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Metabolizable Energy of Supplemental Fat as Related to Dietary Fat Level and Methods of Estimation1
Metabolizable Energy of Supplemental Fat as Related to Dietary Fat Level and Methods of Estimation1 GONZALO G. MATEOS and JERRY L. SELL Department of Animal Science, Iowa State University, Ames, Iowa 50011 (Received for publication December 11, 1980)
1981 Poultry Science 60:1509-1515 INTRODUCTION Supplemental fats are difficult to evaluate in terms of their true energy contribution to practical diets. Two methods have been employed to estimate the energy contribution of fats to diets. 1) The digestibility of the fat is determined by relating the lipid content of diet to the lipid content of excreta. Then, the metabolizable energy (ME) of the supplemental fat is calculated by multiplying the gross energy content of the fat times the digestibility. 2) The fat is substituted into a diet at low levels. A direct measurement is made of the ME of the reference and test diets, and the ME of the fat is calculated by difference. Results obtained by these two methods, however, do not always agree, and, consequently, estimates of the energy contribution of supplemental fats to poultry diets have been inconsistent. The first method assumes that lipid digestibility is independent of the composition of the diet and of the level of supplemental fat used. This assumption, however, may be erroneous. For example, Leeson and Summers (1976) and Sibbald (1978) suggested that a synergism between saturated fatty acids of a supplemental fat of animal origin and unsaturated fatty acids
'journal Paper No. J-10116 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA. Project 2240.
inherent in the grain portion of a diet enhanced the utilization of the supplemental fat. Therefore, added fat may exhibit a greater ME when included in diets containing relatively high levels of unsaturated fatty acids. Indeed, Sibbald and Kramer (1978) observed that the true metabolizable energy of tallow was greater in a corn- than in a wheat-based diet, thereby lending support to the suggestion of fatty acid synergism. Sues (1974) reported an inverse relationship between lipid digestibility and level of fat supplementation. In contrast, Gomez and Polin (1974) reported no change, or slight improvement, in lipid digestibility with increasing levels of fat supplementation. The second method of estimating the energy contribution of fats to diets assumes that the ME's of the dietary ingredients are additive and that supplemental fat does not alter the utilization of other dietary constituents. Numerous authors (Cullen et al., 1962; Gomez and Polin, 1974; Sibbald and Kramer, 1977; Sell et al, 1979; Mateos and Sell, 1980a), however, have reported ME's for fat that exceeded the heat of combustion of this energy source; therefore, it seems that fats and other ingredients probably are not additive with respect to energy utilization by poultry. Kalmbach and Potter (1959) presented evidence that an interaction between dietary constituents and supplemental fat affected ME of the diet. They observed that corn oil and
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ABSTRACT White Leghorn laying hens were fed practical diets containing 0, 5, 10, 15, 20, 25, or 30% yellow grease. The nitrogen-corrected metabolizable energy (ME) of yellow grease was estimated from apparent digestibility data and from ME's of the diet determined experimentally. The ME's of yellow grease were consistently higher when dietary ME data were used in the calculations than when lipid digestibility data were used (8380, 8770, 8567, 9050, 8408, and 8473 kcal/kg vs. 8360, 8344, 8278, 8456, 8212, and 8540 kcal/kgfor diets containing 5, 10, 15, 20, 25, and 30% fat, respectively). A comparison also is presented of ME values for yellow grease obtained by conventional calculation or by use of regression analysis. Regardless of the method of calculation, the ME data indicated that supplemental yellow grease exerted an extrametabolic effect on dietary ME, although the magnitude of this effect varied with the mathematical approach used. (Key words- fat, metabolizable energy, lipid digestibility, laying hens, yellow grease)
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MATERIALS AND METHODS Single Comb White Leghorn hens, 28-weeks of age, were kept in individual cages located in an environmentally controlled room at 27 C. Feed and water were offered ad libitum. Seven experimental groups were used, and five individually caged hens were assigned randomly to each group. One group received a control diet containing no added fat. Increments of 5% yellow grease, ranging from 5 to 30% of the diet, were included in the diets fed to the other six experimental groups (Table 1). Appropriate adjustments were made in the ingredient levels of the fat-supplemented diets so that relatively constant ME to nutrient ratios were maintained. After a 7-day adaptation period, the hens were fed their respective experimental diet for another 5 days. Feed consumption was recorded, and excreta were collected on a quantitative basis every 60 hr. The diets and excreta samples were freeze-dried, were allowed to come to equilibrium with atmospheric moisture, and were analyzed for dry matter, heat of combustion, nitrogen, chromic oxide, and total lipid content. Chromium content of diets and excreta was determined by atomic absorption spectrophotometry (Perkin-Elmer, 1973). The nitrogen-corrected ME's of the diets were calculated from data obtained by both total collection and chromic oxide index methods. The average ME of both methods obtained for each hen was used in subsequent calculations. Ten additional hens were used to determine the ME of the corn and soybean meal used. Two diets were formulated containing corn and
soybean meal exclusively as energy-contributing ingredients (Table 2). Each of these two diets was fed to 5 hens, and excreta were collected totally and were analyzed. The ME of corn and soybean meal was determined by resolution of simultaneous equations using the data obtained. The ME of the sunflower meal was calculated by using the ME values determined for corn, soybean meal and the control (no added fat) diets. It was assumed that the ME's of the corn, soybean meal, and sunflower meal were additive. The ME of yellow grease was obtained by using two different procedures. In procedure 1, lipids were extracted from all diets and feces by the procedure described by the Association of Official Analytical Chemists (Procedure 7047, AOAC, 1975). The apparent digestibility of yellow grease for each level of supplemental fat was calculated and the ME of yellow grease was estimated by multiplying the gross energy content of yellow grease times the apparent digestibility. In procedure 2, the ME of yellow grease was calculated by assuming that ME's of dietary ingredients were additive. Therefore, the ME supplied by the corn, soybean meal, and sunflower meal was subtracted from the ME obtained experimentally for the fat-supplemented diets. The ME of yellow grease was obtained by dividing this difference by the level of supplemental fat. The experimental data for ME of diets and digestibility of lipids were regressed on level of fat in the test diets. Data obtained from these regressions were used for further comparisons. RESULTS The ME of the diets, as determined experimentally, increased with each increment of supplemental yellow grease (Table 3). A comparison of the calculated and determined ME's indicated that changes in dietary ME's associated with supplemental fat were larger in the latter instance. The ME's obtained for corn and soybean meal were 3427 and 2468 kcal/kg, respectively. These values were in good agreement with those listed by the National Research Council (1977) (3430 and 2440 kcal/kg, respectively). The ME data obtained for the corn and soybean meal diets were used to estimate the ME's of yellow grease by three different procedures: 1) conventional method; 2) regression equation, linear model; and 3) regression equation, quadratic model.
