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Effect of Ratio of Unsaturated to Saturated Fatty Acids of the Dietary Lipid Fraction on Utilization and Metabolizable Energy of Added Fats in Young Chicks1
Effect of Ratio of Unsaturated to Saturated Fatty Acids of the Dietary Lipid Fraction on Utilization and Metabolizable Energy of Added Fats in Young Chicks 1 EDDY KETELS and GEORGES DE GROOTE Rijksstation voor Kleinveeteelt, Rijkscentrum voor Landbouwkundig Onderzoek, Ministerie van Landbouw, Burgm. Van Gansberghelaan 92, B-9220 Merelbeke, Belgium (Received for publication January 21, 1988)
1989 Poultry Science 68:1506-1512 INTRODUCTION
Digestibility and AME of fats in broiler diets depend on the fatty acid composition. Renner and Hill (1961) and Young and Garrett (1963) showed that long chain saturated fatty acids were poorly absorbed compared with long chain unsaturated and short chain fatty acids (Hakansson, 1974). Fatty acids are stereospecifically distributed on the glycerol molecule (Brockerhoff, 1971). The importance of these findings for fatty acid utilization was shown by Renner and Hill (1961), Leeson and Summers (1976), and Ketels and De Groote (1988). Saturated fatty acid utilization, however, may be improved by the presence of unsaturated fatty acids in the fat blend (Young and Garrett, 1963; Young, 1965; Lewis and Payne, 1966; Garrett and Young, 1975; Leeson and Summers, 1976). This synergism is caused by the excellent emulsifying capacities of the latter. Sibbald et al. (1961) attributed this
'Research funded by the Institute for the Encouragement of Scientific Research (I.W.O.N.L.).
effect to the presence of lecithin. Since then numerous other authors have pointed to the positive effect of blending vegetable oils with animal fats (Lall and Slinger, 1973; Sibbald, 1978; Scheele, 1986). The nature of the basal diet fat, which is mostly of vegetable origin, may have an important effect on the utilization of added animal fats (Sell et al., 1976; Sibbald and Kramer, 1978, 1980; Fuller and Dale, 1982). Especially at low inclusion levels, interactions between the added fat and the basal diet fat fraction are noticed, resulting in higher animal fat AME values (Ketels et al., 1986; Wiseman et al., 1986). In this study, relations between the degree of saturation of the dietary fat and its utilization and AME value were examined. The objective of this study was to examine to what extent the chemical composition of the dietary fat fraction determines the AME n values of added fats. The ratio of saturated to unsaturated fatty acids (U:S) in the dietary lipid fraction seems very important in affecting fat AMEn, which has been shown in pigs by Stanly (1984).
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ABSTRACT Relationships between the ratio of unsaturated to saturated fatty acids (U:S) in the diet and the utilization and AME„ of added fat were studied using the results of a number of previously reported experiments. Mathematical equations relating fat AME^ fat utilization, and palmitic, oleic, and linoleic acid utilization to the U:S were calculated for young broiler chicks. Best fit regression equations for added fat utilization and fat AMEn were exponential, showing fat utilization increased very steeply in the U:S range of 0 to 2.5, reaching a near asymptotical maximum at a U:S of 4 or more. Synergism between added fats, due to blending vegetable oils with animal fats or using basal diets with unsaturated lipid fractions, led to increased animal fat utilization values. The utilization of saturated fatty acids was especially affected by synergism, whereas utilization of unsaturated fatty acids was not influenced by changing U:S. The origin of effects of factors influencing fat utilization, such as level of fat inclusion and basal diet composition, appeared to be through variation in degree of saturation of the total dietary lipid fraction. For young broilers, about 75% of the variation in fat utilization and AME,, was due to differences in the chemical composition of the fat fraction. (Key words: fats, fatty acids, saturated fatty acids, metabolizable energy, diets)
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DIETARY LIPIDS AND METABOLIZABLE ENERGY TABLE 1. Fatty acid composition offatsr Experiment 2 Experiment 1 Soybean oil
Fatty acid
Tallow
Lard
Lard
Renderers Renders Renderers fat no. 1 fat no. 2 fat (3%) (10%) (10%)
Expeiiment 3 Soybean oil
Tallow
.1 11.6 .1 3.5 20.7 55.0 7.0 1.9 5.45
.1 3.3 25.6 2.8 21.0 36.6 3.4 .8 6.5 .87
C"?J1
.1 10.7 .1 1.8 22.3 51.9 7.0 6.1 6.45
.2 3.3 26.0 3.4 22.2 36.6 3.1 .1 5.1 .84
.1 1.6 22.8 3.0 14.0 45.6 10.4 .1 2.4 1.54
.1 1.7 23.7 2.5 14.1 40.4 8.7 1.0 7.8 1.33
.2 2.0 23.3 3.4 13.1 41.1 7.9 .7 8.3 1.38
.1 1.8 23.4 3.1 13.8 39.2 6.9 .7 11.0 1.28
.1 1.7 22.0 2.8 13.7 38.4 6.6 .7 14.0 1.29
Experiment 1 Ketels et al. (1986); Experiment 2 Ketels et al. (1987); Experiment 3 Ketels et al. (unpublished).
MATERIALS AND METHODS
Best fit regression equations between the U: S of the total fat fraction of the diet and the utilization of the added fat were calculated using data determined in digestibility trials with young Hubbard broilers, during their 2nd wk posthatch. A more detailed description of the applied procedure for these trials is given by Ketels and De Groote (1988). The data used to calculate the mathematical relationships were obtained from the studies described by Ketels et al. (1986; 1987; unpublished results). In these experiments, six different fats were assayed for AME: soybean
oil, beef tallow, lard, and three commercial renderers' fat blends of pork and beef fats. The data points covered the range of 2.5 to 12.5% inclusion. Three basal diets were used, the fat fractions of which differed not only in quantity, but also in degree of saturation. The fatty acid compositions of the fats are given in Table 1 and those of the basal diets in Table 2. In total, 45 diets from three different experiments were used to calculate the regression equations; a list of these diets is given in Table 3. Fat AME values were determined using best fit regression equations between level of fat inclusion and dietary AME (Wiseman et al.,
TABLE 2. Fatty acid composition of the basal diets
Fatty acid
SorghumSorghumSorghumCornWheatsoybean meal soybean meal soybean meal soybean meal soybean me; (Experiment 1) (Experiment 2) (Experiment 3) (Experiment 3) (Experiment 3) tOL\
C12:0 C14:0 C16:0 C16:l C18:0 C18;l C18:2 C18:3 Others Unsaturated:saturated Fat content, g/lOOg
.1 16.0 .4 2.6 28.6 44.3 2.3 5.7 4.04
.1 .1 18.0 .4 3.0 27.7 44.7 3.0 3.0 3.57
.6 .6 17.0 .6 3.0 25.3 43.0 3.6 5.7 3.42 2.46
.5 14.3 .3 3.5 22.7 47.1 3.7 8.1 4.03 2.80
.1 .5 22.9 .3 3.6 17.7 44.3 3.3 7.4 2.42 1.84
'Experiment 1 Ketels et al. (1986); Experiment 2 Ketels et al. (1987); Experiment 3 Ketels et al. (unpublished).
