data, whereas the value of the predictor variable was set as the average day for such a period. Total excreta were collected over 3-d periods, being captured on Al trays directly beneath each cage. Excreta collection times for roosters coincided with those from the broilers. Birds were also weighed at the beginning and end of each collection period to determine mean BW for each collection period. During the collection period, any spilled feed was carefully removed from the excreta and weighed. At the end of the collection period, excreta samples were wrapped in Al foil and then oven-dried (Hot-pack, Waterloo, Ontario, Canada) at 45°C to constant weight and ground to a consistent particle size (1-mm screen size; Wiley Mill, Chicago, IL). Feed samples were ground using a commercial food blender. Samples of the diets and excreta were assayed for GE by C5003 IKA adiabatic oxygen bomb calorimeter (GMBIT and CO KG D-79219, IKA Works Inc., Wilmington, NC). Diets and excreta were assayed for total N using a Leco FP-428 N analyzer (Leco Instruments, Stockport, Cheshire, UK). Nitrogen retention was calculated as the difference between N intake and N in the excreta.
The AMEn 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).
Statistical Analyses
One-way ANOVA was used to test the effect of dietary treatment on response variables within weeks: difference between AME and AMEn per kilogram of diet (
AMEkg), difference between AME and AMEn as a percentage (AME%), live BW gain (ratios NR:LBWG, AME:LBWG, AMEn:LBWG, LBWG:AME, and LBWG:AMEn). Means of response variable, having significant F-statistic (P < 0.05), were further analyzed using Duncan’s multiple range test. The statistical model used was:
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where Yij = average AMEkg, AME%, NR:LBWG, AME:LBWG, AMEn:LBWG, LBWG:AME, and LBWG: AMEn obtained from treatment i in cage j; µ = general mean; 
i = effect of the ith treatment on the response variable Yij, i =1, 2; and 
ij = random variables with mean 0, variance 
2 independent and normally distributed. Under this design, the hypotheses of interest were: Ho:1 = 2 vs. Ha:1 
2.
Response variable was also analyzed as a function of time. In that case, the response variable Yij corresponds to the variable of interest (AME, AMEn, AMEkg, AME%, NR:LBWG, AME:LBWG, AMEn:LBWG, LBWG:AME, and LBWG:AMEn during week i).
The statistical model was:
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where Yij = AME, AMEn, AMEkg, AME%, NR:LBWG, AME:LBWG, AMEn:LBWG, LBWG:AME, and LBWG:AMEn; ij 
n (0, 2), i = the effect of the ith week; and µ = an overall mean.
To study the effect of time on AME, AMEn, daily AME intake (AMEd), daily AMEn intake (AMEnd), percentage N retention (%N), NR, and ratios AME:LBWG, AMEn:LBWG, LBWG:AME, LBWG:AMEn, NR:LBWG, AMEkg, AME%, regression analysis was performed. The given values of AME, AMEn, AMEd, AMEnd, %N, and NR were obtained from sample excreta determinations and were modeled as:
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where Yi = the appropriate response variables of AME, AMEn, AMEd, AMEnd, N, NR, ratios AME:LBWG, AMEn:LBWG, LBWG:AME, LBWG:AMEn, NR:LBWG, AMEkg, AME%, at the ith time; ß0, ß1, ß2 = regression coefficient; Xi = elapsed time (d); and i = random variable assumed to be normal with mean 0 and variance 2.
Analysis was performed by bird type. The SAS GLM procedure (SAS Institute, 2000) was used to fit the regression models and PROC ANOVA was used to obtain the ANOVA.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. The young 7-d-old broiler seems to derive less energy than birds 14 d and older, and the magnitude of this effect is not influenced by N correction (P < 0.001, Table 2). Using the single diet after 7 d, the broiler consistently shows higher AME than does the rooster (P < 0.05, Table 2). However, when N correction is applied, the converse situation is seen, in that roosters consistently exhibit higher AMEn than do broilers at any given age. The higher AME for broilers vs. roosters is due to the greater N retention and the subsequent penalty for N correction in the younger birds.
