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USDA, ARS, U.S. Meat Animal Research Center, Clay Center, NE 68933
1 Correspondence:
P.O. Box 166 (phone: 402-762-4248; fax: 402-762-4209; E-mail:
williams{at}email.marc.usda.gov).
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Key Words: Beef Cattle • Energy Metabolism • Models
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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contains a list of acronyms used in this paper. Acronyms for components of heat energy were taken directly from NRC (1981).
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Metabolizable energy consumed by the animal appears in two forms:
 | [1] |
The first is heat energy (HE), and any ME consumed in excess of HE is retained as part of the body or voided as a useful product. This latter energy is referred to as recovered energy (RE), commonly called energy balance. In growing males and growing nonpregnant, nonlactating females, RE is the energy that is recovered in protein and fat.
Partitioning of HE into meaningful physiologic components is the most complex and controversial aspect of all systems of nomenclature. Components of HE identified by NRC (1981) are basal or fasting heat production (HeE), heat of voluntary activity (HjE), heat of thermal regulation (HcE), heat of digestion, absorption, and assimilation (HdE), heat of fermentation (HfE), heat of waste formation and excretion (HwE), and heat of product formation (HrE). This partitioning is shown in the following equation:
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The components HdE, HfE, HwE, and HrE can be combined and considered to be the heat increment (HiE). In ideal feeding situations in nonstressful environments, HcE would be zero and HjE, the heat associated with activities incident to obtaining food, may be included with HeE. With this simplification, the two main components of HE are HeE and HiE, and Eq. [1
] can be written as
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We propose that the HiE consists of components attributable to maintenance (HiEm) and production (HiEg). Thus, the above equation can be written as follows:
 | [2] |
When production is zero, RE would be zero, and the component HiEg, which is a result of production, would also be zero. In this case, if the animal is in weight equilibrium, this level of MEI would be referred to as the daily ME requirement for maintenance (MEm). When MEI < MEm, body tissues will be mobilized to satisfy the energy deficit, and the animal will lose weight. According to Eq. [2], the components of MEm would be
 | [3] |
Dividing both sides of this equation by MEm, this relationship can be expressed as follows:
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The term HeE/MEm represents the efficiency (km) of ME utilization for maintenance, and the term HiEm/MEm is 1 - km.
When production is greater than zero, all terms in Eq. [2] would be greater than zero; using the information in Eq. [3], we can rewrite Eq. [2] as follows:
 | [4] |
and
 | [5] |
Dividing both sides of Eq. [5] by MEI - MEm gives the following relationship:
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The term RE/(MEI - MEm) represents the net efficiency (kg) or the efficiency of ME utilization for gain, the term HiEg/(MEI - MEm) is 1 - kg, and using the relationship RE/(MEI - MEm) = kg, RE can be expressed as follows:
 | [6] |
The term MEI - MEm is the daily ME that is available for gain (MEg), and according to Eq. [5], the term RE + HiEg would also be equal to MEg. This system of partitioning of MEI is shown in Figure 1, with broken lines to show the summation of RE and HiEg to give MEg.
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In a productive animal, MEI is greater than MEm, and according to Eq. [4], after accounting for MEm, a part of the remaining MEI would be recovered as RE (protein and fat), and the remainder would be lost as heat energy or HiEg. Current systems of energy partitioning stop with Eq. [4], and some allow for adjustments in MEm due to activity and level of feeding. Turner and Taylor (1983) suggested that the same kinds of energetic processes (digestion, circulation, secretion, maintenance of concentration gradients, muscle tone, dynamic turnover of tissues, and food gathering) that food provides for at true maintenance (constant BW and body composition) will be incremented in the productive animal in line with its increased plane of nutrition. Turner and Taylor (1983) and Armstrong and Blaxter (1984) proposed that these increments, which are mainly due to the elevation of vital functions in the productive animal and the costs that are directly involved in the synthesis of RE, are inseparable aspects of a single dynamic pool and cannot be partitioned. This pool is represented by HiEg in Eq. [4], and in current systems of energy partitioning, it is also treated as a single pool.
Milligan and Summers (1986) argued that it would be regrettable if such a singular concept obscured the need for quantitative identification and measurement of the specific changes in support metabolism that accompany production. We propose that partitioning the energy costs associated with production would allow a more accurate means of prediction to be developed. In this system, energy costs that are associated with the elevation of vital functions may be predicted from DMI, and energy costs that are directly involved in the synthesis of RE may be predicted from composition of gain. Using this proposed system of partitioning HiEg we can rewrite Eq. [4] as follows:
 | [7] |
where HiEr represents costs that are directly involved in the synthesis of recovered energy, and HiEv represents costs that are associated with supporting energy-expending processes that are not directly part of the pathways from precursors absorbed to products synthesized. This partitioning of HiEg is also illustrated in Figure 1.
