J. Anim. Sci. 2005. 83:2590-2597
© 2005 American Society of Animal Science
Measurements of heat production for estimation of maintenance energy requirements of Hereford steers1,2
M. Derno3,
W. Jentsch,
M. Schweigel,
S. Kuhla,
C. C. Metges and
H.-D. Matthes
Research Unit Nutritional Physiology "Oskar Kellner," Research Institute for the Biology of Farm Animals (FBN), D-18196 Dummerstorf, Germany
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Abstract
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This study aimed to test the hypothesis that maintenance energy requirement (MEm) can be estimated from continuous heat production measurements during a change from a near maintenance feeding level to far below maintenance for two consecutive days. The MEm of eight Hereford steers weighing 286 ± 5 kg (mean ± SE) was determined in a balance trial. In addition, during the 10-d collection period, the animals were kept in open-circuit respiration chambers to measure 24-h gas exchange continuously at 10-min intervals. During the balance trial, the animals were fed dried chopped grass twice daily at an estimated level of 1.2 x MEm. After termination of the collection period on the 11th d of the balance trial, the steers were offered 2 kg/d of wheat straw while only gas exchange was measured. Estimates of MEm were derived from heat production (HP) data. The analyses included values of 24-h HP, HP of the nocturnal period (0000 to 0630), HP of the nocturnal period (excluding HP caused by standing) during the grass-feeding period and 24-h HP, nocturnal HP, and nocturnal HP (excluding HP caused by standing) during the straw feeding period. The MEm predicted from estimates of HP measurements were 536 ± 9, 470 ± 8, 441 ± 8, 435 ± 8, 393 ± 9, and 373 ± 9 kJ·kg of BW–0.75·d–1, respectively, whereas MEm calculated from data of the balance trial were 416 ± 9 kJ·kg of BW–0.75·d–1. Values predicted for nocturnal HP (excluding HP caused by standing) of grass fed animals, 24-h HP, and nocturnal HP during straw feeding did not differ significantly from MEm. The differences in MEm among animals were reflected by all estimates of HP, whereas the correlation with the 24-h HP during straw feeding reached 0.9 (P = 0.002). We conclude that the method described is adequate to determine MEm with a sufficient degree of accuracy.
Key Words: Maintenance Energy Requirement • Heat Production • Hereford Steers • Straw Feeding
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Introduction
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The need for ME is mainly determined by the maintenance energy requirement (MEm), which represents 50 to 70% of the energy consumed. In contrast to previous beliefs, it has been shown that energy requirements for maintenance per unit of metabolic body size are not constant (Turner and Taylor, 1983
). Maintenance requirements may vary by 10 to 30% because of genetic differences, suggesting substantial among- and within-breed differences for this trait (Hotovy et al., 1991; Johnson et al., 2003).
The utilization of such differences in maintenance requirements offers opportunities to improve the economic efficiency of beef cattle production, as the use of breeds or selection for animals with lower MEm is expected to decrease feed costs, the largest variable cost in beef cattle production. To realize these opportunities, the accurate determination of MEm at various production levels is required (Hotovy et al., 1991). Conventional balance techniques (Schiemann et al., 1971) are very time-consuming and expensive. Thus, it is desirable to develop a method that allows the determination of MEm within a shorter time. In animals fed at near maintenance levels followed by a day of straw feeding (below maintenance requirements), a short transition period occurs where the animal’s metabolism switches from an anabolic to a catabolic state. Heat production (HP) measured in this period may be an accurate estimate of MEm, as this entity is defined as the energy required to exactly balance HP (Webster, 1978; Wenk et al., 2001). Thus, the present study was designed such that specific HP values could be determined by measurement of HP during specific periods during the transition phase with or without adjustment for physical activity and without individual animal measurement of diet metabolizibility. These HP values will be compared with MEm determined from a conventional balance trial.
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Materials and Methods
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This study was conducted with the approval by the Animal Care Committee of the Ministry of Nutrition, Agriculture, Forestry, and Fishery, Schwerin, State Mecklenburg-Vorpommern, Germany (No. VI-522a-7221.31-1-002/99).
