RELATIONSHIPS AMONG DIET, METABOLIZABLE ENERGY UTILIZATION AND NET ENERGY VALUES OF FEEDSTUFFS

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1 RELATIONSHIPS AMONG DIET, METABOLIZABLE ENERGY UTILIZATION AND NET ENERGY VALUES OF FEEDSTUFFS W. N. Garrett University of California 1 Davis Summary A comparative slaughter experiment was conducted with beef steers to determine the energy utilization of six specific mixtures of a roughage basal and a concentrate basal diet. Of special interest were comparisons of relationships among metabolizable energy (ME) utilization, net energy values and diet composition. ME (kcal/g) was linearly related to proportion of roughage in the diet (r = -.96) and to mean net energy values determined for the diets (r = -.99 for NE m and r = -.97 for NEg). None of these regressions had significant curvilinear components. Since the efficiency of ME utilization for maintenance (km) or for gain (kg) is the product of the reciprocal of ME and NEm or NEg, a curvilinear relationship could be expected between the proportion of roughage in the diet and km or kg. However, these relationships were also linear (r =.96 and -.93, respectively). These findings along with some observations made from the literature form the basis for the suggestion that experimental variability (technical and animal) together with the model chosen to present the results of a particular trial may be responsible for some reports of the associative effects of feeds. (Key Words: Net Energy, Metabolizable Energy, Energy Utilization, Associative Effects.) Introduction The early work of Forbes et al. (1931, 1933) led Kriss (1943) to conclude that net energy values of individual feeds are fundamentally variable in the sense that their value depends on the combination in which they are fed with other feeding stuffs. Blaxter and Wainman (1964) reported, "Our results agree with the t Dept. of Animal Science. statement by Kriss in that the energy retained by an animal depends on the amount and composition of its ration as a whole, and that net energy values can only be applied to single foods when they are given as the sole food." These statements deal with "associative effects" and by implication suggest that a system of feed evaluation based on assigning a specific net energy value to each feedstuff (Lofgreen and Garrett, 1968) may not be appropriate. Vance et al. (1972) and Byers et al. (1976) also attributed the variability they have observed in net energy values of corn and corn silage mixtures to associative effects. The objective of this investigation was to determine the energy utilization of specific combinations of roughage and concentrate by beef steers giving special consideration to the relationships among diet composition, net energy values and the efficiency of metabolizable energy utilization. Experimental Methods This experiment used the comparative slaughter technique to estimate energy retention in beef steers fed diets differing in proportion of roughage and concentrate. Initial and final empty body composition was determined from carcass density (Garrett and Hinman, 1969). The 86 experimental animals, purchased from commercial sources, were of Hereford and Angus parentage (initial empty body weight 234 kg). Six steers were halter broken for use in digestion trials, eight were slaughtered to obtain initial body composition and the remainder (72) received one of six diets formulated from the roughage and concentrate basal mixtures (table 1) to have various levels of hay and grain. The extreme diets were 90:10 and 10:90 ratios of the roughage and concentrate basals. Between these extremes the diets contained either 76:24, 58:42, 42:58, or 26:74 proportions of roughage basal to concentrate basal. The level 1403 JOURNAL OF ANIMAL SCIENCE, Vol. 49, No. 5, 1979

2 1404 GARRETT of roughage (hay) varied from 78.3 to 8.7%. All steers were implanted with Synovex-S (Syntex Laboratories, Palo Alto, CA 94302) at the beginning of the feeding period and those fed the highest roughage diets (90:10, 76:24, 58:42 and 42:58) were reimplanted about midway during the trial. Each basal diet was formulated on an as-fed basis to contain approximately 11.3% crude protein (about 12.5% of dry matter). Calcium, phosphorus and vitamin A were added by appropriate supplementation to maintain a minimum of.45% calcium,.3% phosphorus and 544 1U of vitamin A/kg of vitamin A (see footnote table 1). All animals were housed and fed in individual pens (3x6.5m). One-half the animals (six each diet) had free access to their feed for 24 hr and the other half for 16 hr (approximately 1600 to 0800). The 16 hr treatment was to provide information relative to animal performance and feeding behavior when time of access to feed was limited. This information was requested by scientists dealing with starling control in California feedlots and will not be discussed in detail in this paper. The feeding periods varied in length in an attempt to slaughter all dietary groups at approximately the same average empty body weight (425 kg). The digestibility of three basal feed mixtures (10:90, 50:50 and 90:10, roughage basal: concentrate basal) was determined by total collection of feces from four steers (random assignment to treatment except that no steer received the same diet twice). The steers received a constant amount of feed (about 1.2 times their maintenance requirement) for 14 days after at least 10 days of adjustment to any diet change. Fecal collections were the final 7 days of the constant feed intake period. Gross energy was determined on all feed and fecal samples with an adiabatic bomb calorimeter. Metabolizable