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tallow had higher ME's when substituted for a portion of a basal diet than when added at the expense of cerelose. Mateos and Sell (1980b) reported that the ME of yellow grease varied with the specific carbohydrate used as a main dietary constituent. Recently, Mateos (1980) postulated that supplemental fats may enhance energy utilization from diets by slowing rate of food passage (ROP). A reduction in ROP could result in a more complete digestion and utilization of the diet (Tuckey et al, 1958). The objective of the research reported here was to estimate the ME of yellow grease in practical diets for laying hens by using actual ME determination and lipid digestibility data. Also studied was the influence of the level of fat inclusion on the relative changes in dietary ME and the estimated ME of yellow grease.
ME OF FAT AND FAT LEVEL
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TABLE 1. Composition of the experimental diets, % Suppl emental fat (%) Ingredient
10
5
20
15
25
30
25.00 4.60 61.28
25.00 8.32 52.13 5.00
25.00 11.87 43.10 10.00
25.00 15.55 33.90 15.00
25.00 19.18 24.80 20.00
25.00 22.87 15.70 25.00
25.00 26.51 6.55 30.00
.01 .06 .30
.03 .02 .30
.03
.05
.07
.08
.09
.30
.30
.30
.30
.30
1.30 6.65
1.40 7.00
1.60 7.30
1.70 7.70
1.85 8.00
1.90 8.3 5
2.00 8.75
.30 .50
.30 .50
.30 .50
.30 .50
.30 .50
.30 .50
.30 .50
2600 14.62
2772 15.62
2944 16.55
3113 17.52
3285 18.48
3457 19.47
3628 20.44
.28 .61
.31 .65
.32 .73
.35 .82
.38 .92
.39
.41
1.02
1.11
Yellow grease (Association of American Feed Control Officials, 1981, number 33.20, IFC number 4-00-409, Animal Fat, Feed Grade), obtained from National Byproducts Co., Des Moines, IA had by analysis the following fatty acid composition: myristic, 1.8%; palmitic, 24.7%; palmitoleic, 2.6%; stearic, 15.8%; linoleic, 9.7%; oleic, 44.4%; linolenic, .6%; others, .4%. Supplied the following per kilogram of diet: vitamin A, 8000 IU; vitamin D 3 , 2400 IU; vitamin B 1 2 , 5 Mg; riboflavin, 6.6 mg; calcium pantothenate, 6.6 mg; niacin, 2.2 mg; choline, 440 mg; ethoxyquin, 11 mg. c
On the basis of National Research Council (1977) values for soybean meal and corn. The ME's of 7900 and 1540 kcal/kg were used for yellow grease and high-fiber sunflower meal, respectively. The moisture contents of corn, soybean meal, and sunflower meal were 10.6, 10.2, and 8.8%, respectively.
Conventional Methods for Estimating the ME of Yellow Grease The ME determined for the control (no added fat) diet was 2608 kcal/kg. Assuming additivity of ME values of dietary ingredients in the control diet, the ME of sunflower meal was obtained by subtracting the ME supplied by the corn (.6128 X 3427 = 2100 kcal/kg) and by the soybean meal (.460 X 2468 = 113 kcal/kg) from the ME of the control diet (2608 kcal/kg). Thus, the energy supplied by sunflower meal to the diets was 2608 - 2213 = 395 kcal/kg, and the ME of sunflower meal was 395/25 X 100 = 1580 kcal/kg. By using these data and the dietary ME values obtained for each test diet, the ME of yellow grease was estimated. For example, in the diet containing 10% yellow grease, corn supplied 1477 kcal of ME to the diet (.4310 X 3427), soybean meal supplied 293 kcal (.1187 X 2468), and sunflower meal supplied 395 kcal (.25 X 1580). The ME contributed by yellow grease to this diet was assumed to be the difference between the determined ME of the diet (3042 kcal/kg) and the energy supplied by
the other three ingredients (1477 + 293 + 395 = 2165 kcal/kg). Thus, the ME of yellow grease in diet 3 (10% yellow grease), assuming additivity of ME's of dietary ingredients, was (3042 — 2165/10) X 100 = 8770 kcal/kg. The ME values of yellow grease obtained by this method of calculation were 8380, 8770, 8567, 9050, 8408, and 8473 kcal/kg for the 5, 10, 15, 20,
TABLE 2. Composition of the diets used to determine the metabolhable energy (ME) of soybean meal and com, % Ingredient
Diet A
Diet B
Yellow corn Soybean meal, 48.5% protein Calcium carbonate Dicalcium phosphate Salt Vitamin premix a
70.0 20.0
50.0 40.0
7.8 1.4 .3 .5
7.8 1.4 .3 .5
Supplied the same quantities per kilogram of diet as described in Table 1.
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Sunflower meal, 28% protein Soybean meal, 48.5% protein Yellow corn Yellow grease a DL-methionine L-lysine Salt Dicalcium phosphate Calcium carbonate Chromic oxide Vitamin premix 0 Calculated analysis: ME (kcal/kg)c Crude protein, % Methionine, % Lysine, %
0
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TABLE 3. Nitrogen-corrected metabolizable energy (ME) of the experimental diets, kcal/kg (air dry basis) Determined ME
Calculated 3
Supplemental fat (%)
Total collection
ME
2600 2772 2944 3113 3285 3457 3628
0 5 10 15 20 25 30
2602 ± 2 8 2816 + 18 3023 ± 31 3226 ± 33 3550 ± 57 3588 ± 4 1 3812+36
b
Cr203
Average
2613 2800 3062 3226 3507 3611 3819
2608 2805 3043 3226 3528 3599 .3815
± 28 ± 29 ± 46 ±23 ±40 ±21 + 32
±12 ± 24 + 36 ± 28 ±47 ±25 ±25
25, and 30% yellow grease-supplemented diets, respectively (Table 4). The ME of yellow grease also was estimated by multiplying the lipid digestibility coefficient (Table 5) times the gross energy of yellow grease (9375 kcal/kg). The resulting ME's were 8360, 8344, 8278, 8456, 8212, and 8540 kcal/kg for the 5, 10, 15, 20, 25, and 30% yellow grease-supplemented diets, respectively. In general, the ME values obtained for yellow grease from lipid digestibility data were smaller than those obtained by assuming additivity of ME's of dietary ingredients.
Slinger and Muztar (1980) advocated the use of regression-equation analysis (ME of diet on level of supplemental fat) to obtain more reliable estimates of the ME values of supplemental fats. The data obtained from the experiment reported here were used for exploring this statistical approach. Apparent lipid digestibility and dietary ME data each were regressed on level of supplemental fat in the diet. The model used was y = a + bx + ex 2 + 2 , where y equals lipid digestibility or dietary ME and x equals level of supplemental fat. The apparent lipid digestibility of diets was independent of level of supplemental yellow grease. The linear (b-factor) and quadratic (c-factor) components of the regression model were small and not significantly different from zero (P>.10). Therefore, the apparent lipid
Estimation of ME's of Yellow Grease with Regression Analysis, Linear Model Recently, Mateos and Sell (1980a,b) and
TABLE 4. Nitrogen-corrected metabolizable energy (ME) of yellow grease, assuming additivity of ME of ingredients, kcal/kg (air dry basis)
Supplemental fat (%) 0 5 10 15 20 25 30
ME supplied byb
Determined 3 ME of diet
Corn
A
B
2608 2805 3042 3226 3528 3599 3815
2100 1786 1477 1162 850 538 224
SBM C
SFM D
113 205 293 384 473 564 654
395 395 395 395 395 395 395
Experimental data are averages of 5 hens per diet ME (kcal/kg) of each ingredient times percentage of ingredient in diet. Column A —columns B + C + D.