Animals
Eight Hereford steers (approximate age = 530 d; BW = 286 ± 5 kg) selected at random from the farm at Gut Dalwitz (Dalwitz, Germany) were used. After arriving at the experiment station, the animals were halter-trained and adapted to handling and to the respiration chambers for 5 mo before the trials started. The remaining clause is just a fragment.]. During this time, they were housed and fed individually at 1.6 x their estimated MEm. Mean BW of the eight animals was used to predict maintenance intake. The feed amount was adjusted to increasing BW every 2 wk. The diet consisted of barley grain and dried chopped grass (20:80 wt/wt, as-fed basis) supplemented with 15 g (as-fed basis) of a vitamin and mineral mixture (22% Ca; 3% P; 10% Na; 3% Mg; 600,000 IU/kg vitamin A; 80,000 IU/kg vitamin D3; 750 ppm vitamin E; 6,000 ppm Zn; 6,500 ppm Mn; 1,000 ppm Cu; 100 ppm I; 40 ppm Se; and 25 ppm Co, DM basis).
Experimental Design
Experiments were performed in two cycles with four animals each. During the balance trials, an experimental diet consisting only of dried chopped grass supplemented with the vitamin and mineral mixture was fed at a level of 1.2 x MEm (assumed MEm of 550 kJ· kg of BW–0.75· d–1, where BW = mean BW of the four animals in each cycle). This feeding level was maintained during the complete balance period, which consisted of a 10-d adaptation period and a 10-d collection period. Subsequent to the collection period, 2 kg (as-fed basis) of long-stem wheat straw was fed for 1 d. The chemical composition of the diets is given in Table 1.
The balance trials were performed in four open-circuit respiration chambers, with a volume of 11 m3 each at 18°C (Schiemann et al., 1971). Relative humidity was 60%. After the adaptation period, the animals were moved to the respiration chambers, where they were kept for the total collection and the straw feeding periods. The steers were fed meals of equal size twice daily (0730 and 1330) and were given free access to water. The chambers were equipped with a hydraulic feeding system allowing feeding without opening the chamber. Feed intake was monitored daily, and any spilled and uneaten feed was recorded and considered when daily feed intake was calculated.
Feces and urine were sampled daily in the morning during the collection period. After homogenization, weighed aliquots of feces (20%) and urine (3%) were taken and stored at 4°C for subsequent analyses. Feed and excreta composition was analyzed using standard procedures to calculate energy balance (Schiemann et al., 1971).
Gas exchange was measured continuously throughout the collection and straw feeding periods at 10-min intervals (Figure 1) by infrared absorption-based CO2- and CH4- and paramagnetic-based O2-gas analyzers (Maihak AG, Germany), respectively. Data were collected using Simatic hardware and Win CC software (Siemens AG, Germany). This allowed monitoring of the effects of activity and posture on diurnal variation of HP. Daily HP (23.5 h) was estimated based on measurements of O2 consumption, CO2 and CH4 production, and daily urine excretion (Brouwer, 1965; Schiemann et al., 1971). Measurements were interrupted for 0.5 h every day to clean the chambers and to perform the collections. Accordingly, no corrections of the gas exchange measurements relative to human contributions were necessary. Immediately after cleaning (approximately 10 min), the chambers were closed to allow for equilibration of the gas concentrations before taking measurements of gaseous exchange. Standing and lying times were registered by a photoelectric cell; other physical activity was recorded by a motion-monitoring device based on infrared light reflection. Constant movement throughout a single measurement period of 10 min would amount to 6,000 counts. The animals were weighed before and after the collection and straw feeding periods. Additionally, when animals were kept in respiration chambers, their heart rate was monitored. Data were taken at 1-min intervals by a heart rate monitor (Polar, Kempele, Finland) that was placed in a belt tied around the thorax behind the forelegs. Heart rate data sets were incomplete for some animals because of difficulties in attaching the monitor electrodes on the steers. Therefore, only heart rate data collected after the 17-h overnight feed withdrawal period were averaged for the first 20 min of each daily trial (0700 to 0720) before statistical analysis. Rectal temperature was measured daily in the morning, 16 h after feed withdrawal.