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C12:0 C14:0 C16:0 C16:l C18:0 C18:l C18:2 C18:3 Others Unsaturated:saturated
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KETELS AND DE GROOTE TABLE 3. Fat AMEn and utilization values, related to the unsaturated:saturated ratio (U:S)
Basal diet
Added fat
Sorghum Soybean oil
Lard
Tallow
Renderers fat (3% FFA2)
Renderers fat no. 1 (10% FFA)
Sorghum Renderers fat no. 2 (10% FFA)
Wheat
Soybean oil Tallow
Com
Soybean oil Tallow
Sorghum Soybean oil Tallow
Fat and fatty acid utilization Experiment
5 7.5 10 12.5 2.5 5 7.5 10 12.5 2.5 5 7.5 10 12.5 2.5 5 7.5 10 2.5 5 7.5 10 2.5 5 7.5 10 2.5 5 7.5 10 2.5 7.5 2.5 5 7.5 2.5 7.5 2.5 5 7.5 2.5 7.5 2.5 5 7.5
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
AME„
U:S 5.68 5.89 6.01 6.09 2.77 2.34 2.12 1.99 1.91 2.41 1.86 1.59 1.42 1.31 2.42 2.03 1.87 1.72 2.43 2.04 1.85 1.73 2.40 2.00 1.80 1.68 2.41 2.01 1.81 1.69 3.70 4.57 1.31 1.15 1.05 4.50 4.71 1.70 1.38 1.23 4.34 4.83 1.85 1.39 1.26
(kcal/kg) 8,101 8,101 8,101 8,101 7,850 7,850 7,850 7,850 7,850 8,612 7,100 5,928 5,326 8,044 7,625 7,207 6,788 6,817 6,817 6,817 6,817 7,654 7,124 6,669 6,275 7,859 7,070 6,438 5,927 8,244 8,244 5,768 5,594 5,391
7,162 5,868 8,183 8,183 8,018 6,544 5,871
Total fat C16:0
C18:l
C18:2
f"M
93.8 91 89.7 91.5 85.3 83.1 77.6 82.8 86.3 83.9 69.2 63.9 64.1 61.7 92.4 85.6 80.2 77.3 91.7 80.9 76.4 81.5 80. i 79 78.9 88.6 74.6 70.8 70.7 79 91.7 72.9 68.8 58.2 95 90.4 75.8 68.5 57.9 93.4 91.1 65.2 66.3 65.2
99.8 91.7 89.7 84.5 81.8 80.6 76.1 84.9 84.2 76.7 62.8 55.8 52.9 57.4 80.5 74.6 77.8 74.9 81.1 71.4 67.1 73.2 88.7 71.6 66.6 70.9 77.8 64.1 62.1 64.6 75.6 84.6 63.5 62.6 58.4 99.9 91.3 69.5 91.9 59.2 89.3 89.6 54.2 58.6 61.6
95.4 93.4 92.2 93.9 89.9 90.3 84.6 90.3 92.5 99.4 84.9 80.9 83.9 81.9 95.2 92.7 88.6 84.5 96.6 89.3 86.7 89.5 89.2 89.8 86.7 93.8 85.3 83 84.5 80.2 94.5 91.5 92.6 89.9 96.6 90.3 84.9 84.7 86.4 90.4 89.3 78.8 82.7 83.8
94.2 93.1 92.2 94.7 102.8 100.4 96.2 94.5
86.4 100.2
89.7 94.8 91.8
86 91.2
Experiment 1 Ketels et al. (1986); Experiment 2 Ketels et al. (1987); Experiment 3 Ketels et al. (unpublished). FFA = Free fatty acid.
2
1986), in order to reduce the high standard errors associated with calculation of fat AME at low inclusion levels by simple extrapolation (Muztar et ai, 1981). Standard errors of fat
AME, calculated by the multi-level technique, decreased three to seven fold (Wiseman et al, 1986) at inclusion levels of 0 to 10%. The figures used in this experiment were thus
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Sorghum Pork fat
Level of use
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DIETARY LIPIDS A N D METABOLIZABLE ENERGY TABLE 4. Mathematical relationships between the ratio of unsaturated to saturated fatty acids (U:S) and fat-utilization parameters Parameter
Equation
Fat AME,,, kcal/kg
= 8,227 ± ± .2079 = 92.02 ± ± .2072 = 93.43 ± ± .1896 = 84.76 ± = 94.07 ± ± .4962 = 95.77 ±
Added fat utilization, % Palmitic acid utilization, % Oleic acid utilization, %
Linoleic acid utilization, %
168 1 - 10,318 ± 2,557 EXP [-1.1685 (U:S)] 2.02 - 103.42 ± 25.60 EXP [-1.0507 (U:S)] 3.68 - 77.96 ± 15.36 E X P [-.6578 (U:S)] 1.34 + 1.57 ± .46 (U:S) 10.64 - 14.78 ± 6.23 EXP [- .4677 (U:S)] 3.66 - .48 ± .85 (U:S)
.768
451 kcal
<.001
.779
5.3%
<.001
.733 .218
6.8% 4.5%
<.001
.253 .024
4.5% 5.2%
<.01 <.05
<01
SE.
interactions, was seen as an increase in added fat utilization. In order to compute the ideal mathematical relationships between the U:S and fat utilization parameters, the best fit equations were selected out of linear, quadratic, cubic, and exponential curves. These were evaluated by means of R^, Sxy% and F values. RESULTS
Best fit regression equations are listed in Table 4. The best fit relations between U:S and AMEn (Figure 1) and total fat utilization (Figure 2) were exponential (P<.001). Best fit
100.