The term HiEv may be grouped with MEm and referred to as maintenance. In current feeding systems, HiEv is contained in both HeE and HiEg. Turner and Taylor (1983) and Armstrong and Blaxter (1984) proposed that the increased energy expenditures that occur when the plane of nutrition is increased to achieve production are caused by the productive state and should therefore be charged against the productive process. In the partitioning system shown in Figure 1, HiEv is left as a separate component, and it will be referred to as the heat production associated with support metabolism in productive animals. The above equation can be rewritten as follows:
 | [8] |
Dividing both sides of this equation by MEI - MEm - HiEv will give the following relationship:
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The term RE/(MEI - MEm - HiEv) is kg, the term HiEr/(MEI - MEm - HiEv) is 1 - kg, and RE can be expressed as follows:
 | [9] |
The term MEI - MEm - HiEv represents the daily ME that is available for gain, or MEg. According to Eq. [8], the term RE + HiEr would also be equal to MEg; this is shown in Figure 1 with solid lines. For the same values of MEI and MEm in Eq. [6] and [9], the estimate of kg in Eq. [9] would have to be greater to give the same value for RE as that in Eq. [6] since MEg calculated with Eq. [9] would be smaller by the amount HiEv. In addition, the impact of previous levels of nutrition on heat production can also be modeled through HiEv. In the next three sections, we will discuss the development of models and methods to predict MEm, HiEv, and the impact of previous levels of nutrition on heat production.
Estimating Maintenance Requirements
The definition of MEm used in model development is the amount of ME that will exactly balance heat production and produce no loss or gain in body energy reserves in an animal that is in equilibrium with respect to full BW (FBW) and body composition. In this case, the total MEI is used to support the equilibrium FBW. The theoretical basis for this definition of MEm is that growing or mature, nonpregnant, nonlactating cattle fed constant amounts of a fixed diet would have a decreasing rate of growth and, over time, would use all of the food consumed to maintain a constant BW. This response was demonstrated by Taylor and Young (1968), who fed six groups of Ayrshire females fixed food intakes to achieve daily MEI that varied from 5 to 20 Mcal in 3-Mcal increments. The heifers were kept on these fixed feeding levels for up to 7 yr, so that they achieved weight stasis at immature BW. These data showed that FBW maintained at equilibrium was simply proportional to food intake, and that MEI at FBW stasis for the six groups in order of increasing MEI were 31.2, 35.6, 32.3, 31.5, 32.0, and 31.7 kcal/kg of FBW. This evidence was extended by Taylor et al. (1981) to show that equilibrium BW was simply proportional to food intake within animals held at weights from 0.25 mature through maturity. This suggests that for a particular diet, MEm/kg of FBW is constant for an individual, and this requirement for an animal in any state is the heat production per kg of FBW, that it would exhibit if at equilibrium under standard conditions at any FBW. This implies that a particular animal, regardless of its present FBW, pattern of energy metabolism, or body composition, would always exhibit the same heat production per kilogram of FBW at equilibrium, and this heat production may be estimated as
 | [10] |
where kMEm is given as kcal/kg of FBW.
Based on the definition of MEm, it follows that experiments in which cattle are fed fixed amounts of a fixed diet until FBW stasis is achieved would provide the most accurate estimates of MEm. In these experiments, MEI would be known, and at weight stasis, the estimate of kMEm would be MEI/FBW. Two sets of experimental data that met these requirements were used to estimate kMEm. In the first experiment, Taylor et al. (1986) divided mature, nonpregnant, nonlactating cows from each of five breeds into four feeding groups per breed, and fed each group a fixed level of a diet containing 2.36 Mcal of ME/kg of DM. The feeding levels were arranged to make the weight of fat in the whole body about 5, 15, 25, and 35% of FBW, and animals were kept for up to 2 yr on these fixed feeding levels. In the second experiment, Jenkins and Ferrell (1997) divided 6- to 10-yr-old mature, nonpregnant, nonlactating cows from each of nine breeds into four groups per breed and fed each group a fixed level of an experimental diet (77.5% ground alfalfa, 17.5% corn, 5% corn silage) containing 2.25 Mcal of ME/kg of DM. The fixed feeding levels were 58, 76, 93, or 111 g/kg of FBW0.75 of the experimental diet, and the experiment ended when the average FBW change for eight contiguous weeks was zero. These two experiments provide data on feed consumption of mature, nonpregnant, nonlactating cows at FBW stasis.