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Figure 1. Time course of heat production by Animal No. 2 over the entire experimental period. MEm = maintenance energy requirement.
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Calculations and Statistical Analyses
In the balance trial, maintenance energy requirement (MEmBT) was calculated as the difference between the energy value of feed intake (MEI) and the MEI at zero gain according to equation [1]. According to Schiemann et al. (1971) and Hoffmann et al. (1993), a coefficient of utilization for growth (kg) of ME of 0.55 was used to calculate MEm as
 | [1] |
where ER is energy retention.
The following values of HP over defined periods of time were calculated to compare the MEm values derived from the balance trial (MEmBT) with those from continuous HP measurements:
- 24-h HP during grass feeding (HPgrass);
- HP of the period from 0000 to 0630 during grass feeding (HPnocturnal grass);
- Minimum HP value of the period from 0000 to 0630 (HPnocturnal min grass);
- 24-h HP during straw feeding (HPstraw1);
- HP of the period from 0000 to 0630 during straw feeding (HPnocturnal straw); and
- Minimum HP value of the period from 0000 to 0630 during straw feeding (HPnocturnal min straw).
Because the last meal was offered 10.5 h before HP measurements and because the animals consumed the feed in <30 min, it was assumed that HP in the nocturnal period (0000 to 0630) was not influenced by HP components associated with ingestion and digestion. For this reason, HPnocturnal was taken as a measure for the energy metabolism associated with nutrient metabolism. Peaks of HP occurring in this period were associated with physical activity of the animals. Therefore, the minimum values of HP in the nocturnal period reflect energy metabolism only because of metabolic processes. For comparison with the other HP estimates, the mean of the 10 minimum HP values in the nocturnal periods was computed.
Energy expenditure for standing was calculated as the difference between HPnocturnal and HPnocturnal min for both feeding regimens (Schrama et al., 1995). To facilitate the comparison with other energy metabolism variables, data were converted into 24-h values. Data are given as means (±SE).
Heat production data were analyzed with the following linear repeated measures model: yij = µ + 
i + 
j + 
; where is the effect of animal i, is the effect of day j, and is the random experimental error. Differences among animals and days of the collection period per measurement of the animal were tested as repeated measures using the MIXED procedure of SAS (Version 8; SAS Inst., Inc, Cary, NC). Differences among methods and animals were tested by two-way ANOVA using the GLM procedure of SAS. If the F-test was significant (P < 0.05), differences were evaluated by Tukey-HSD-test. Individual correlations between MEm estimated by the various methods, as well as correlations between MEm and ME, CH4 energy, age, BW, rectal temperature, and heart rate were calculated using the CORR procedure of SAS.
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Results
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Energy metabolism data for each animal are presented in Table 2. The variation among animals was not related to MEm. A typical HP profile of an individual animal for the total 11-d experimental period (10-d collection period, 1-d straw feeding period) shows two distinct daily peaks in response to the meals (Figure 1
). Following feed distribution in the morning, HP increased from the minimum level to a first peak reached after approximately 1 h and 15 min. A second peak in HP was observed after the consumption of the second meal. As expected, the straw feeding resulted in a decrease in the mean daily HP (Table 3). Daily total HP (HPgrass) and the HP values of the nocturnal period (HPnocturnal grass) were greater than MEmBT. The values of MEm estimates derived from HPnocturnal grass min, HPstraw, and HPnocturnal straw did not differ from MEmBT. Maintenance energy requirement for the individual animals derived from the various HP measurements were all positively correlated with MEmBT; MEm estimated during the straw feeding day (HPstraw) showed the strongest association (r = 0.90). There were considerable differences in MEm among animals estimated in the balance trial, with the greatest difference being 22.8% (between Animal No. 1 and Animal No. 3).
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Table 2. Energy digestibility, methane energy, ME, respiratory quotient, and maintenance energy requirement determined in the balance trial
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Table 3. Maintenance energy requirement (MEm) of Hereford steers determined in a balance trial and predicted from continuous measurements of heat production (HP; kJ·kg of BW–0.75·d–1) during grass and wheat straw feeding
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The differences among animals were not related to age (r = –0.12; P = 0.092); however, BW at the end of the balance trial showed a negative correlation with MEmBT (r = –0.81; P = 0.013). The BW gain over the 10 d ranged from 0 to 7 kg. Feeding straw for 1 d resulted in BW loss of up to 7 kg (Table 4).