9000
M«
01 ^
X
8000
t
90
ID 0
"«
>»• C 7000
**/
DQ r-
LU
5000
«
«
"I
o Q
'
/ " */
(f) 111
6000
«*
/i
/«."
60
/•
50
4000
u:s RATIO
u:s RATIO FIGURE 1. Relation between the AMEn o f f a t w i dietary unsaturated:saturated fatty acids (U:S) ratio.
^
FIGURE 2. Relation between the digestibility of added fat and the dietary unsaturated:saturated fatty acids (U:S) ratio.
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calculated by extrapolating the regression equations at the experimentally applied fat incorporation levels of 2.5, 5, 7.5, 10, and 12.5% (Ketels et al., 1987). Fat AME values were corrected for nitrogen retention (AMEn). Fat and fatty acid utilization data were calculated by difference using fat absorption values of birds fed die basal diet and birds fed the experimental, fat-supplemented diets. The difference in fat retention represents the absorbed amount of added fat, and is a measure of fat utilization. The utilization of the fat in the basal diet was assumed to be constant. Therefore, every improvement in basal diet fat utilization, e.g., by synergistic
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KETELS AND DE GROOTE
equations for oleic and linoleic acid were linear (P<.01), whereas palmitic acid utilization was curvilinearly related to the U:S (P<.001). Residual errors were relatively low. The Sxy^ values for fat AMEn and fat utilization were 450 kcal/kg and 5.5%, respectively. About 75% of the variation in fat AMEn, fat utilization, and palmitic acid utilization can be explained by differences in the U:S. DISCUSSION
As a fat blend contains less saturated acids and more easily digested unsaturated fatty acids, the relative extent of synergism becomes smaller. If the U:S exceeds 4, the fat is utilized at the bird's maximal capacity. Synergism is very important for the absorption of long chain saturated fatty acids, such as palmitic and stearic acids. Unsaturated fatty acids are easily absorbed, regardless of the presence of other fatty acids (Table 4), whereas saturates are to be emulsified prior to absorption (Krogdahl, 1985). Studies of Young and Garrett (1963) and of Garrett and Young (1975) with fatty acids in chick diets, indicated that increasing the U:S of a fatty acid mixture consisting of palmitic, oleic, and linoleic acid improved palmitic acid
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The exponential equations relating AMEn and utilization of added fat to the U:S were very steep in the U:S range of 0 to 2.5. At higher ratios, the curve tended to asymptotically reach a maximum. This maximal value is 8,227 kcal/kg for fat AME n and 92% for fat utilization (a coefficient of the exponential regression equation). Other authors found higher AMEn values for well digestible oils, and even for animal fats (Horani and Sell, 1977; Mateos and Sell, 1980). Most of these values, however, were determined with adult birds or with older broilers. Fat AME n appears to be significandy affected with age (Whitehead and Fisher, 1975; Sibbald, 1978; Lessire et al., 1982; Sell et al, 1986). The present data were obtained using young broilers, which might explain the low fat AMEn values. Most basal diets, composed of cereals, contain a small amount of unsaturated fat, with a U:S of about 3.5 to 4.5 (Table 2). These data points are in fact the "hinge points" of the equations; at lower U:S, fat utilization parameters change quickly, if, for example, saturated animal fat is added. If a vegetable oil is added to a basal diet, the resulting diets have an U:S higher than 4. At such high ratios, fat utilization and AME n are almost unaffected (Table 3). Consequently, the level of fat inclusion is very important in determining the AME n of the added fat. Gradually adding amounts of animal fat to a diet results in a stepwise decreasing AME n of that fat. The effects of inclusion level have been observed by Mateos and Sell (1980, 1981), Halloran and Sibbald (1979), Wiseman et al. (1986), and Ketels et al. (1986). The small amount of unsaturated basal diet fatty acids may improve absorption of added animal fats (Leeson and Summers, 1976), mainly at low inclusion levels (Wiseman et ai, 1986). The AMEn values of added animal fats are higher in corn based diets, which are
characterized by a high U:S (Sibbald and Kramer, 1978, 1980) than if other cereals are used (with a lower U:S). Three different basal diets were used to calculate these regression equations (Ketels et al., unpublished results). The concept of synergism between animal fats and vegetable oils has been recognized for many years (Sibbald et al., 1962; Lewis and Payne, 1966; Lall and Slinger, 1973). Addition of small amounts of vegetable oils to animal fats results in an AME n of the fat mixture higher than the expected calculated value (Sibbald, 1978). However, Wiseman and Lessire (1987) fail to observe synergistic interactions in beef tallow-rapeseed oil blends. Synergism between fats may also determine to a large extent me curvature of these regression equations. In all the authors' experiments, synergistic interactions between basal diet unsaturated fatty acids and small amounts of added saturated animal fat were observed (Ketels et al., 1986, 1987). Although in the experimental data used to calculate the equations no data points of mixtures of fats were included, the effect of synergism between added saturated and unsaturated fats may also appear from the derived regression equations. Increasing the U:S of a 10% beef tallowsupplemented diet by replacing 10% of the fat by soybean oil, increases the U:S of the dietary blend from 1.35 to 1.80. This replacement results in an improvement of the added fat AME n from 6,100 to about 7,000 kcal/kg, which is higher than the theoretically calculated value of 6,300 kcal/kg. These results confirm the observations of Lall and Slinger (1973) and of Sibbald (1978), who found that replacing 10 to 20% of tallow by vegetable oils was sufficient to increase fat AMEn to levels greater than anticipated.
DIETARY LIPIDS AND METABOLIZABLE ENERGY
ACKNOWLEDGMENTS
The authors wish to thank J. De Deken, W. Bogaert, L. Vermeulen, P. Trossaert, and L. Slagmulder for their technical assistance.