Column C divided by the percentage of fat inclusion.
ME supplied bv c yellow grease
MEof d yellow grease
E
F
419 877
1285 1810 2102 2542
8380 8770 8567 9050 8408 8473
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On the basis of NRC (1977) values for corn and soybean meal. The ME values 1540 and 7900 kcal/kg were used for high-fiber sunflower meal and yellow grease, respectively. Mean ± SE.
ME OF FAT AND FAT LEVEL
Determination of the ME's of Yellow Grease •with Regression Analysis, Quadratic Model Although t h e regression line relating ME and level of fat of t h e diets c o n f o r m e d t o a linear m o d e l , t h e quadratic c o m p o n e n t (c-factor) a p p r o a c h e d significance (P=.09). When t h e quadratic effect was included in t h e model, t h e ME's of t h e diets were defined by t h e e q u a t i o n y = 2 5 9 1 + 4 8 x - . 2 4 x 2 . On t h e basis of ME for diets calculated from this regression e q u a t i o n a n d following t h e p r o c e d u r e described previously, a n o t h e r set of ME's was calculated for
yellow grease. T h e data presented in Table 7 s h o w t h a t t h e resulting ME's varied from 9 1 2 0 kcal/kg ( 5 % fat) t o 8 5 2 0 kcal/kg (30% fat). There was n o quadratic effect of level of supplemental fat on lipid digestibility, b u t for comparative purposes, t h e ME's of yellow grease estimated by using data from regression analysis of lipid digestibility are presented in Table 7. DISCUSSION In general, t h e ME values obtained for yellow grease were smaller w h e n lipid digestibility data were used in t h e calculations t h a n w h e n data of d e t e r m i n e d metabolizable energy were used. Similar differences b e t w e e n M E of fats estimated b y these t w o procedures have b e e n r e p o r t e d with broiler chicks ( G o m e z and Polin, 1 9 7 4 ) and turkeys (Whitehead and Fisher, 1 9 7 5 ) . These differences m a y represent t h e effects of fat on t h e utilization of energy from o t h e r dietary constituents. If t h e ME of yellow grease estimated from lipid digestibility data is a reliable estimate of t h e ME c o n t e n t of t h e fat, t h e n t h e " e x t r a " ME generally associated with yellow grease o n t h e basis of m e a s u r e d changes in dietary ME m u s t be derived from o t h e r dietary c o m p o n e n t s . T h e m a g n i t u d e of t h e differences b e t w e e n t h e ME's of yellow grease estimated b y t h e t w o m e t h o d s varied with level of supplemental fat when t h e conventional a p p r o a c h t o ME d e t e r m i n a t i o n was used ( 2 0 , 4 2 6 , 2 8 9 , 594, 196, and - 6 7 kcal/kg of yellow grease for t h e diets s u p p l e m e n t e d with 5, 10, 1 5 , 20, 2 5 , and 30% yellow grease, respectively) (Tables 4 and 5). When t h e linear
TABLE 5. Apparent lipid digestibility of the experimental diets and ME of yellow grease estimated from lipid digestibility data
Supplemental fat (%)
Total lipid ingested (g in 5 days)
0 5 10 15 20 25 30
15.51 + 40.80 ± 58.53 + 74.11 ± 103.23 ± 106.69 + 123.74 ±
1.671 2.01 3.81 2.86 10.15 7.25 5.70
Apparent lipid digestibility
ME* (kcal/kg)
77.32 ± 87.99 ± 88.14+ 87.61 ± 89.74+ 87.09 ± 90.69 ±
8360 8344 8278 8456 8212 8540
4.27 1.14 1.43 2.58 3.95 4.30 1.51
The gross energy of the yellow grease was 9375 kcal/kg. The ME was calculated by multiplying lipid digestibility times gross energy and dividing by 100. b. Mean ± SE.
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digestibility coefficients of t h e diets were assumed t o be a c o n s t a n t 8 8 . 5 4 % . T h e ME of yellow grease, calculated o n this basis, was 8 3 0 1 kcal/kg (.8854 X 9 3 7 5 kcal/kg), irrespective of supplemental fat level. Regression of dietary ME on level of supplem e n t a l fat showed t h a t t h e ME's of t h e diets were defined by t h e e q u a t i o n y (kcal/kg) = 2 6 2 1 + 4 0 . 7 X- T h e linear c o m p o n e n t (b-factor) was highly significant ( P < . 0 0 0 1 ) , b u t t h e quadratic c o m p o n e n t (c-factor) did n o t reach a significant level ( P » . 0 5 ) . Therefore, only t h e linear regression e q u a t i o n was used t o estimate t h e ME of yellow grease in this instance. T h e ME of yellow grease for each level of fat was o b t a i n e d by subtracting t h e ME supplied by t h e o t h e r dietary c o n s t i t u e n t s from t h e ME of t h e test diets calculated b y t h e linear regression e q u a t i o n . T h e p r o c e d u r e used was analogous t o t h a t described previously in section I of t h e results and is presented in Table 6. T h e data s h o w t h a t t h e ME of yellow grease was 8 5 0 0 kcal/kg, irrespective of level of fat used.
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TABLE 6. Metabolizable energy (ME) of yellow grease by using linear regression equations and assuming additivity of the nitrogen-corrected ME's of ingredients, kcal/kg (air dry basis) ME supplied byb
Supplemental fat (%) A
MEof a diet B
Corn C
SBM D
SFM E
ME su pplied b y c yellow' grease F
MEof d yellow grease G
ME of YG based e on LD data H
0 5 10 15 20 25 30
2621 2824 3028 3232 3435 3639 3843
2100 1786 1477 1162 850 538 224
113 205 293 384 473 564 654
408 408 408 408 408 408 408
425 850 1275 1700 2125 2550
8500 8500 8500 8500 8500 8500 8500
8301 8301 8301 8301 8301 8301 8301
ME of each ingredient (kcal/kg) times percentage of ingredient in diet. Column B — Columns C + D + E. Column F divided by column A. ME of yellow grease on the basis of lipid digestibility data; ME (kcal/kg) = apparent lipid digestibility X gross energy of yellow grease, divided by 100.