energy was estimated as.82 times digestible energy (NRC, 1969; ARC, 1965). Net energy values for maintenance and gain were calculated according to the methods of Garrett et al. (1971) in which fasting heat production is assumed to be 77W 90 kcal/day for all steers (Lofgreen and Garrett, 1968). Analysis of variance (6x 2 factorial arrangement) was used to determine the existence of statistically different means in all criteria of performance from the feeding trial. When different means were found the Duncan multiple range test was applied. Step-wise polynominal regression procedures which added variables by significance provided the statistical techniques to relate various dietary parameters (percentage roughage or ME concentration, for example) to several measures associated with animal response [NE m, NEg or efficiency of ME use for maintenance (k m) and gain (kg)]. One imperfection of the comparative slaugh- TABLE 1. INGREDIENTS OF THE ROUGHAGE AND CONCENTRATE BASAL DIETS Percentage composition, as fed basis lnternat'l Roughage Concentrate Ingredient Ref. No. basal basal Alfalfa hay a Sudan hay a Corn a Cane molasses Fat Urea Trace mineral salt b 15 ".5 Mineral supplements c Vitamin A, IU/kg d d athe hay was chopped by hammermill (1.6 cm screen). The corn was steam rolled. bcontains: NaCI, 96%; Zn,.49% Mn,.39%; Fe,.23%; Cu,.33%; Co,.012%; I,.007%. CDicalcium phosphate ( ), sodium tripoly phosphate ( ), ground limestone ( ) or calcium sulfate ( ). The actual quantity of these ingredients varied to maintain at least.45% Ca and.3% P in the diets (as fed basis). dvitamin A: 544 IU/kg of feed (as fed basis).

3 RELATIONSHIPS AMONG DIET, METABOLIZABLE ENERGY AND NET ENERGY 1405 ter technique is that energy retention for an individual animal is obtained by difference between its final energy content and estimated initial energy content (based on the mean of the initial slaughter group). Thus, an unknown portion of the individual animal variability in energy retention (which influences the determination of NE m, NEg, k m and kg) is due to this technical anomaly. For this reason treatment means as well as individual values were used in the regression analyses. Certainly the individual animal values give the only appropriate statistics relative to the total variation present. The use of means in conjunction with the analyses of the particular parameters associated with energy retention also has justification since the basic experimental unit for the comparative slaughter technique is a group of animals. Results and Discuccion Regression analysis of the data from the digestion trial (table 2) indicates a strong linear relationship (no significant quadratic affect) between the percentage roughage or roughage basal mixture in the diet and the digestible energy concentration. This evidence for no measurable associative effects on the digestibility of these diets is strengthened by the information that the use of mean values to calculate the regression line rather than the individual observations results in an identical intercept and regression coefficient with a coefficient of determination of 99%. Digestion coefficients for the proximate constituents were also linearly related to percentage roughage in the diet, and all coefficients of determination were over 99% when the mean digestion coefficients were used in the regression analysis. The DE and ME contents (kcal/g) of the roughage and concentrate basal mixtures estimated from the regression equations were 2.73, 2.24 and 3.84, 3.15, respectively. DE and ME values (kcal/g) for the six diets (calculated from either regression equation of table 2) were (by level of roughage basal): 10, 3.73 and 3.06; 26, 3.55 and 2.91; 42, 3.38 and 2.77; 58, 3.20 and 2.62; 76, 3.00 and 2.46; 90, 2.84 and A linear relationship between the percentage roughage or concentrate in a series of mixed diets and the digestibility of dietary energy is in agreement with the experiments of Blaxter and Wainman (1964) and Asplund and Harris (1971). However, Byers et al. (1976) found this relationship to be curvilinear for mixtures of corn and corn silage. Some measures of animal response to the feeding regimens are shown in table 3. The attempt to feed all animals to the same final empty body weight (FEBW) was successful in that no differences (P<.05) were observed as being due to diet. There was a general trend for increasing amounts of hay to decrease daily gain (a 1% increase in hay decreased daily gain 4.6g) and to increase the feed to gain ratio (1% more hay increased F to G ratio by.06). There were no significant interactions between hours access to feed and diet for any of the response criteria in table 3. Steers with access to feed for 24 hr were heavier (440 vs 425 kg), gained faster (1.02 vs.94 kg/day) and consumed more feed (7.94 vs 7.40 kg/day) than those with 16 hr access (P<.05). Information related to energy utilization of the diets is given in table 4 and regression equations determined between percentage roughage basal in the diet with net energy values and the efficiency of metabolizable energy utilization are in table 5. TABLE 2. DIGESTIBLE ENERGY (-+ SE) CONTENT OF THE 10:90 50:50 AND ROUGHAGE AND CONCENTRATE BASAL MIXTURES AND THE REGRESSION EQUATION USED TO PREDICT DE CONTENT OF THE MIXED DIETS Diet code Digestible and % energy, Regression Correlation roughage) kcal/g equations a coefficient SE 10:90 (8.7) :50 (43.5) Y = X :10 (78.3) Y = X ay is DE concentration, kcal/g; X 1 is percentage roughage basal in diet; X 2 is percentage roughage (hay) in the diet. The equations were calculated with four observations per diet.