REFERENCES Brockerhoff, H., 1971. Stereospecific analysis of triglycerides. Lipids 6:942-956. De Schrijver, R., 1985. Feeding free fatty acids as energy source. Page 137 in: Proc. 5th Eur. Symp. Poult. Nutr. Jerusalem, Israel. Fuller, H. L., and N. M. Dale, 1982. Effect of ratio of basal diet fat to test fat on the true metabolizable energy of the test fat. Poultry Sci. 61:914-918. Garrett, R. L., and R. J. Young, 1975. Effect of micelle formation on the absorption of neutral fat and fatty acids by the chicken. J. Nutr. 105:827-838. Hakansson, J., 1974. Factors affecting the digestibility of fats and fatty acids in chicks and hens. Swed. J. Agric. Res. 4:33-47. Halloran, H. R., and I. R. Sibbald, 1979. Metabolizable energy values of fats measured by several procedures. Poultry Sci. 58:1299-1307. Horani, F., and J. L. Sell, 1977. Effect of feed grade animal fat on laying hen performance and on metabolizable energy of rations. Poultry Sci. 56:1972-1980. Ketels, E., and G. De Groote, 1988. The nutritional value for broilers of fats characterized by short-chain fatty acids as affected by level of inclusion and age. Anim. Feed Sci. Technol. 22:105-118. Ketels, E., G. Huyghebaert, and G. De Groote, 1986. The effect of the level of inclusion on the AME„ value of soybean oil, lard and tallow. Pages 460-463 in: Proc. 7th Eur. Poult. Conf. Paris, France. Ketels, E., G. Huyghebaert, and G. De Groote, 1987. The nutritional value of commercial fat blends in broiler diets. 1. Effect of the incorporation level on the metabolizable energy content. Arch. Gefliigelkd. 51: 59-64. Krogdahl, A., 1985. Digestion and absorption of lipids in poultry. J. Nutr. 115:675-685. Lall, S. P., and S. J. Slinger, 1973. The metabolizable energy content of rapeseed oils and rapeseed oil foots and the effect of blending with other fats. Poultry Sci. 52: 143-151. Leeson, S., and J. D. Summers, 1976. Fat ME-values: the effect of fatty acid saturation. Feedstuffs 48(46): 26-28. Lessire, M„ B. Leclercq, and L. Conan, 1982. Metabolisable energy value of fats in chicks and adult cockerels. Anim. Feed Sci. Technol. 7:365-374. Lewis, D., and C. G. Payne, 1966. Fats and amino acids in broiler rations. 6. Synergistic relationships in fatty acid utilisation. Br. Poult. Sci. 7:209-218. Mateos, G. G., and J. L. Sell, 1980. 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, 1981. Metabolizable energy of supplemental fat as related to dietary fat level and methods of estimation. Poultry Sci. 60:1509-1515. Muztar, A. J., S. Leeson, and S. J. Slinger, 1981. Effect of blending and level of inclusion on the metabolizable energy of tallow and tower rapeseed soapstocks. Poultry Sci. 60:365-372. Renner, R., and F. W. Hill, 1961. Factors affecting the absorbability of saturated fatty acids. J. Nutr. 74: 254-258. Scheele, C. W., 1986. The effect of the addition of fats and oils with varying fatty acid composition to Tenderers fat on the AME-value and digestibility (in Dutch). Spelderholt Institute for Poultry Research and Exten-
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and total fatty acid utilization, following an asymptotically increasing function. Similar results were obtained for palmitic acid and total fatty acid utilization in the present experiment. The prediction equations were calculated using data of experiments with young broilers and with long chain fats and oils, composed of mainly C16 and C18 fatty acids. Short chain fatty acids are readily absorbed in the intestine, due to their higher polarity (Hakansson, 1974). Fats, composed of these fatty acids, are highly digestible, although their U:S is unfavorable. Coconut oil, for instance, with a U:S smaller than 1, has an AME n value of about 6,970 (Ketels and De Groote, 1988). Fats used to compute the regression equations were all triglyceride fats, with low amounts of free fatty acids. Completely hydrolyzed fats are more poorly utilized than triglyceride fats (Leeson and Summers, 1976; De Schrijver, 1985), because by hydrolysis in the intestine mono and diglycerides are formed, which have excellent emulsifying capacities. These equations are indirectly influenced by the emulsification in the intestine. The capacity of young birds to digest fats is finite (Krogdahl, 1985), because the secretion of bile salts and lipase is limited in young broilers. Consequently, fat utilization, as described by the regression equations, is also affected by the variable influence of the quantity of bile salts and lipase secreted by the chick, depending on fat inclusion levels. The chemical composition of the fat appears to be a very important factor in determining fat AMEn. Th e degree of saturation (U:S), distinctly affects fat utilization parameters. It appears that most of the effects associated with the use of different basal diets, with different inclusion levels and with the use of different fats, are due to the effects of varying chemical composition of the dietary fat fraction. For young broilers of 2 wk of age, the chemical composition of the dietary fat fraction, and especially the degree of saturation, accounts for about 75% of the variation in AMEn and utilization of added fats.
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KETELS AND DE GROOTE Sibbald, I. R., S. J. Slinger, and G. C. Ashton, 1962. The utilization of a number of fats, fatty acid materials, and mixtures thereof evaluated in terms of metabolizable energy, chick weight gains and gain:feed ratios. Poultry Sci. 41:46-61. Stanly, T. S., 1984. Use of fats in diets for growing pigs. Pages 313-331 in: Fats in Animal Nutrition. J. Wiseman, ed. Butterworths, London, England, U.K. Whitehead, C. C , and C. Fisher, 1975. The utilisation of various fats by turkeys of different ages. Br. Poult. Sci. 16:481-485. Wiseman, J., D.J.A. Cole, F. G. Perry, B. G. Vernon, and B. C. Cook, 1986. Apparent metabolisable energy values of fats for broiler chicks. Br. Poult. Sci. 27:561-576. Young, R. J., 1965. Fats and fatty acids in animal nutrition. Pages 61-71 in: Proc. Maryland Nutr. Conf. Feed Mfr., University of Maryland, College Park, MD. Young, R. J., and R. L. Garrett, 1963. The effect of environment, diet composition and the ratio of fatty acids in the mixture on the absorption of fatty acids by the chick. Pages 71-79 in: Proc. Cornell Nutr. Conf. Feed Mfr., Cornell University, Ithaca, NY.
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sion, Beekbergen, The Netherlands. Sell, J. L., F. Horani, and R. L. Johnson, 1976. The 'extracaloric' effect of fat in laying hen rations. Feedstuffs 48(27):28-29. Sell, J. L., A. Krogdahl, and N. Hanyu, 1986. Influence of age on utilization of supplemental fats by young turkeys. Poultry Sci. 65:546-554. Sibbald, I. R., 1978. The true metabolizable energy values of mixtures of tallow with either soybean oil or lard. Poultry Sci. 57:473^177. Sibbald, I. R., and J.K.G. Kramer, 1978. The effect of die basal diet on the true metabolizable energy value of fat. Poultry Sci. 57:685-691. Sibbald, I. R., and J.K.G. Kramer, 1980. The effect of the basal diet on the utilization of fat as a source of true metabolizable energy, lipid and fatty acids. Poultry Sci. 59:316-324. Sibbald, I. R., S. J. Slinger, and G. C. Ashton, 1961. Factors affecting the metabolizable energy content of poultry feeds. 2. Variability in the ME values attributed to samples of tallow and undegummed soybean oil. Poultry Sci. 40:303-308.