Data on food consumption at FBW stasis for growing cattle are extremely rare; however, some data were published by Foot and Tulloh (1977) on yearling Angus steers, and by Tudor and O’Rourke (1980) on 1-wk-old male and female Hereford calves. In the data of Foot and Tulloh (1977), 18 Angus steers were fed a diet of 2.25 Mcal of ME/kg of DM to maintain a FBW of 330 kg, over a feeding period of 120 d. Data for calves held at their birth weight were obtained from Tudor and O’Rourke (1980), who restricted feed intake of three male and three female Hereford calves at birth for 200 d. From d 81 to 200 of this experiment, the diet had an ME density of 2.86 Mcal of ME/kg of DM. These two experiments provided data on feed consumption of young calves and growing weaned cattle that were at FBW stasis.
Estimating Heat Production of Support Metabolism
Data from Webster et al. (1974) showed that the fasting heat production of British Friesian and Aberdeen Angus steers decreased from 44 kcal/kg of FBW at approximately 100 kg of FBW to 23 kcal/kg of FBW at approximately 400 kg of FBW. Similar results were also obtained by Freetly et al. (1995), who showed that the fasting heat production of Suffolk and Texel ewes decreased from about 50 kcal/kg of FBW at 35% of their mature weight, to about 25 kcal/kg of FBW at maturity. Components of heat production in these data would be HeE and HiEv. If we assume that for a particular diet, MEm/kg of FBW is constant, then HeE/kg of FBW would also be constant. Hence, these data support a model in which HiEv/kg of FBW is initially high in immature animals and gradually decreases to approach zero in mature animals as equilibrium weight is approached. A theoretical model for MEm and HiEv that fits this proposal is shown in Figure 2.
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Studies in which fasting (FHP) and fed heat production were measured showed that the ME equivalent of fed heat production increased by 13.6% (Birkelo et al., 1991; cattle), 15 to 18% (Gray and McCracken, 1979; pigs), and 15% (Thorbek and Henkel, 1976; cattle) per multiple of the ME equivalent of FHP intake. In these studies, the ME equivalent of FHP may be regarded as equivalent to MEm; hence, using MEm as the base, level of nutrition was defined in terms of multiples of MEm intake (MM) as follows:
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and the following relationship was used to model HiEv/kg of FBW as a linear function of MM:
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where kHiEv is in units of kcal ME/kg of FBW. This equation can be written as follows:
 | [11] |
With this formulation, HiEv would be zero when MEI is the same as MEm (MM = 1), and would gradually increase in value as MEI exceeds MEm. To estimate HiEv, breed estimates for kHiEv would be needed, and the approach used to obtain these estimates will now be discussed.
Starting with Eq. [7], the terms MEm, HiEv, and HiEr + RE were replaced by the right hand sides of Eq. [10] and [11], and MEg, respectively. These substitutions are shown in the following equation:
 | [12] |
This equation was used as the basis for obtaining breed estimates for kHiEv. The approach used was to predict MEI with varying values of kHiEv to find the kHiEv value that minimized the sum of squared deviations between observed and predicted MEI. For this method to work, kHiEv must be the only unknown in Eq. [12]; hence, we would need breed estimates for kMEm and experimental data on observed MEI and other variables needed to predict MEI with Eq. [12]. Breed estimates for kMEm were independently obtained, as discussed in the previous section, and experimental data in which FBW, dFBW, and MEI were observed were obtained from Smith et al. (1976) and Cundiff et al. (1981; 1984) on the average performance of 17 different biological types of steers in the first three cycles of the Germplasm Evaluation (GPE) project (a comprehensive investigation conducted to characterize production performance of diverse breeds of cattle) at the U.S. Meat Animal Research Center.