4 1406 GARRETT Steers receiving the 26:74 diet (22.6% hay) consumed more (P<.05) ME than those fed any other diet and animals fed the highest roughage (78.3% hay) diet consumed the least (P<.05) ME. Retained energy (RE) increased as the amount of hay in the diet decreased. Differences in RE, however, were small and not significant among diets of intermediate ME energy content (36.6 through 66.1% hay), but the high roughage (78.3%) and low roughage (8.7 and 22.6%) diets resulted in less and more RE, respectively, (P<.05) when compared to the intermediate energy diets. Data in table 4 and regression relationships in table 5 illustrate the strong linear relationships (P<.O01) between percentage roughage basal in the diet and the NEm and NEg values (no significant curvilinear component). If mean values for NE m and NEg are used in the regression analysis, the coefficients of determinations are 99% and 95%, respectively. These data can, therefore, be interpreted as indicating that NE m and NEg values can appropriately be assigned to individual feedstuffs. Blaxter and Wainman (1964)reported linear regressions of percentage corn (in diets varying from 0 to 100% corn) against the metabolizability of the gross energy or against the net availability of ME for maintenance and for gain. Since NE m and NEg (expressed as kcal/g of diet) are the products of the availability of ME (for maintenance or for gain) and the ME concentration in the diet, it follows that the relationships between percentage corn in the diets and the net energy values (NE m or NEg) would (assuming the regressions were without error) be curvilinear. However, analysis of the data of Blaxter and Wainman (1964), using predicted ME values (from linear equations given in the paper) for the diets and the efficiencies of ME utilization given in their table 13, also indicates strong linear relationships between percentage corn and NE m (r =.989) and NEg (r =.977). These data can be compared to correlation coefficients calculated (data from their table 13) between percentage corn in the diets and the efficiency of ME utilization for maintenance and gain of.791 and.984, respectively. Thus, it seems that an equally appropriate conclusion from their data would have been that percentage corn was linearly related to NEm and to NEg concentration and, therefore, net energy values assigned to single ingredients can be used to predict the net energy value of mixtures of these ingredients. This conclusion would necessarily mean that the efficiency of ME use was, theoretically at least, curvilinearly related to the amount of corn in the diet (assuming that ME concentration and percentage corn are linearly related). The conclusion of Blaxter and Wainman (1964) that the production of 2 kg of corn added to a maintenance ration of hay would be less than when added to a maintenance ration of corn is correct (but opposite to the conclusions of Forbes et al. (1931) that corn added to a basal diet of corn had a smaller net energy value than when added to a diet of alfalfa hay). Therefore, the statement that net energy values TABLE 3. FINAL WEIGHTS, DALLY WEIGHT GAINS, FEED CONSUMPTIONS, FEED TO GAIN RATIOS AND CARCASS WEIGHTS OF STEERS FED DIETS WITH VARIOUS LEVELS OF ROUGHAGE AND CONCENTRATE f Diet code and Days Final Daily Daily Feed to Carcass (% roughage) fed weightg,kg gain, kg feed, kg gain weight, kg 10:90 (8.7) a 1.12 a 6.78 a 6.1 a 288 a 26:74 (22.6) a 1.09 a 7.65b c 7.1 b 280 a 42:58 (36.6) a 1.03 ah 7.41b 7.2 b 308 a 58:42 (50.5) a.96 b 7.73b c 8.1 c 300 a 76:24 (66. I ) a.93 b 8.20 c 8.9 d 304 a 90:10 (78.3) a.78 c 8.20 c 10.6 e 292 a a'b'c'd'ewithin a column means having the same letter in the superscript do not differ (P <.05). fall animal weights are on an empty body basis and feed weights on a dry matter basis. Twelve observations per mean for all treatments with the exception that one steer was removed from treatment 10:90 due to an unidentified illness. glnitial empty body weights were similar for all dietary treatments the overall mean SD is kilograms.