1989 Poultry Science 68:1506-1512 INTRODUCTION
Digestibility and AME of fats in broiler diets depend on the fatty acid composition. Renner and Hill (1961) and Young and Garrett (1963) showed that long chain saturated fatty acids were poorly absorbed compared with long chain unsaturated and short chain fatty acids (Hakansson, 1974). Fatty acids are stereospecifically distributed on the glycerol molecule (Brockerhoff, 1971). The importance of these findings for fatty acid utilization was shown by Renner and Hill (1961), Leeson and Summers (1976), and Ketels and De Groote (1988). Saturated fatty acid utilization, however, may be improved by the presence of unsaturated fatty acids in the fat blend (Young and Garrett, 1963; Young, 1965; Lewis and Payne, 1966; Garrett and Young, 1975; Leeson and Summers, 1976). This synergism is caused by the excellent emulsifying capacities of the latter. Sibbald et al. (1961) attributed this
'Research funded by the Institute for the Encouragement of Scientific Research (I.W.O.N.L.).
effect to the presence of lecithin. Since then numerous other authors have pointed to the positive effect of blending vegetable oils with animal fats (Lall and Slinger, 1973; Sibbald, 1978; Scheele, 1986). The nature of the basal diet fat, which is mostly of vegetable origin, may have an important effect on the utilization of added animal fats (Sell et al., 1976; Sibbald and Kramer, 1978, 1980; Fuller and Dale, 1982). Especially at low inclusion levels, interactions between the added fat and the basal diet fat fraction are noticed, resulting in higher animal fat AME values (Ketels et al., 1986; Wiseman et al., 1986). In this study, relations between the degree of saturation of the dietary fat and its utilization and AME value were examined. The objective of this study was to examine to what extent the chemical composition of the dietary fat fraction determines the AME n values of added fats. The ratio of saturated to unsaturated fatty acids (U:S) in the dietary lipid fraction seems very important in affecting fat AMEn, which has been shown in pigs by Stanly (1984).
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ABSTRACT Relationships between the ratio of unsaturated to saturated fatty acids (U:S) in the diet and the utilization and AME„ of added fat were studied using the results of a number of previously reported experiments. Mathematical equations relating fat AME^ fat utilization, and palmitic, oleic, and linoleic acid utilization to the U:S were calculated for young broiler chicks. Best fit regression equations for added fat utilization and fat AMEn were exponential, showing fat utilization increased very steeply in the U:S range of 0 to 2.5, reaching a near asymptotical maximum at a U:S of 4 or more. Synergism between added fats, due to blending vegetable oils with animal fats or using basal diets with unsaturated lipid fractions, led to increased animal fat utilization values. The utilization of saturated fatty acids was especially affected by synergism, whereas utilization of unsaturated fatty acids was not influenced by changing U:S. The origin of effects of factors influencing fat utilization, such as level of fat inclusion and basal diet composition, appeared to be through variation in degree of saturation of the total dietary lipid fraction. For young broilers, about 75% of the variation in fat utilization and AME,, was due to differences in the chemical composition of the fat fraction. (Key words: fats, fatty acids, saturated fatty acids, metabolizable energy, diets)
1507
DIETARY LIPIDS AND METABOLIZABLE ENERGY TABLE 1. Fatty acid composition offatsr Experiment 2 Experiment 1 Soybean oil
Fatty acid
Tallow
Lard
Lard
Renderers Renders Renderers fat no. 1 fat no. 2 fat (3%) (10%) (10%)
Expeiiment 3 Soybean oil
Tallow
.1 11.6 .1 3.5 20.7 55.0 7.0 1.9 5.45
.1 3.3 25.6 2.8 21.0 36.6 3.4 .8 6.5 .87
C"?J1
.1 10.7 .1 1.8 22.3 51.9 7.0 6.1 6.45
.2 3.3 26.0 3.4 22.2 36.6 3.1 .1 5.1 .84
.1 1.6 22.8 3.0 14.0 45.6 10.4 .1 2.4 1.54
.1 1.7 23.7 2.5 14.1 40.4 8.7 1.0 7.8 1.33
.2 2.0 23.3 3.4 13.1 41.1 7.9 .7 8.3 1.38
.1 1.8 23.4 3.1 13.8 39.2 6.9 .7 11.0 1.28
.1 1.7 22.0 2.8 13.7 38.4 6.6 .7 14.0 1.29
Experiment 1 Ketels et al. (1986); Experiment 2 Ketels et al. (1987); Experiment 3 Ketels et al. (unpublished).
MATERIALS AND METHODS
Best fit regression equations between the U: S of the total fat fraction of the diet and the utilization of the added fat were calculated using data determined in digestibility trials with young Hubbard broilers, during their 2nd wk posthatch. A more detailed description of the applied procedure for these trials is given by Ketels and De Groote (1988). The data used to calculate the mathematical relationships were obtained from the studies described by Ketels et al. (1986; 1987; unpublished results). In these experiments, six different fats were assayed for AME: soybean
oil, beef tallow, lard, and three commercial renderers' fat blends of pork and beef fats. The data points covered the range of 2.5 to 12.5% inclusion. Three basal diets were used, the fat fractions of which differed not only in quantity, but also in degree of saturation. The fatty acid compositions of the fats are given in Table 1 and those of the basal diets in Table 2. In total, 45 diets from three different experiments were used to calculate the regression equations; a list of these diets is given in Table 3. Fat AME values were determined using best fit regression equations between level of fat inclusion and dietary AME (Wiseman et al.,
TABLE 2. Fatty acid composition of the basal diets
Fatty acid
SorghumSorghumSorghumCornWheatsoybean meal soybean meal soybean meal soybean meal soybean me; (Experiment 1) (Experiment 2) (Experiment 3) (Experiment 3) (Experiment 3) tOL\
C12:0 C14:0 C16:0 C16:l C18:0 C18;l C18:2 C18:3 Others Unsaturated:saturated Fat content, g/lOOg
.1 16.0 .4 2.6 28.6 44.3 2.3 5.7 4.04
.1 .1 18.0 .4 3.0 27.7 44.7 3.0 3.0 3.57
.6 .6 17.0 .6 3.0 25.3 43.0 3.6 5.7 3.42 2.46
.5 14.3 .3 3.5 22.7 47.1 3.7 8.1 4.03 2.80
.1 .5 22.9 .3 3.6 17.7 44.3 3.3 7.4 2.42 1.84
'Experiment 1 Ketels et al. (1986); Experiment 2 Ketels et al. (1987); Experiment 3 Ketels et al. (unpublished).