regression model was used to estimate the ME of yellow grease, the difference was 199 kcal/kg of fat, irrespective of fat level (Table 6). The use of data obtained by the quadratic regression equation (Table 7) yielded differences of 819, 679, 579, 464, 335, and 219 kcal/kg of yellow grease for supplemental levels of 5, 10, 15, 20, 25, and 30%, respectively. These data illustrate two points. First,
supplemental yellow grease exerted a fairly consistent extracaloric effect in practical laying hen diets when changes in dietary ME's associated with supplemental fat were used as the criteria. Also, the results clearly show that the magnitude of this extracaloric effect varied considerably, depending on the procedure used in evaluating the data. Two of the procedures used herein to
TABLE 7. Metabolizable energy (ME) of yellow grease by using regression equation including the quadratic component in estimating the nitrogen-corrected ME of the experimental diets, kcal/kg (air dry basis) ME supplied byh
Supplemental fat (%) A
MEof a diet B
Corn C
SBM D
0 5 10 15 20 25 30
2591 2825 3046 3256 3454 3639 3812
2100 1786 1477 1162 850 538 224
113 205 293 384 473 564 654
SFM E
ME supplied by c yellow' grease F
MEof d yellow grease G
ME of YG based e on LD data H
378 378 378 378 378 378 378
456 898 1332 1753 2159 2556
9120 8980 8880 8765 8636 8520
8301 8301 8301 8301 8301 8301
ME (kcal/kg) = 2591 + 48 (% supplemental fat) - .24 (% supplemental fat) 2 . ME of each ingredient (kcal/kg) times percentage of ingredient in diet. Determined ME values were: corn, 3427 kcal/kg; soybean meal 2468 kcal/kg; sunflower meal, 1512 kcal/kg. C
B - (C + D + E).
d
F divided by A.
On the basis of lipid digestibility data, ME yellow grease (kcal/kg) = 88.54 X gross energy of yellow grease divided by 100.
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ME of diet (kcal/kg) = 2621 + 40.7 (% supplemental fat).
ME OF FAT AND FAT LEVEL
ACKNOWLEDGMENT T h e research was s u p p o r t e d , in part, by a grant from t h e Fats and Protein Research F o u n d a t i o n , Des Plaines, IL.
REFERENCES Association of American Feed Control Officials, 1981. Pages 116-117 in Official publication, 1981. Ass. Feed Control Offic, Inc. Association of Official Analytical Chemists, 1975. Official methods of analysis. 12th ed. Washington, DC. Cullen, M. P., O. G. Rassmussen, and O.H.M. Wilder, 1962. Metabolizable energy value and utilization of different types and grades of fat by the chick. Poultry Sci. 41:360-367. Gomez, M. X., and D. Polin, 1974. Influence of cholic acid on the utilization of fats in the growing chicken. Poultry Sci. 53:733-781. Halloran, M. R„ and I. R. Sibbald, 1979. Metabolizable
energy values of fats measured by several procedures. Poultry Sci. 58:1299-2307. Kalmbach, M. P., and L. M. Potter, 1959. Studies in evaluating energy content of feeds for the chick. 3. The comparative values of corn oil and tallow. Poultry Sci. 38:1217. (Abstr.) Leeson, S., and J. D. Summers, 1976. Fat metabolizable energy values: the effect of fatty acid saturation. Feedstuffs 48(46): 2 6 - 2 8 . Mateos, G. G., 1980. Nature of the extrametabolic effect of supplemental fat in diets for laying hens. Unpublished Ph.D. thesis, Library, Iowa State University, Ames, IA. Mateos, G. G., and J. L. Sell, 1980a. True and apparent metabolizable energy value of fat for laying hens: influence of level of use. Poultry Sci. 59:369—373. Mateos, G. G., and J. L. Sell, 1980b. Influence of graded levels of fat on utilization of pure carbohydrate by the laying hen. J. Nutr. 110:1894— 1903. National Research Council, 1977. Nutrient requirements of poultry. Nat. Acad. Sci., Washington, DC. Perkin-Elmer, 1973. Analytical methods for atomic absorption spectrophotometry. Norwalk, CT. Sell, J. L., L. G. Tenesaca, and G. L. Bales, 1979. Influence of dietary fat on energy utilization by laying hens. Poultry Sci. 58:900-905. Sibbald, I. R., 1978. The true metabolizable energy values of mixtures of tallow with either soybean oil or lard. Poultry Sci. 57:473-477. Sibbald, I. R., and J.K.G. Kramer, 1977. The true metabolizable energy values of fats and fat mixtures. Poultry Sci. 56:2079-2086. Sibbald, I. R., and J.K.G. Kramer, 1978. The effect of the basal diet on the true metabolizable energy of fat. Poultry Sci. 57:685-691. Sibbald, I. R., and K. Price, 1977. The effects of level of dietary inclusion and of calcium on the true metabolizable energy values of fat. Poultry Sci. 56:2070-2078. Sibbald, I. R., and S. J. Slinger, 1963. A biological assay for metabolizable energy in poultry feed ingredients together with findings which demonstrate some of the problems associated with the evaluation of fat. Poultry Sci. 42:313-325. Slinger, S. J., and A. J. Mutzar, 1980. Energy measures for poultry and swine, practical application. Pages 103-109 in Proc. 16th Nutr. Conf. Feed Manuf., Univ. Guelph, Guelph, Ontario. Sues, M., 1974. Digestibility and metabolizable energy content of beef tallow for laying hens. Pages 367-369 in Proc. 25th World Poultry Congr., New Orleans, LA. Tuckey, R., B. E. March, and J. Biely, 1958. Diet and the rate of food passage in the growing chick. Poultry Sci. 37:786-792. Whitehead, G. C , and C. Fisher, 1975. The utilization of various fats by turkeys of different ages. Brit. Poultry Sci. 16:481-485.
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evaluate t h e d a t a indicated t h a t level of fat s u p p l e m e n t a t i o n influenced t h e extracaloric effect. This influence, in t u r n , resulted in estimated ME's for yellow grease t h a t varied with level of fat s u p p l e m e n t a t i o n . Sibbald and Price ( 1 9 7 7 ) , Halloran a n d Sibbald ( 1 9 7 9 ) , Slinger and Muztar ( 1 9 8 0 ) , a n d Mateos and Sell ( 1 9 8 0 a ) have r e p o r t e d t h e variable effect of level of fat s u p p l e m e n t a t i o n o n t h e estimated ME values of t h e s u p p l e m e n t a l fat. Because s u p p l e m e n t a l fats usually are evaluated at relatively l o w levels of t h e diet, small errors m a d e at any stage of an assay will be magnified greatly when conventional calculations of ME are m a d e ( S i b b a l d a n d Slinger, 1 9 6 3 ) . Therefore, Sibbald and Slinger ( 1 9 6 3 ) a n d Slinger< a n d Muztar ( 1 9 8 0 ) suggested t h a t experiments c o n d u c t e d t o d e t e r m i n e t h e ME value of supplemental fats be designed so t h a t regression analysis could be used in mathematically evaluating t h e results. T h e findings of our research lend general s u p p o r t t o this suggestion, especially when relatively low levels of fat s u p p l e m e n t a t i o n (up t o 1 5 % of t h e diet) are used. This approach should yield t h e m o s t reliable estimate of t h e c o n t r i b u t i o n of a supplemental fat to t h e ME of p o u l t r y diets, although t h e values will n o t necessarily represent t h e actual ME c o n t e n t of t h e supplemental fat per se.