5 RELATIONSHIPS AMONG DIET, METABOLIZABLE ENERGY AND NET ENERGY 1407 TABLE 4. METABOLIZABLE ENERGY (ME) INTAKE, RETAINED ENERGY (RE), NET ENERGY VALUES (NE m AND NEg) AND THE EFFICIENCY OF ME USE FOR MAINTENANCE AND FOR GAIN AS RELATED TO VARIOUS LEVELS OF ROUGHAGE IN THE DIET (Means -+ SE)g Net energy values and the Maintenance efficiency of ME use h Diet code ME requirement Maintenance Gain and (% intake RE mcal/day/ roughage) mcal/day mcal/day W sa NEm, kcal/g Eft. % NEg, kcal/g Eft. % j, J 10:90 (8.7) b c a a a :74 (22.6) c c b b b :58 (36.6) 20.51ab b c c i.22 c ;42 (50.5) 20.28ab b c d d :24 (66.1) 20.18ab b c e d :10 (78.3) a 3.51 d d f e a'b'c'd'e'fwithin a column means having the same letter in the superscript do not differ (P <.05). gtwelve observations/treatment except 10:90 (see footnote table 3). hefficiency of ME use as calculated from NEm/ME and NEg/ME ratios X 100 (NE and ME values are kcal/g of dry matter). can be applied only to single foods given as the sole food appears to be the result of the method of analysis rather than a statistical certainty. In fact the assignment of specific NEm and NEg values to individual feedstuffs does not necessarily mean that different increments of one ingredient added to the other will result in an equivalent animal response, since the mixture will have its own NE values with an efficiency of ME utilization unlike either ingredient when fed alone. An analysis similar to that used by Blaxter and Wainman (1964) can be applied to the data from our experiment. That is, there is a linear relationship between percentage roughage and the efficiency of utilization of ME for maintenance (r = -.97) and for gain (r = -.93) when calculated from group means. However, as shown in table 5, the correlation coefficients are higher if NEm (r =.996) and NEg (r =.974) are used in the regression analysis instead of the efficiency of ME utilization, In this experiment, the conclusion that net energy values are more strongly related to percentage roughage in the diet than were the efficiencies of metabolizable energy utilization seems the most appropriate. (Note: The replacement of percentage roughage basal with percentage roughage does not alter the correlation coefficients or the SE of predicted Y). Vance et al. (1972) reported that NE m values were linearly related to percentage corn silage in silage to concentrate mixtures, but found a significant quadratic influence on the NEg content. Byers et al. (1976) found that the efficiency of ME use for maintenance and production was linearly related to levels of corn in corn to corn silage mixtures, but reported negative associative effects on digestibility and metabolizability for intermediate (37 and 67% added corn) diets. Several experiments (Lofgreen et al., 1963; Garrett et al., 1964, 1971; Hall et al, 1968) show linear relationships between retained energy and feed intake above maintenance. The net energy values of dried grass: barley mixtures reported by Wainman et al. (1970) are also linearly related to the proportion of barley in the mixtures (r =.99). The lack of uniformity in the literature regarding the relationship between proportions of an ingredient in a mixture and its net energy values or the efficiency with which its energy is used for maintenance and production is not confined to the recent experiments cited (see reviews by Mitchell, 1942; Blaxter, 1956; Reid, 1962). It may be that technical errors in the determination of metabolizability and energy balance, as well as the variation in methods used to interpret the results of any single experiment are as much responsible for the different conclusions as some aspect of the associative effect phenomenon. Certainly the conclusion of Forbes et al. (1931), Kriss (1943) and Blaxter and Wainman (1964) that the value of an individual feedstuff depends on the amount and composition of the diet is a well documented fact. The results of this trial indicate the convention which assigns NE m and