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summarizes regression analyses of changes in AME and AMEn over time for broilers and roosters. By virtue of experimental design, an increase in AME and AMEn is expected over time (P > 0.05) with the multiple diet series, because this was inherent in formulation. Using a single diet, the rooster AME and AMEn values are unaffected by time, and this is to be expected considering the relatively short time of 49 d within the life cycle of roosters (P > 0.05). Broilers using a single diet, on the other hand, exhibit a quadratic relationship for both AME and AMEn over time (P < 0.001, Table 3), and this is clearly depicted in Figure 1. It should be noticed that although some R2 are not high (e.g., R2 = 0.50), the regression coefficient is still significant (P < 0.01). The AME and AMEn for the multiple diets fed over time for roosters and broilers are shown in Figures 2 and 3, respectively. Because roosters are expected to be at close to zero N balance, AME and AMEn are similar over time (Figure 2) with the day coefficient being negative.
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. As expected, N retention of roosters was rarely different from zero (P > 0.05). For broilers, there was a significant (P < 0.01) increase in N retained each day over time, although when expressed as a percentage of N intake, there was decline over time, especially after 28 d of age. These data are shown more clearly in Figures 4 and 5, respectively.
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. Values were positive, indicating lower values for AMEn. Results from Table 5 show AMEn values were consistently proportional to AME values when single diets are used over time (4 to 5%). When a commercial series of diets was used, the correction declined from 5.3% at 7 d to 3.8% at 49 d. In terms of providing data for modeling of broiler energy and N metabolism over time, Tables 6, 7, and 8 provide details of bird response relative to growth rate, period, or both. Table 6 depicts AME and AMEn intake for broilers over time, whereas Table 7 details actual N retention per unit of live weight gain. Table 6 shows that broilers constantly increase their daily AME or AMEn intake regardless of the feeding program. This AME intake showed a quadratic trend during the experimental period (P < 0.05, Table 6). This information indicates that in broilers, feed intake continues to increase, whereas the N retention (or lean growth capacity) declines as birds get fatter over time.
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indicates that N retention per unit of weight gain varies linearly as a function of time and that such retention is significantly different for different times (d; P < 0.001). The grams of N retained (birds/d) per unit of live BW gain increases as birds age (P < 0.001, Table 7), especially in broilers fed with a single diet. Table 8 details weekly changes in AME and AMEn intake per unit of live weight gain, indicating an almost 100% increase in energy requirement per unit of gain in 49-d vs. 7-d broilers. Comparable estimated variance from the regression analysis provided values of 1.236 vs. 1.234 for single diets and 1.236 vs. 1.135 for multiple diets when defining AME or AMEn:LBWG, suggesting the N correction adds little to precision in defining this particular measure of energy efficiency. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The argument against N retention is that it is not a biological norm, and with specialization in poultry nutrition today, species comparison for energy evaluation is not too important. If all broilers in a bioassay therefore retained N at similar rates, then a classical uncorrected AME would seem to be a more logical basis for feed formulation. As shown in Table 5, the N correction imposes a 4 to 5% penalty on the AME evaluation. When commercial-type multiple diets are used, the correction declines from 5.3% at 7 d to 3.8% at 49 d, reflecting a decline in protein content of the diet and a decline in N retention (as a fraction of N intake) over time (Figure 5). These values for the magnitude of N correction agree reasonably well with the 4.2% value reported by De Groote (1974) and values of 3 to 5% determined with slower-growing Leghorn and Barred Plymouth Rock strains (Lopez, 2006). It is well-recognized that ME content of dietary protein is greater when used for deposition of body protein than for catabolism to yield nutrients for purposes other than protein deposition (Kleiber, 1975). In poultry, assuming no protein turnover, the maximal ME per gram of protein for production of tissue is 5.7 kcal (equal to the GE of protein, assuming 100% efficiency of utilization), whereas for protein that is fully catabolized, the value is 4.39 kcal [5.7– (0.16 x 8.22); De Groote, 1974], where 0.16 = the average N content of protein; 8.22 = kilocalories per gram of N; and 4.39 = the ME value of digestible protein for purposes other than protein deposition. The difference (1.31 kcal) is the energy excreted as uric acid per gram of protein catabolized (8.22/6.25). Assuming that protein turnover occurs independent of the diet, supplying the energy for maintenance (and thus turnover) would imply an additional energy cost to synthesize uric acid. It has been estimated that the energy expenditure associated with uric acid synthesis is about 330 kcal/mol (Buttery and Boorman, 1976). In addition, there is an indication that dietary protein per se stimulates protein turnover (Kita et al., 1993), increasing the energy cost to the bird and concomitantly lowering the available energy value of the protein source.