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1981 Poultry Science 60:1509-1515 INTRODUCTION Supplemental fats are difficult to evaluate in terms of their true energy contribution to practical diets. Two methods have been employed to estimate the energy contribution of fats to diets. 1) The digestibility of the fat is determined by relating the lipid content of diet to the lipid content of excreta. Then, the metabolizable energy (ME) of the supplemental fat is calculated by multiplying the gross energy content of the fat times the digestibility. 2) The fat is substituted into a diet at low levels. A direct measurement is made of the ME of the reference and test diets, and the ME of the fat is calculated by difference. Results obtained by these two methods, however, do not always agree, and, consequently, estimates of the energy contribution of supplemental fats to poultry diets have been inconsistent. The first method assumes that lipid digestibility is independent of the composition of the diet and of the level of supplemental fat used. This assumption, however, may be erroneous. For example, Leeson and Summers (1976) and Sibbald (1978) suggested that a synergism between saturated fatty acids of a supplemental fat of animal origin and unsaturated fatty acids
'journal Paper No. J-10116 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA. Project 2240.
inherent in the grain portion of a diet enhanced the utilization of the supplemental fat. Therefore, added fat may exhibit a greater ME when included in diets containing relatively high levels of unsaturated fatty acids. Indeed, Sibbald and Kramer (1978) observed that the true metabolizable energy of tallow was greater in a corn- than in a wheat-based diet, thereby lending support to the suggestion of fatty acid synergism. Sues (1974) reported an inverse relationship between lipid digestibility and level of fat supplementation. In contrast, Gomez and Polin (1974) reported no change, or slight improvement, in lipid digestibility with increasing levels of fat supplementation. The second method of estimating the energy contribution of fats to diets assumes that the ME's of the dietary ingredients are additive and that supplemental fat does not alter the utilization of other dietary constituents. Numerous authors (Cullen et al., 1962; Gomez and Polin, 1974; Sibbald and Kramer, 1977; Sell et al, 1979; Mateos and Sell, 1980a), however, have reported ME's for fat that exceeded the heat of combustion of this energy source; therefore, it seems that fats and other ingredients probably are not additive with respect to energy utilization by poultry. Kalmbach and Potter (1959) presented evidence that an interaction between dietary constituents and supplemental fat affected ME of the diet. They observed that corn oil and
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ABSTRACT White Leghorn laying hens were fed practical diets containing 0, 5, 10, 15, 20, 25, or 30% yellow grease. The nitrogen-corrected metabolizable energy (ME) of yellow grease was estimated from apparent digestibility data and from ME's of the diet determined experimentally. The ME's of yellow grease were consistently higher when dietary ME data were used in the calculations than when lipid digestibility data were used (8380, 8770, 8567, 9050, 8408, and 8473 kcal/kg vs. 8360, 8344, 8278, 8456, 8212, and 8540 kcal/kgfor diets containing 5, 10, 15, 20, 25, and 30% fat, respectively). A comparison also is presented of ME values for yellow grease obtained by conventional calculation or by use of regression analysis. Regardless of the method of calculation, the ME data indicated that supplemental yellow grease exerted an extrametabolic effect on dietary ME, although the magnitude of this effect varied with the mathematical approach used. (Key words- fat, metabolizable energy, lipid digestibility, laying hens, yellow grease)
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MATERIALS AND METHODS Single Comb White Leghorn hens, 28-weeks of age, were kept in individual cages located in an environmentally controlled room at 27 C. Feed and water were offered ad libitum. Seven experimental groups were used, and five individually caged hens were assigned randomly to each group. One group received a control diet containing no added fat. Increments of 5% yellow grease, ranging from 5 to 30% of the diet, were included in the diets fed to the other six experimental groups (Table 1). Appropriate adjustments were made in the ingredient levels of the fat-supplemented diets so that relatively constant ME to nutrient ratios were maintained. After a 7-day adaptation period, the hens were fed their respective experimental diet for another 5 days. Feed consumption was recorded, and excreta were collected on a quantitative basis every 60 hr. The diets and excreta samples were freeze-dried, were allowed to come to equilibrium with atmospheric moisture, and were analyzed for dry matter, heat of combustion, nitrogen, chromic oxide, and total lipid content. Chromium content of diets and excreta was determined by atomic absorption spectrophotometry (Perkin-Elmer, 1973). The nitrogen-corrected ME's of the diets were calculated from data obtained by both total collection and chromic oxide index methods. The average ME of both methods obtained for each hen was used in subsequent calculations. Ten additional hens were used to determine the ME of the corn and soybean meal used. Two diets were formulated containing corn and
soybean meal exclusively as energy-contributing ingredients (Table 2). Each of these two diets was fed to 5 hens, and excreta were collected totally and were analyzed. The ME of corn and soybean meal was determined by resolution of simultaneous equations using the data obtained. The ME of the sunflower meal was calculated by using the ME values determined for corn, soybean meal and the control (no added fat) diets. It was assumed that the ME's of the corn, soybean meal, and sunflower meal were additive. The ME of yellow grease was obtained by using two different procedures. In procedure 1, lipids were extracted from all diets and feces by the procedure described by the Association of Official Analytical Chemists (Procedure 7047, AOAC, 1975). The apparent digestibility of yellow grease for each level of supplemental fat was calculated and the ME of yellow grease was estimated by multiplying the gross energy content of yellow grease times the apparent digestibility. In procedure 2, the ME of yellow grease was calculated by assuming that ME's of dietary ingredients were additive. Therefore, the ME supplied by the corn, soybean meal, and sunflower meal was subtracted from the ME obtained experimentally for the fat-supplemented diets. The ME of yellow grease was obtained by dividing this difference by the level of supplemental fat. The experimental data for ME of diets and digestibility of lipids were regressed on level of fat in the test diets. Data obtained from these regressions were used for further comparisons. RESULTS The ME of the diets, as determined experimentally, increased with each increment of supplemental yellow grease (Table 3). A comparison of the calculated and determined ME's indicated that changes in dietary ME's associated with supplemental fat were larger in the latter instance. The ME's obtained for corn and soybean meal were 3427 and 2468 kcal/kg, respectively. These values were in good agreement with those listed by the National Research Council (1977) (3430 and 2440 kcal/kg, respectively). The ME data obtained for the corn and soybean meal diets were used to estimate the ME's of yellow grease by three different procedures: 1) conventional method; 2) regression equation, linear model; and 3) regression equation, quadratic model.
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tallow had higher ME's when substituted for a portion of a basal diet than when added at the expense of cerelose. Mateos and Sell (1980b) reported that the ME of yellow grease varied with the specific carbohydrate used as a main dietary constituent. Recently, Mateos (1980) postulated that supplemental fats may enhance energy utilization from diets by slowing rate of food passage (ROP). A reduction in ROP could result in a more complete digestion and utilization of the diet (Tuckey et al, 1958). The objective of the research reported here was to estimate the ME of yellow grease in practical diets for laying hens by using actual ME determination and lipid digestibility data. Also studied was the influence of the level of fat inclusion on the relative changes in dietary ME and the estimated ME of yellow grease.