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C12:0 C14:0 C16:0 C16:l C18:0 C18:l C18:2 C18:3 Others Unsaturated:saturated
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KETELS AND DE GROOTE TABLE 3. Fat AMEn and utilization values, related to the unsaturated:saturated ratio (U:S)
Basal diet
Added fat
Sorghum Soybean oil
Lard
Tallow
Renderers fat (3% FFA2)
Renderers fat no. 1 (10% FFA)
Sorghum Renderers fat no. 2 (10% FFA)
Wheat
Soybean oil Tallow
Com
Soybean oil Tallow
Sorghum Soybean oil Tallow
Fat and fatty acid utilization Experiment
5 7.5 10 12.5 2.5 5 7.5 10 12.5 2.5 5 7.5 10 12.5 2.5 5 7.5 10 2.5 5 7.5 10 2.5 5 7.5 10 2.5 5 7.5 10 2.5 7.5 2.5 5 7.5 2.5 7.5 2.5 5 7.5 2.5 7.5 2.5 5 7.5
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
AME„
U:S 5.68 5.89 6.01 6.09 2.77 2.34 2.12 1.99 1.91 2.41 1.86 1.59 1.42 1.31 2.42 2.03 1.87 1.72 2.43 2.04 1.85 1.73 2.40 2.00 1.80 1.68 2.41 2.01 1.81 1.69 3.70 4.57 1.31 1.15 1.05 4.50 4.71 1.70 1.38 1.23 4.34 4.83 1.85 1.39 1.26
(kcal/kg) 8,101 8,101 8,101 8,101 7,850 7,850 7,850 7,850 7,850 8,612 7,100 5,928 5,326 8,044 7,625 7,207 6,788 6,817 6,817 6,817 6,817 7,654 7,124 6,669 6,275 7,859 7,070 6,438 5,927 8,244 8,244 5,768 5,594 5,391
7,162 5,868 8,183 8,183 8,018 6,544 5,871
Total fat C16:0
C18:l
C18:2
f"M
93.8 91 89.7 91.5 85.3 83.1 77.6 82.8 86.3 83.9 69.2 63.9 64.1 61.7 92.4 85.6 80.2 77.3 91.7 80.9 76.4 81.5 80. i 79 78.9 88.6 74.6 70.8 70.7 79 91.7 72.9 68.8 58.2 95 90.4 75.8 68.5 57.9 93.4 91.1 65.2 66.3 65.2
99.8 91.7 89.7 84.5 81.8 80.6 76.1 84.9 84.2 76.7 62.8 55.8 52.9 57.4 80.5 74.6 77.8 74.9 81.1 71.4 67.1 73.2 88.7 71.6 66.6 70.9 77.8 64.1 62.1 64.6 75.6 84.6 63.5 62.6 58.4 99.9 91.3 69.5 91.9 59.2 89.3 89.6 54.2 58.6 61.6
95.4 93.4 92.2 93.9 89.9 90.3 84.6 90.3 92.5 99.4 84.9 80.9 83.9 81.9 95.2 92.7 88.6 84.5 96.6 89.3 86.7 89.5 89.2 89.8 86.7 93.8 85.3 83 84.5 80.2 94.5 91.5 92.6 89.9 96.6 90.3 84.9 84.7 86.4 90.4 89.3 78.8 82.7 83.8
94.2 93.1 92.2 94.7 102.8 100.4 96.2 94.5
86.4 100.2
89.7 94.8 91.8
86 91.2
Experiment 1 Ketels et al. (1986); Experiment 2 Ketels et al. (1987); Experiment 3 Ketels et al. (unpublished). FFA = Free fatty acid.
2
1986), in order to reduce the high standard errors associated with calculation of fat AME at low inclusion levels by simple extrapolation (Muztar et ai, 1981). Standard errors of fat
AME, calculated by the multi-level technique, decreased three to seven fold (Wiseman et al, 1986) at inclusion levels of 0 to 10%. The figures used in this experiment were thus
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Sorghum Pork fat
Level of use
1509
DIETARY LIPIDS A N D METABOLIZABLE ENERGY TABLE 4. Mathematical relationships between the ratio of unsaturated to saturated fatty acids (U:S) and fat-utilization parameters Parameter
Equation
Fat AME,,, kcal/kg
= 8,227 ± ± .2079 = 92.02 ± ± .2072 = 93.43 ± ± .1896 = 84.76 ± = 94.07 ± ± .4962 = 95.77 ±
Added fat utilization, % Palmitic acid utilization, % Oleic acid utilization, %
Linoleic acid utilization, %
168 1 - 10,318 ± 2,557 EXP [-1.1685 (U:S)] 2.02 - 103.42 ± 25.60 EXP [-1.0507 (U:S)] 3.68 - 77.96 ± 15.36 E X P [-.6578 (U:S)] 1.34 + 1.57 ± .46 (U:S) 10.64 - 14.78 ± 6.23 EXP [- .4677 (U:S)] 3.66 - .48 ± .85 (U:S)
.768
451 kcal
<.001
.779
5.3%
<.001
.733 .218
6.8% 4.5%
<.001
.253 .024
4.5% 5.2%
<.01 <.05
<01
SE.
interactions, was seen as an increase in added fat utilization. In order to compute the ideal mathematical relationships between the U:S and fat utilization parameters, the best fit equations were selected out of linear, quadratic, cubic, and exponential curves. These were evaluated by means of R^, Sxy% and F values. RESULTS
Best fit regression equations are listed in Table 4. The best fit relations between U:S and AMEn (Figure 1) and total fat utilization (Figure 2) were exponential (P<.001). Best fit
100.
9000
M«
01 ^
X
8000
t
90
ID 0
"«
>»• C 7000
**/
DQ r-
LU
5000
«
«
"I
o Q
'
/ " */
(f) 111
6000
«*
/i
/«."
60
/•
50
4000
u:s RATIO
u:s RATIO FIGURE 1. Relation between the AMEn o f f a t w i dietary unsaturated:saturated fatty acids (U:S) ratio.