An argument in favor of not correcting for N retention is that AME more closely resembles biological process in the growing broiler. However, such a system would only be useful if assays were precise and accurate, a situation that initially led to correction to zero N retention being accepted as the standard system. Kleiber (1975) did not support N correction as a means of reducing variance within bioassays. Likewise, amino acid balance of diets may affect AME, and certainly differential growth rates among birds within a bioassay will impart variance. Leeson et al. (1977) also showed that AME values for diets or ingredients varied depending upon the protein, amino acid, or both, level of the diet and that N correction resolved this issue. It is obvious that if AME values are to be considered, then there needs to be much more uniform control over bird type and diet formulation. The actual variation seen in the current results (Table 2, means ± SE values) supports the contention that independent of time, the variability among birds is high and that N correction slightly improves accuracy of results, reducing residual variance by 15% and improving R2 of the estimated model.
Alternatives have been proposed for correction other than to zero N retention. Larbier and Leclercq (1994) suggested a correction based on the BW of the bird, being in the order of 32 g of N/kg of weight gain. These authors indicated improved precision of the assay results with such a correction vs. AME. This approach assumes that the correction factor is static regardless of bird age or bird weight. Our data (Table 5) indicates the magnitude of the correction to change over the commercial life cycle of the broiler, albeit by a small amount. However, in attempting to minimize all sources of variance, a single correction factor based on weight may contribute to differences in young vs. older broilers. Another approach has been to adjust N balance to a standard biological norm rather than to zero (Morgan, 1972). This approach favors species-specific value. Even over the relatively short lifespan of the broiler, the percentage of N retention changes (Figure 5) and so a single correction factor, such as to 60% retention, would be something of a compromise.
Another issue raised by the current research is the validity of using roosters to determine diet energy values for use in broiler nutrition. Roosters are often used for ME bioassays, because they can be maintained for long periods of time, eat relatively much feed, and adapt quickly to diet change. If AME values are accepted, then roosters provide a good estimate of values obtained with broilers, and where differences occur, the rooster values are lower (Table 2). However, when AMEn is calculated, the rooster values are consistently higher than determined with broilers. For the 7-d broiler, the difference is around 7% in favor of the rooster, whereas at other ages, the difference is around 2%.
Because there is less variance in energy values expressed as AMEn rather than AME, it seems as though there is sufficient bird-to-bird variation in growth, N retention, or both, to warrant the use of the correction factor. If N correction was to be held to a more biologically norm, rather than zero, then 60% retention seems a good compromise. Correction to zero N retention has the most effect on high-protein diets (e.g., starter diet, Table 2), high-protein ingredients, or both. In most poultry diets therefore, the N correction system penalizes ingredients such as soybean meal compared with corn. In associated studies, we are determining the AME and AMEn of corn and soybean meal and then using these values to formulate diets and examining the economics of broiler production and predictability of broiler performance.
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ACKNOWLEDGMENTS |
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Received for publication December 8, 2006. Accepted for publication April 16, 2007.
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