ME OF FAT AND FAT LEVEL
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TABLE 1. Composition of the experimental diets, % Suppl emental fat (%) Ingredient
10
5
20
15
25
30
25.00 4.60 61.28
25.00 8.32 52.13 5.00
25.00 11.87 43.10 10.00
25.00 15.55 33.90 15.00
25.00 19.18 24.80 20.00
25.00 22.87 15.70 25.00
25.00 26.51 6.55 30.00
.01 .06 .30
.03 .02 .30
.03
.05
.07
.08
.09
.30
.30
.30
.30
.30
1.30 6.65
1.40 7.00
1.60 7.30
1.70 7.70
1.85 8.00
1.90 8.3 5
2.00 8.75
.30 .50
.30 .50
.30 .50
.30 .50
.30 .50
.30 .50
.30 .50
2600 14.62
2772 15.62
2944 16.55
3113 17.52
3285 18.48
3457 19.47
3628 20.44
.28 .61
.31 .65
.32 .73
.35 .82
.38 .92
.39
.41
1.02
1.11
Yellow grease (Association of American Feed Control Officials, 1981, number 33.20, IFC number 4-00-409, Animal Fat, Feed Grade), obtained from National Byproducts Co., Des Moines, IA had by analysis the following fatty acid composition: myristic, 1.8%; palmitic, 24.7%; palmitoleic, 2.6%; stearic, 15.8%; linoleic, 9.7%; oleic, 44.4%; linolenic, .6%; others, .4%. Supplied the following per kilogram of diet: vitamin A, 8000 IU; vitamin D 3 , 2400 IU; vitamin B 1 2 , 5 Mg; riboflavin, 6.6 mg; calcium pantothenate, 6.6 mg; niacin, 2.2 mg; choline, 440 mg; ethoxyquin, 11 mg. c
On the basis of National Research Council (1977) values for soybean meal and corn. The ME's of 7900 and 1540 kcal/kg were used for yellow grease and high-fiber sunflower meal, respectively. The moisture contents of corn, soybean meal, and sunflower meal were 10.6, 10.2, and 8.8%, respectively.
Conventional Methods for Estimating the ME of Yellow Grease The ME determined for the control (no added fat) diet was 2608 kcal/kg. Assuming additivity of ME values of dietary ingredients in the control diet, the ME of sunflower meal was obtained by subtracting the ME supplied by the corn (.6128 X 3427 = 2100 kcal/kg) and by the soybean meal (.460 X 2468 = 113 kcal/kg) from the ME of the control diet (2608 kcal/kg). Thus, the energy supplied by sunflower meal to the diets was 2608 - 2213 = 395 kcal/kg, and the ME of sunflower meal was 395/25 X 100 = 1580 kcal/kg. By using these data and the dietary ME values obtained for each test diet, the ME of yellow grease was estimated. For example, in the diet containing 10% yellow grease, corn supplied 1477 kcal of ME to the diet (.4310 X 3427), soybean meal supplied 293 kcal (.1187 X 2468), and sunflower meal supplied 395 kcal (.25 X 1580). The ME contributed by yellow grease to this diet was assumed to be the difference between the determined ME of the diet (3042 kcal/kg) and the energy supplied by
the other three ingredients (1477 + 293 + 395 = 2165 kcal/kg). Thus, the ME of yellow grease in diet 3 (10% yellow grease), assuming additivity of ME's of dietary ingredients, was (3042 — 2165/10) X 100 = 8770 kcal/kg. The ME values of yellow grease obtained by this method of calculation were 8380, 8770, 8567, 9050, 8408, and 8473 kcal/kg for the 5, 10, 15, 20,
TABLE 2. Composition of the diets used to determine the metabolhable energy (ME) of soybean meal and com, % Ingredient
Diet A
Diet B
Yellow corn Soybean meal, 48.5% protein Calcium carbonate Dicalcium phosphate Salt Vitamin premix a
70.0 20.0
50.0 40.0
7.8 1.4 .3 .5
7.8 1.4 .3 .5
Supplied the same quantities per kilogram of diet as described in Table 1.
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Sunflower meal, 28% protein Soybean meal, 48.5% protein Yellow corn Yellow grease a DL-methionine L-lysine Salt Dicalcium phosphate Calcium carbonate Chromic oxide Vitamin premix 0 Calculated analysis: ME (kcal/kg)c Crude protein, % Methionine, % Lysine, %
0
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TABLE 3. Nitrogen-corrected metabolizable energy (ME) of the experimental diets, kcal/kg (air dry basis) Determined ME
Calculated 3
Supplemental fat (%)
Total collection
ME
2600 2772 2944 3113 3285 3457 3628
0 5 10 15 20 25 30
2602 ± 2 8 2816 + 18 3023 ± 31 3226 ± 33 3550 ± 57 3588 ± 4 1 3812+36
b
Cr203
Average
2613 2800 3062 3226 3507 3611 3819
2608 2805 3043 3226 3528 3599 .3815
± 28 ± 29 ± 46 ±23 ±40 ±21 + 32
±12 ± 24 + 36 ± 28 ±47 ±25 ±25
25, and 30% yellow grease-supplemented diets, respectively (Table 4). The ME of yellow grease also was estimated by multiplying the lipid digestibility coefficient (Table 5) times the gross energy of yellow grease (9375 kcal/kg). The resulting ME's were 8360, 8344, 8278, 8456, 8212, and 8540 kcal/kg for the 5, 10, 15, 20, 25, and 30% yellow grease-supplemented diets, respectively. In general, the ME values obtained for yellow grease from lipid digestibility data were smaller than those obtained by assuming additivity of ME's of dietary ingredients.
Slinger and Muztar (1980) advocated the use of regression-equation analysis (ME of diet on level of supplemental fat) to obtain more reliable estimates of the ME values of supplemental fats. The data obtained from the experiment reported here were used for exploring this statistical approach. Apparent lipid digestibility and dietary ME data each were regressed on level of supplemental fat in the diet. The model used was y = a + bx + ex 2 + 2 , where y equals lipid digestibility or dietary ME and x equals level of supplemental fat. The apparent lipid digestibility of diets was independent of level of supplemental yellow grease. The linear (b-factor) and quadratic (c-factor) components of the regression model were small and not significantly different from zero (P>.10). Therefore, the apparent lipid
Estimation of ME's of Yellow Grease with Regression Analysis, Linear Model Recently, Mateos and Sell (1980a,b) and
TABLE 4. Nitrogen-corrected metabolizable energy (ME) of yellow grease, assuming additivity of ME of ingredients, kcal/kg (air dry basis)
Supplemental fat (%) 0 5 10 15 20 25 30
ME supplied byb
Determined 3 ME of diet
Corn
A
B
2608 2805 3042 3226 3528 3599 3815
2100 1786 1477 1162 850 538 224
SBM C
SFM D
113 205 293 384 473 564 654
395 395 395 395 395 395 395
Experimental data are averages of 5 hens per diet ME (kcal/kg) of each ingredient times percentage of ingredient in diet. Column A —columns B + C + D.