^
FIGURE 2. Relation between the digestibility of added fat and the dietary unsaturated:saturated fatty acids (U:S) ratio.
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calculated by extrapolating the regression equations at the experimentally applied fat incorporation levels of 2.5, 5, 7.5, 10, and 12.5% (Ketels et al., 1987). Fat AME values were corrected for nitrogen retention (AMEn). Fat and fatty acid utilization data were calculated by difference using fat absorption values of birds fed die basal diet and birds fed the experimental, fat-supplemented diets. The difference in fat retention represents the absorbed amount of added fat, and is a measure of fat utilization. The utilization of the fat in the basal diet was assumed to be constant. Therefore, every improvement in basal diet fat utilization, e.g., by synergistic
1510
KETELS AND DE GROOTE
equations for oleic and linoleic acid were linear (P<.01), whereas palmitic acid utilization was curvilinearly related to the U:S (P<.001). Residual errors were relatively low. The Sxy^ values for fat AMEn and fat utilization were 450 kcal/kg and 5.5%, respectively. About 75% of the variation in fat AMEn, fat utilization, and palmitic acid utilization can be explained by differences in the U:S. DISCUSSION
As a fat blend contains less saturated acids and more easily digested unsaturated fatty acids, the relative extent of synergism becomes smaller. If the U:S exceeds 4, the fat is utilized at the bird's maximal capacity. Synergism is very important for the absorption of long chain saturated fatty acids, such as palmitic and stearic acids. Unsaturated fatty acids are easily absorbed, regardless of the presence of other fatty acids (Table 4), whereas saturates are to be emulsified prior to absorption (Krogdahl, 1985). Studies of Young and Garrett (1963) and of Garrett and Young (1975) with fatty acids in chick diets, indicated that increasing the U:S of a fatty acid mixture consisting of palmitic, oleic, and linoleic acid improved palmitic acid
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The exponential equations relating AMEn and utilization of added fat to the U:S were very steep in the U:S range of 0 to 2.5. At higher ratios, the curve tended to asymptotically reach a maximum. This maximal value is 8,227 kcal/kg for fat AME n and 92% for fat utilization (a coefficient of the exponential regression equation). Other authors found higher AMEn values for well digestible oils, and even for animal fats (Horani and Sell, 1977; Mateos and Sell, 1980). Most of these values, however, were determined with adult birds or with older broilers. Fat AME n appears to be significandy affected with age (Whitehead and Fisher, 1975; Sibbald, 1978; Lessire et al., 1982; Sell et al, 1986). The present data were obtained using young broilers, which might explain the low fat AMEn values. Most basal diets, composed of cereals, contain a small amount of unsaturated fat, with a U:S of about 3.5 to 4.5 (Table 2). These data points are in fact the "hinge points" of the equations; at lower U:S, fat utilization parameters change quickly, if, for example, saturated animal fat is added. If a vegetable oil is added to a basal diet, the resulting diets have an U:S higher than 4. At such high ratios, fat utilization and AME n are almost unaffected (Table 3). Consequently, the level of fat inclusion is very important in determining the AME n of the added fat. Gradually adding amounts of animal fat to a diet results in a stepwise decreasing AME n of that fat. The effects of inclusion level have been observed by Mateos and Sell (1980, 1981), Halloran and Sibbald (1979), Wiseman et al. (1986), and Ketels et al. (1986). The small amount of unsaturated basal diet fatty acids may improve absorption of added animal fats (Leeson and Summers, 1976), mainly at low inclusion levels (Wiseman et ai, 1986). The AMEn values of added animal fats are higher in corn based diets, which are
characterized by a high U:S (Sibbald and Kramer, 1978, 1980) than if other cereals are used (with a lower U:S). Three different basal diets were used to calculate these regression equations (Ketels et al., unpublished results). The concept of synergism between animal fats and vegetable oils has been recognized for many years (Sibbald et al., 1962; Lewis and Payne, 1966; Lall and Slinger, 1973). Addition of small amounts of vegetable oils to animal fats results in an AME n of the fat mixture higher than the expected calculated value (Sibbald, 1978). However, Wiseman and Lessire (1987) fail to observe synergistic interactions in beef tallow-rapeseed oil blends. Synergism between fats may also determine to a large extent me curvature of these regression equations. In all the authors' experiments, synergistic interactions between basal diet unsaturated fatty acids and small amounts of added saturated animal fat were observed (Ketels et al., 1986, 1987). Although in the experimental data used to calculate the equations no data points of mixtures of fats were included, the effect of synergism between added saturated and unsaturated fats may also appear from the derived regression equations. Increasing the U:S of a 10% beef tallowsupplemented diet by replacing 10% of the fat by soybean oil, increases the U:S of the dietary blend from 1.35 to 1.80. This replacement results in an improvement of the added fat AME n from 6,100 to about 7,000 kcal/kg, which is higher than the theoretically calculated value of 6,300 kcal/kg. These results confirm the observations of Lall and Slinger (1973) and of Sibbald (1978), who found that replacing 10 to 20% of tallow by vegetable oils was sufficient to increase fat AMEn to levels greater than anticipated.
DIETARY LIPIDS AND METABOLIZABLE ENERGY
ACKNOWLEDGMENTS
The authors wish to thank J. De Deken, W. Bogaert, L. Vermeulen, P. Trossaert, and L. Slagmulder for their technical assistance.