Predicted values for MEg were obtained by first using the model of Williams et al. (1992) to predict dEBW from observed values of dFBW in the experimental data set. Predicted values for dEBW were then used as input to the model of Williams and Jenkins (1998) to predict MEg with varying values for kHiEv. The model of Williams and Jenkins (1998) uses dEBW as input to predict the amount of fat-free matter (FFM) and fat in dEBW, and MEg was calculated from these predicted values as follows:
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where FP is the fraction of protein in FFM, and 5.7 and 9.5 represent the amount of energy in Mcal/kg of DM of protein and fat, respectively (Brouwer, 1965). Methods to predict FP and kg are developed and discussed in a companion paper (Williams and Jenkins, 2003). In the actual estimation process, growth and body composition of the 17 biological types of steers in the experimental data were simulated with the model of Williams and Jenkins (1998) until the steers achieved the EBW observed in the data. This model consists of a system of differential equations that are numerically integrated on a daily basis. During the finishing phase, MEI was predicted on a daily basis according to Eq. [12], using input values for kHiEv and kMEm and predicted values for MEg. For each breed type, the input value for kHiEv was varied from 5 to 15 kcal in 0.1-kcal increments, in separate runs. Deviations of predicted from observed values for MEI were squared and summed, and the kHiEv input value that resulted in the minimum sum of squared deviations was selected as the breed estimate of kHiEv.
Estimating the Impact of Previous Plane of Nutrition on Heat Production
Turner and Taylor (1983) analyzed data from Monterio (1972) and Ledger and Sayers (1977) on metabolic responses with changes in food intake and showed a delayed response in metabolism with a change in food intake. These results support a delayed response in heat production with a change in level of feeding. According to Eq. [7], the components of HE that would vary with level of feeding are HiEv and HiEr since MEm is constant on a unit FBW basis. For a particular diet, the component HiEv is a direct function of level of nutrition as in Eq. [11], whereas the component HiEr is a function of composition of gain, which is impacted by level of nutrition. This suggests that HiEv would be a good candidate to model the impact of previous level of nutrition on heat production, and this can be accomplished with the following equation:
 | [13] |
where the value for HiEv on day t (HiEv[t]) is calculated as the value of HiEv on day t - 1 (HiEv[t - 1]) plus a fraction 
of the difference between HiEv calculated with Eq. [11] (HiEv[11]) on day t, and HiEv(t - 1). Keele et al. (1992) used the same concept to model the impact of nutritional changes on body composition in growing cattle, with an value of 0.03, and we used the same value for .
Other Components of Heat Production
In developing the ME partitioning system in Figure 1, it was assumed that animals were in a nonstressful environment with respect to climatic conditions and food availability. The consequences of this assumption were that heat of thermal regulation was zero, and heat production associated with activities that were incident to obtaining food were included in basal heat production. In stressful environments, allowances would have to be made for heat production associated with thermal regulation and/or increased levels of activity. The ARC (1980) uses an allowance for standing and walking, and CSIRO (1990) developed methods to calculate allowances for thermal regulation and activity. The impact of thermal regulation and increased levels of activity would be to increase the maintenance requirement of the animal. We suggest that this increased requirement be reflected in MEm and that published methods be used to calculate allowances for heat production associated with thermal regulation and increased activity in stressful environments.
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Results of the experiments by Taylor et al. (1986) and Jenkins and Ferrell (1997) are shown in Table 2. Estimates of kMEm from Jenkins and Ferrell (1997) ranged from a low of 27.6 kcal of ME for Pinzgauer to a high of 33.6 kcal of ME for Red Poll, and estimates for Hereford, Angus, and Charolais were the same (30.5 kcal of ME). The mean estimate of kMEm for Aberdeen Angus and Hereford cows from Taylor et al. (1986) was 26.85 kcal of ME, compared with an estimate of 30.5 kcal of ME from Jenkins and Ferrell (1997) for Angus and Hereford cows. Dietary ME concentrations in the two experiments were not much different (2.25 vs. 2.36 Mcal/kg of DM); hence, the lower estimate obtained in the experiment of Taylor et al. (1986) is probably due to a lower level of animal activity in this experiment (cows on the highest level of feeding were fed through electronic feeding gates, and cows on the other three feeding levels were fed in tiestalls).
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, together with results for Hereford and Angus cows from Jenkins and Ferrell (1997). The ratio of partial efficiencies of ME utilization for maintenance calculated according to NRC (2000) were used to adjust the kMEm values to a common dietary ME density value of 2.84 Mcal/kg of DM. This energy density is the same as the average energy density of the diets used in the first three cycles of the GPE project. The adjusted kMEm values were very similar for the different ages and sexes. The higher estimate 28.4 kcal of ME from Foot and Tulloh (1977) is probably due to the fact that feed consumption was still decreasing, but at a very small rate after 120 d of treatment. These results suggest that there were no differences due to sex in estimates of kMEm and support the theoretical model in Figure 2 that kMEm is constant from birth to mature weights.