Column C divided by the percentage of fat inclusion.
ME supplied bv c yellow grease
MEof d yellow grease
E
F
419 877
1285 1810 2102 2542
8380 8770 8567 9050 8408 8473
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On the basis of NRC (1977) values for corn and soybean meal. The ME values 1540 and 7900 kcal/kg were used for high-fiber sunflower meal and yellow grease, respectively. Mean ± SE.
ME OF FAT AND FAT LEVEL
Determination of the ME's of Yellow Grease •with Regression Analysis, Quadratic Model Although t h e regression line relating ME and level of fat of t h e diets c o n f o r m e d t o a linear m o d e l , t h e quadratic c o m p o n e n t (c-factor) a p p r o a c h e d significance (P=.09). When t h e quadratic effect was included in t h e model, t h e ME's of t h e diets were defined by t h e e q u a t i o n y = 2 5 9 1 + 4 8 x - . 2 4 x 2 . On t h e basis of ME for diets calculated from this regression e q u a t i o n a n d following t h e p r o c e d u r e described previously, a n o t h e r set of ME's was calculated for
yellow grease. T h e data presented in Table 7 s h o w t h a t t h e resulting ME's varied from 9 1 2 0 kcal/kg ( 5 % fat) t o 8 5 2 0 kcal/kg (30% fat). There was n o quadratic effect of level of supplemental fat on lipid digestibility, b u t for comparative purposes, t h e ME's of yellow grease estimated by using data from regression analysis of lipid digestibility are presented in Table 7. DISCUSSION In general, t h e ME values obtained for yellow grease were smaller w h e n lipid digestibility data were used in t h e calculations t h a n w h e n data of d e t e r m i n e d metabolizable energy were used. Similar differences b e t w e e n M E of fats estimated b y these t w o procedures have b e e n r e p o r t e d with broiler chicks ( G o m e z and Polin, 1 9 7 4 ) and turkeys (Whitehead and Fisher, 1 9 7 5 ) . These differences m a y represent t h e effects of fat on t h e utilization of energy from o t h e r dietary constituents. If t h e ME of yellow grease estimated from lipid digestibility data is a reliable estimate of t h e ME c o n t e n t of t h e fat, t h e n t h e " e x t r a " ME generally associated with yellow grease o n t h e basis of m e a s u r e d changes in dietary ME m u s t be derived from o t h e r dietary c o m p o n e n t s . T h e m a g n i t u d e of t h e differences b e t w e e n t h e ME's of yellow grease estimated b y t h e t w o m e t h o d s varied with level of supplemental fat when t h e conventional a p p r o a c h t o ME d e t e r m i n a t i o n was used ( 2 0 , 4 2 6 , 2 8 9 , 594, 196, and - 6 7 kcal/kg of yellow grease for t h e diets s u p p l e m e n t e d with 5, 10, 1 5 , 20, 2 5 , and 30% yellow grease, respectively) (Tables 4 and 5). When t h e linear
TABLE 5. Apparent lipid digestibility of the experimental diets and ME of yellow grease estimated from lipid digestibility data
Supplemental fat (%)
Total lipid ingested (g in 5 days)
0 5 10 15 20 25 30
15.51 + 40.80 ± 58.53 + 74.11 ± 103.23 ± 106.69 + 123.74 ±
1.671 2.01 3.81 2.86 10.15 7.25 5.70
Apparent lipid digestibility
ME* (kcal/kg)
77.32 ± 87.99 ± 88.14+ 87.61 ± 89.74+ 87.09 ± 90.69 ±
8360 8344 8278 8456 8212 8540
4.27 1.14 1.43 2.58 3.95 4.30 1.51
The gross energy of the yellow grease was 9375 kcal/kg. The ME was calculated by multiplying lipid digestibility times gross energy and dividing by 100. b. Mean ± SE.
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digestibility coefficients of t h e diets were assumed t o be a c o n s t a n t 8 8 . 5 4 % . T h e ME of yellow grease, calculated o n this basis, was 8 3 0 1 kcal/kg (.8854 X 9 3 7 5 kcal/kg), irrespective of supplemental fat level. Regression of dietary ME on level of supplem e n t a l fat showed t h a t t h e ME's of t h e diets were defined by t h e e q u a t i o n y (kcal/kg) = 2 6 2 1 + 4 0 . 7 X- T h e linear c o m p o n e n t (b-factor) was highly significant ( P < . 0 0 0 1 ) , b u t t h e quadratic c o m p o n e n t (c-factor) did n o t reach a significant level ( P » . 0 5 ) . Therefore, only t h e linear regression e q u a t i o n was used t o estimate t h e ME of yellow grease in this instance. T h e ME of yellow grease for each level of fat was o b t a i n e d by subtracting t h e ME supplied by t h e o t h e r dietary c o n s t i t u e n t s from t h e ME of t h e test diets calculated b y t h e linear regression e q u a t i o n . T h e p r o c e d u r e used was analogous t o t h a t described previously in section I of t h e results and is presented in Table 6. T h e data s h o w t h a t t h e ME of yellow grease was 8 5 0 0 kcal/kg, irrespective of level of fat used.
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TABLE 6. Metabolizable energy (ME) of yellow grease by using linear regression equations and assuming additivity of the nitrogen-corrected ME's of ingredients, kcal/kg (air dry basis) ME supplied byb
Supplemental fat (%) A
MEof a diet B
Corn C
SBM D
SFM E
ME su pplied b y c yellow' grease F
MEof d yellow grease G
ME of YG based e on LD data H
0 5 10 15 20 25 30
2621 2824 3028 3232 3435 3639 3843
2100 1786 1477 1162 850 538 224
113 205 293 384 473 564 654
408 408 408 408 408 408 408
425 850 1275 1700 2125 2550
8500 8500 8500 8500 8500 8500 8500
8301 8301 8301 8301 8301 8301 8301
ME of each ingredient (kcal/kg) times percentage of ingredient in diet. Column B — Columns C + D + E. Column F divided by column A. ME of yellow grease on the basis of lipid digestibility data; ME (kcal/kg) = apparent lipid digestibility X gross energy of yellow grease, divided by 100.