6 1408 GARRETT TABLE 5. LINEAR REGRESSION EQUATIONS RELATIVE TO THE UTILIZATION OF ENERGY FROM DIETS CONTAINING VARYING PROPORTIONS OF ROUGHAGES AND CONCENTRATES Variables Regression coefficients Correlation SE Intercept SE Slope SE Coefficients Y Roughage basala(x) versus (Y); NE m (-.996) b.062 (.026) b NEg (-.974).132 (.064) k m (-.963).023 (.009) kg (-.925).047 (.021) aroughage basal is expressed as the decimal fraction. NE m and NER are kcal/g, k m and kg are the efficiency of utilization of ME for maintenance and gain, respectively, expressed a~the decimal fraction.-totai observations 71. bcorrelation coefficients and standard errors in parenthesis were obtained when diet means were used in the regression analysis. NEg values to single feedstuffs has considerable statistical validity for application to ruminants, but because of variability (technical and animal, Garrett, 1978) in this and published experiments, it is not appropriate to unequivocally rule out other interpretations (Blaxter, 1974; Blaxter and Boyne, 1978 ; INRA, 1978). Literature Cited ARC Nutrient Requirements of Farm Livestock. No. 2 Ruminants. Agr. Res. Council, London. Asplund, J. M. and L. E. Harris Associative effects on the digestibility of energy and the utilization of nitrogen in sheep fed simplified rations. J. Anita. Sci. 32:152. Blaxter, K. L The nutritive value of feeds as shources of energy: A review. J. Dairy Sci. 39:1396. Blaxter, K. L Metabolisable energy and feeding systems for ruminants. P 3. Nutrition Conf. for Feed Manufacturers, No. 7. H. Swan and D. Lewis (Ed.) Butterworths, London. Blaxter, K. L. and A. W. Boyne The estimation of the nutritive value of feeds as energy sources for ruminants and the derivation of feeding systems. J. Agr. Sci. Camb. 90:47. Blaxter, K. L. and F. W. Wainman The utilization of the energy of different rations by sheep and cattle for maintenance and for fattening. J. Agr. Sci. Camb. 63:113. Byers, F. M., D. E. Johnson and J. K. Matsushima Associative effects between corn and corn silage on energy partitioning by steers. In M. Vermorel (Ed.) Energy Metabolism of Farm Animals, E.A.A.P. Pub. 19, p Forbes, E. B., W. W. Braman, M. Kriss and R. W. Swift The metabolizable energy and net energy values of corn meal when fed exclusively in combination with alfalfa hay, J. Agr. Res. 43:1015. Forbes, E. B., W. W. Braman, M. Kriss and R. W. Swift The associative effects of feeds in relation to the utilization of feed energy. J. Agr. Res. 46:753. Garrett, W. N Energetic efficiency of beef and dairy steers. J. Anim. Sci. 32:451. Garret-t, W. N Factors influencing energetic efficiency of beef production. Symposium paper. Joint ASAS/ADSA meet. July. (Submitted). Garrett, W. N., G. P. Lofgreen and J. H. Meyer A net energy comparison of barley and milo for fattening cattle. J. Anita. Sci. 23:470. Garrett, W. N., G. P. Lofgreen and J. L. Itull Influence of processing methods on the energy utilization of feed grains. Hilgardia 41:123. Garrett, W. N. and N. Hinman Re-evaluation of the relationship between carcass density and the body composition of beef steers. J. Anita. Sci. 28:1. Hall, G. A. B., C. W. Absher, R. Totusek and A. D. Tillman Net energy of sorghum grain and corn for fattening cattle. J. Anim. Sci. 27:165. INRA Alimentation des Ruminants. Ed. INRA Publications (Route de Saint-Cyr.) Versailles. Kriss, M Evaluation of feeds on the basis of net available nutrients. J. Anita. Sci. 2:63. Lofgreen, G. P., D. L. Bath and II. T. Strong Net energy of successive increments of feed above maintenance for beef cattle. J. Anim. Sci. 22:598. Lofgreen, G. P. and W, N. Garrett A system for expressing net energy requirements and feed values for growing and finishing beef cattle. J. AninL Sci. 27:793. Mitchell, tt The evaluation of feeds on the basis of digestible and metabolizable nutrients. J. Anim. Sci. 1 : 159. NRC United States-Canadian Tables of Feed Composition. Pub National Academy of Science-Nadonai Research Council, Washington, DC. Reid, J. T Energy values of feeds-past, present, and future. P 54 In Symposium Papers on

7 RELATIONSHIPS AMONG DIET, METABOLIZABLE ENERGY AND NET ENERGY 1409 Animal Nutritions Contributions to Modern Animal Agriculture. Cornet[ Univ., Ithaca, NY. Vance, R. D., R. L. Preston, V. R. Cahill and E. W. Klosteman Net energy evaluation of cattle-finishing rations containing varying proportions of corn grain and corn silage. J. Anita. Sci. 34:851. Wainman, F. W., K. L. Blaxter, J. S. Smith and P. J. S. Dewey Calorimetric studies of the nutritional value of dried grass. P 17 In A. Schurch and G. Wenk (Ed.) Energy Metabolism of Farm Animals. EAAP Pub. 13.