REFERENCES Brockerhoff, H., 1971. Stereospecific analysis of triglycerides. Lipids 6:942-956. De Schrijver, R., 1985. Feeding free fatty acids as energy source. Page 137 in: Proc. 5th Eur. Symp. Poult. Nutr. Jerusalem, Israel. Fuller, H. L., and N. M. Dale, 1982. Effect of ratio of basal diet fat to test fat on the true metabolizable energy of the test fat. Poultry Sci. 61:914-918. Garrett, R. L., and R. J. Young, 1975. Effect of micelle formation on the absorption of neutral fat and fatty acids by the chicken. J. Nutr. 105:827-838. Hakansson, J., 1974. Factors affecting the digestibility of fats and fatty acids in chicks and hens. Swed. J. Agric. Res. 4:33-47. Halloran, H. R., and I. R. Sibbald, 1979. Metabolizable energy values of fats measured by several procedures. Poultry Sci. 58:1299-1307. Horani, F., and J. L. Sell, 1977. Effect of feed grade animal fat on laying hen performance and on metabolizable energy of rations. Poultry Sci. 56:1972-1980. Ketels, E., and G. De Groote, 1988. The nutritional value for broilers of fats characterized by short-chain fatty acids as affected by level of inclusion and age. Anim. Feed Sci. Technol. 22:105-118. Ketels, E., G. Huyghebaert, and G. De Groote, 1986. The effect of the level of inclusion on the AME„ value of soybean oil, lard and tallow. Pages 460-463 in: Proc. 7th Eur. Poult. Conf. Paris, France. Ketels, E., G. Huyghebaert, and G. De Groote, 1987. The nutritional value of commercial fat blends in broiler diets. 1. Effect of the incorporation level on the metabolizable energy content. Arch. Gefliigelkd. 51: 59-64. Krogdahl, A., 1985. Digestion and absorption of lipids in poultry. J. Nutr. 115:675-685. Lall, S. P., and S. J. Slinger, 1973. The metabolizable energy content of rapeseed oils and rapeseed oil foots and the effect of blending with other fats. Poultry Sci. 52: 143-151. Leeson, S., and J. D. Summers, 1976. Fat ME-values: the effect of fatty acid saturation. Feedstuffs 48(46): 26-28. Lessire, M„ B. Leclercq, and L. Conan, 1982. Metabolisable energy value of fats in chicks and adult cockerels. Anim. Feed Sci. Technol. 7:365-374. Lewis, D., and C. G. Payne, 1966. Fats and amino acids in broiler rations. 6. Synergistic relationships in fatty acid utilisation. Br. Poult. Sci. 7:209-218. Mateos, G. G., and J. L. Sell, 1980. 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, 1981. Metabolizable energy of supplemental fat as related to dietary fat level and methods of estimation. Poultry Sci. 60:1509-1515. Muztar, A. J., S. Leeson, and S. J. Slinger, 1981. Effect of blending and level of inclusion on the metabolizable energy of tallow and tower rapeseed soapstocks. Poultry Sci. 60:365-372. Renner, R., and F. W. Hill, 1961. Factors affecting the absorbability of saturated fatty acids. J. Nutr. 74: 254-258. Scheele, C. W., 1986. The effect of the addition of fats and oils with varying fatty acid composition to Tenderers fat on the AME-value and digestibility (in Dutch). Spelderholt Institute for Poultry Research and Exten-
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and total fatty acid utilization, following an asymptotically increasing function. Similar results were obtained for palmitic acid and total fatty acid utilization in the present experiment. The prediction equations were calculated using data of experiments with young broilers and with long chain fats and oils, composed of mainly C16 and C18 fatty acids. Short chain fatty acids are readily absorbed in the intestine, due to their higher polarity (Hakansson, 1974). Fats, composed of these fatty acids, are highly digestible, although their U:S is unfavorable. Coconut oil, for instance, with a U:S smaller than 1, has an AME n value of about 6,970 (Ketels and De Groote, 1988). Fats used to compute the regression equations were all triglyceride fats, with low amounts of free fatty acids. Completely hydrolyzed fats are more poorly utilized than triglyceride fats (Leeson and Summers, 1976; De Schrijver, 1985), because by hydrolysis in the intestine mono and diglycerides are formed, which have excellent emulsifying capacities. These equations are indirectly influenced by the emulsification in the intestine. The capacity of young birds to digest fats is finite (Krogdahl, 1985), because the secretion of bile salts and lipase is limited in young broilers. Consequently, fat utilization, as described by the regression equations, is also affected by the variable influence of the quantity of bile salts and lipase secreted by the chick, depending on fat inclusion levels. The chemical composition of the fat appears to be a very important factor in determining fat AMEn. Th e degree of saturation (U:S), distinctly affects fat utilization parameters. It appears that most of the effects associated with the use of different basal diets, with different inclusion levels and with the use of different fats, are due to the effects of varying chemical composition of the dietary fat fraction. For young broilers of 2 wk of age, the chemical composition of the dietary fat fraction, and especially the degree of saturation, accounts for about 75% of the variation in AMEn and utilization of added fats.
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KETELS AND DE GROOTE Sibbald, I. R., S. J. Slinger, and G. C. Ashton, 1962. The utilization of a number of fats, fatty acid materials, and mixtures thereof evaluated in terms of metabolizable energy, chick weight gains and gain:feed ratios. Poultry Sci. 41:46-61. Stanly, T. S., 1984. Use of fats in diets for growing pigs. Pages 313-331 in: Fats in Animal Nutrition. J. Wiseman, ed. Butterworths, London, England, U.K. Whitehead, C. C , and C. Fisher, 1975. The utilisation of various fats by turkeys of different ages. Br. Poult. Sci. 16:481-485. Wiseman, J., D.J.A. Cole, F. G. Perry, B. G. Vernon, and B. C. Cook, 1986. Apparent metabolisable energy values of fats for broiler chicks. Br. Poult. Sci. 27:561-576. Young, R. J., 1965. Fats and fatty acids in animal nutrition. Pages 61-71 in: Proc. Maryland Nutr. Conf. Feed Mfr., University of Maryland, College Park, MD. Young, R. J., and R. L. Garrett, 1963. The effect of environment, diet composition and the ratio of fatty acids in the mixture on the absorption of fatty acids by the chick. Pages 71-79 in: Proc. Cornell Nutr. Conf. Feed Mfr., Cornell University, Ithaca, NY.
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sion, Beekbergen, The Netherlands. Sell, J. L., F. Horani, and R. L. Johnson, 1976. The 'extracaloric' effect of fat in laying hen rations. Feedstuffs 48(27):28-29. Sell, J. L., A. Krogdahl, and N. Hanyu, 1986. Influence of age on utilization of supplemental fats by young turkeys. Poultry Sci. 65:546-554. Sibbald, I. R., 1978. The true metabolizable energy values of mixtures of tallow with either soybean oil or lard. Poultry Sci. 57:473^177. Sibbald, I. R., and J.K.G. Kramer, 1978. The effect of die basal diet on the true metabolizable energy value of fat. Poultry Sci. 57:685-691. Sibbald, I. R., and J.K.G. Kramer, 1980. The effect of the basal diet on the utilization of fat as a source of true metabolizable energy, lipid and fatty acids. Poultry Sci. 59:316-324. Sibbald, I. R., S. J. Slinger, and G. C. Ashton, 1961. Factors affecting the metabolizable energy content of poultry feeds. 2. Variability in the ME values attributed to samples of tallow and undegummed soybean oil. Poultry Sci. 40:303-308.