regression model was used to estimate the ME of yellow grease, the difference was 199 kcal/kg of fat, irrespective of fat level (Table 6). The use of data obtained by the quadratic regression equation (Table 7) yielded differences of 819, 679, 579, 464, 335, and 219 kcal/kg of yellow grease for supplemental levels of 5, 10, 15, 20, 25, and 30%, respectively. These data illustrate two points. First,
supplemental yellow grease exerted a fairly consistent extracaloric effect in practical laying hen diets when changes in dietary ME's associated with supplemental fat were used as the criteria. Also, the results clearly show that the magnitude of this extracaloric effect varied considerably, depending on the procedure used in evaluating the data. Two of the procedures used herein to
TABLE 7. Metabolizable energy (ME) of yellow grease by using regression equation including the quadratic component in estimating the nitrogen-corrected ME of the experimental diets, kcal/kg (air dry basis) ME supplied byh
Supplemental fat (%) A
MEof a diet B
Corn C
SBM D
0 5 10 15 20 25 30
2591 2825 3046 3256 3454 3639 3812
2100 1786 1477 1162 850 538 224
113 205 293 384 473 564 654
SFM E
ME supplied by c yellow' grease F
MEof d yellow grease G
ME of YG based e on LD data H
378 378 378 378 378 378 378
456 898 1332 1753 2159 2556
9120 8980 8880 8765 8636 8520
8301 8301 8301 8301 8301 8301
ME (kcal/kg) = 2591 + 48 (% supplemental fat) - .24 (% supplemental fat) 2 . ME of each ingredient (kcal/kg) times percentage of ingredient in diet. Determined ME values were: corn, 3427 kcal/kg; soybean meal 2468 kcal/kg; sunflower meal, 1512 kcal/kg. C
B - (C + D + E).
d
F divided by A.
On the basis of lipid digestibility data, ME yellow grease (kcal/kg) = 88.54 X gross energy of yellow grease divided by 100.
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ME of diet (kcal/kg) = 2621 + 40.7 (% supplemental fat).
ME OF FAT AND FAT LEVEL
ACKNOWLEDGMENT T h e research was s u p p o r t e d , in part, by a grant from t h e Fats and Protein Research F o u n d a t i o n , Des Plaines, IL.
REFERENCES Association of American Feed Control Officials, 1981. Pages 116-117 in Official publication, 1981. Ass. Feed Control Offic, Inc. Association of Official Analytical Chemists, 1975. Official methods of analysis. 12th ed. Washington, DC. Cullen, M. P., O. G. Rassmussen, and O.H.M. Wilder, 1962. Metabolizable energy value and utilization of different types and grades of fat by the chick. Poultry Sci. 41:360-367. Gomez, M. X., and D. Polin, 1974. Influence of cholic acid on the utilization of fats in the growing chicken. Poultry Sci. 53:733-781. Halloran, M. R„ and I. R. Sibbald, 1979. Metabolizable
energy values of fats measured by several procedures. Poultry Sci. 58:1299-2307. Kalmbach, M. P., and L. M. Potter, 1959. Studies in evaluating energy content of feeds for the chick. 3. The comparative values of corn oil and tallow. Poultry Sci. 38:1217. (Abstr.) Leeson, S., and J. D. Summers, 1976. Fat metabolizable energy values: the effect of fatty acid saturation. Feedstuffs 48(46): 2 6 - 2 8 . Mateos, G. G., 1980. Nature of the extrametabolic effect of supplemental fat in diets for laying hens. Unpublished Ph.D. thesis, Library, Iowa State University, Ames, IA. Mateos, G. G., and J. L. Sell, 1980a. True and apparent metabolizable energy value of fat for laying hens: influence of level of use. Poultry Sci. 59:369—373. Mateos, G. G., and J. L. Sell, 1980b. Influence of graded levels of fat on utilization of pure carbohydrate by the laying hen. J. Nutr. 110:1894— 1903. National Research Council, 1977. Nutrient requirements of poultry. Nat. Acad. Sci., Washington, DC. Perkin-Elmer, 1973. Analytical methods for atomic absorption spectrophotometry. Norwalk, CT. Sell, J. L., L. G. Tenesaca, and G. L. Bales, 1979. Influence of dietary fat on energy utilization by laying hens. Poultry Sci. 58:900-905. Sibbald, I. R., 1978. The true metabolizable energy values of mixtures of tallow with either soybean oil or lard. Poultry Sci. 57:473-477. Sibbald, I. R., and J.K.G. Kramer, 1977. The true metabolizable energy values of fats and fat mixtures. Poultry Sci. 56:2079-2086. Sibbald, I. R., and J.K.G. Kramer, 1978. The effect of the basal diet on the true metabolizable energy of fat. Poultry Sci. 57:685-691. Sibbald, I. R., and K. Price, 1977. The effects of level of dietary inclusion and of calcium on the true metabolizable energy values of fat. Poultry Sci. 56:2070-2078. Sibbald, I. R., and S. J. Slinger, 1963. A biological assay for metabolizable energy in poultry feed ingredients together with findings which demonstrate some of the problems associated with the evaluation of fat. Poultry Sci. 42:313-325. Slinger, S. J., and A. J. Mutzar, 1980. Energy measures for poultry and swine, practical application. Pages 103-109 in Proc. 16th Nutr. Conf. Feed Manuf., Univ. Guelph, Guelph, Ontario. Sues, M., 1974. Digestibility and metabolizable energy content of beef tallow for laying hens. Pages 367-369 in Proc. 25th World Poultry Congr., New Orleans, LA. Tuckey, R., B. E. March, and J. Biely, 1958. Diet and the rate of food passage in the growing chick. Poultry Sci. 37:786-792. Whitehead, G. C , and C. Fisher, 1975. The utilization of various fats by turkeys of different ages. Brit. Poultry Sci. 16:481-485.
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evaluate t h e d a t a indicated t h a t level of fat s u p p l e m e n t a t i o n influenced t h e extracaloric effect. This influence, in t u r n , resulted in estimated ME's for yellow grease t h a t varied with level of fat s u p p l e m e n t a t i o n . Sibbald and Price ( 1 9 7 7 ) , Halloran a n d Sibbald ( 1 9 7 9 ) , Slinger and Muztar ( 1 9 8 0 ) , a n d Mateos and Sell ( 1 9 8 0 a ) have r e p o r t e d t h e variable effect of level of fat s u p p l e m e n t a t i o n o n t h e estimated ME values of t h e s u p p l e m e n t a l fat. Because s u p p l e m e n t a l fats usually are evaluated at relatively l o w levels of t h e diet, small errors m a d e at any stage of an assay will be magnified greatly when conventional calculations of ME are m a d e ( S i b b a l d a n d Slinger, 1 9 6 3 ) . Therefore, Sibbald and Slinger ( 1 9 6 3 ) a n d Slinger< a n d Muztar ( 1 9 8 0 ) suggested t h a t experiments c o n d u c t e d t o d e t e r m i n e t h e ME value of supplemental fats be designed so t h a t regression analysis could be used in mathematically evaluating t h e results. T h e findings of our research lend general s u p p o r t t o this suggestion, especially when relatively low levels of fat s u p p l e m e n t a t i o n (up t o 1 5 % of t h e diet) are used. This approach should yield t h e m o s t reliable estimate of t h e c o n t r i b u t i o n of a supplemental fat to t h e ME of p o u l t r y diets, although t h e values will n o t necessarily represent t h e actual ME c o n t e n t of t h e supplemental fat per se.
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