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. The adjusted values were obtained by multiplying the observed kMEm values by the ratio of km calculated from the ME density of the experimental diet to the km value calculated for a diet containing 2.84 Mcal of ME/kg of DM, according to NRC (2000). Cundiff et al. (1993) grouped 25 breeds of cattle into seven biological types based on growth rate and mature size, lean-to-fat ratio, age at puberty, and milk production. This grouping information, together with the information from Jenkins and Ferrell (1997) on kMEm estimates, was used to estimate kMEm values shown in Table 4 for an additional 12 breeds of cattle that were evaluated at the U.S. Meat Animal Research Center. Estimates drawn from data published by Smith et al. (1976) and Cundiff et al. (1981; 1984) for 15 types of crossbred cattle that were evaluated in the first three cycles of the GPE program are also shown in Table 4. These estimates were calculated as half the sum of the purebred and the Hereford-Angus estimate of kMEm, assuming no heterosis for kMEm, and using 27.8 kcal for the Hereford-Angus crossbred value for kMEm.
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The input values of kHiEv that minimized the residual sums of squares between observed and predicted MEI values for the 17 breeds in Cycles 1, 2, and 3 of the GPE data (Smith et al., 1976; Cundiff et al., 1981; 1984) are shown in Table 5. Estimates for Angus, Hereford, and Hereford-Angus crossbreds were 8.8, 8.6, and 8.8 kcal, respectively. These estimates show very little evidence for heterosis in kHiEv, and an estimate of 8.7 kcal was used for the Hereford-Angus crossbreds. Values for kHiEv for purebreds were calculated by doubling the crossbred value and subtracting 8.7. These values are also shown in Table 5 for 19 purebreds and for purebreds that were not included in the GPE data; kHiEv values were obtained by use of breed groupings from Cundiff et al. (1993).
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Calculated and Simulated Responses
Results of calculations in Table 6 show the theoretical response in HiEv, HiEv/kg of FBW, and (MEm +HiEv)/kg of FBW to changes in level of nutrition and stage of maturity. Input values used for kMEm and kHiEv were 30 kcal/kg of FBW and 8.5 kcal/[kg of FBW x (MM - 1)]. Increasing MEI from 18 to 24 Mcal at 200 kg of FBW increased MM from three to four, and this resulted in an increase in HiEv from 3.4 to 5.1 Mcal and an increase in HiEv/kg of FBW from 17 to 25.5 kcal. Heat production accounted for by the sum of MEm and HiEv increased by 18% from 9.4 to 11.1 Mcal. This response is similar to observed increases in heat production of 13.6% (Birkelo et al., 1991; cattle), 15 to 18% (Gray and McCracken, 1979; pigs), and 15% (Thorbek and Henkel, 1976; cattle) per multiple of the ME equivalent of the FHP intake. For a fixed MEI level of 24 Mcal, the calculated MEm was 6, 12, and 24 Mcal at FBW values of 200, 400, and 800 kg; hence, at 800 kg of FBW, this level of MEI would be equal to MEm, MM would be 1, and HiEv would be zero (see Eq. [6]). The value of (MEm + HiEv)/kg of FBW was 55.5, 38.5, and 30.0 kcal at FBW of 200, 400, and 800 kg, respectively. This decrease was entirely accounted for by the decrease in HiEv/kg of FBW, since kMEm was constant at 30 kcal/kg of FBW. This response is a result of MEm increasing from 6 to 24 Mcal, which caused MM to decrease from 4 to 1 with the constant MEI level of 24 Mcal. These results demonstrate the ability of the model to respond to increasing levels of maturity and are similar to the theoretical responses illustrated in Figure 2.
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were used to estimate MEm and HiEv on a daily basis. The response in (MEm + HiEv)/kg of FBW for the three cattle types with stage of maturity is illustrated in Figure 3. The response for Simmental and Charolais crossbreds was similar, with Simmental being about 3 kcal greater at 50% of mature weight. The (MEm + HiEv)/kg of FBW for Hereford-Angus crossbreds decreased at a faster rate than Simmental and Charolais crossbreds as proportion of mature weight increased. These responses are similar to the theoretical model in Figure 2, and they follow the same pattern as the experimental data from Webster et al. (1974) and Freetly et al. (1995). Theoretically, if these cattle were fed the same diet for an indefinite period of time, the values of (MEm + HiEv)/kg of FBW would approach at the kMEm value for each breed type.
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Received for publication September 19, 2002. Accepted for publication February 20, 2003.
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