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Table 4. Age and BW characteristics before and during the balance trial, as well as heart rate (HR) and rectal temperature (RT) during the balance trial of Hereford steers
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The mean heart rate measured over 20 min before the first feeding was 58 beats/min. The values were not correlated with the MEmBT measurements (r = 0.38; P = 0.38). The mean rectal temperature measured daily in the morning after a 16-h feed withdrawal period was 38.5 ± 0.05°C. Rectal temperature was positively associated with MEm (r = 0.70; P = 0.056).
Differences in activity-related thermogenesis may contribute to both within- and among-animal variation in MEm. In the respiration chamber, standing was the pre-dominant component of physical activity. Animals spent 29% (range, 23 to 37%) of the day standing (Table 5). The duration of standing periods was unrelated to MEmBT for both diets, but animals stood longer (P = 0.002) during the straw feeding day. Energy expenditure for standing did not differ (P = 0.86) among animals (Table 3). The physical activity of the animals in the respiration chamber also was recorded. On average, 1,200 activity counts per 10 min were registered; there was no difference (P = 0.072) between diets (Table 5). Activity counts were not associated with MEmBT.
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Table 5. Physical activity and standing time per day in relation to maintenance energy requirement (MEm) and heat production (HP) of Hereford steers fed the grass diet and wheat straw
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Discussion
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We hypothesized that MEm can be estimated from continuous HP measurements during a transition phase from near maintenance energy supply to feed withdrawal. Our value for MEm based on data from the balance trial, 416 ± 9 kJ·kg of BW–0.75·d–1 is in agreement with literature data, which range from 370 to 620 kJ·kg of BW–0.75·d–1 (Birkelo et al., 1991; Jentsch et al., 1995; Williams and Jenkins, 2003). Steers gained approximately 300 g/d, indicating that feed intake exceeded maintenance. The calculation of MEm by extrapolation of ME intake to zero energy balance assumes a linear relationship between MEI and energy retention above maintenance and, thus, a constant efficiency of ME utilization, which is possibly incorrect. In previous investigations in oxen and wethers, a linear relationship was established for intakes 
2 x MEm (Nehring et al., 1961, Schiemann et al., 1971). In this study, a supposed MEm of 550 kJ·kg of BW–0.75·d–1 was used to calculate the feed amount for the collection trial. This estimate was derived from BW gain (approximately 750 g/d) and feed consumption during the adaptation period. Thus, we conclude for the calculation of feed amount that MEm of the Hereford steers used in the present experiment was in the upper range reported for cattle. In contrast, the recalculation of MEI by use of the experimentally determined values for ME and MEm (equation [1]) showed that the animals were actually fed at 1.6 x MEm rather than at the planned level of 1.2 x MEm. Despite this, and assuming a linear relationship between energy retention and MEI in animals fed above maintenance, it seems possible to extend the feeding level in the collection period to higher levels than near maintenance.
Estimates of MEm calculated from the measurement of the minimum HP of the nocturnal period during grass feeding (HPnocturnal min grass 441 ± 8 kJ·kg of BW–0.75·d–1), the 24-h HP during straw feeding (HPstraw 435 ± 8 kJ·kg of BW–0.75·d–1), and the HP of the nocturnal period during straw feeding (HPnocturnal straw 393 ± 9 kJ·kg of BW–0.75·d–1) did not differ significantly from MEmBT. Correlations suggest that this equivalence holds at the individual animal level, thereby supporting our hypothesis that MEm can be determined by one of these methods. The 24-h HP measured at the straw feeding day (HPstraw; Table 2) shows the strongest correlation (r = 0.90) with MEmBT. These results agree with our hypothesis that animals fed at near maintenance levels enter a transient phase of zero energy balance during the feed withdrawal period. This phase is equal to the mean minimum HP of the nocturnal period calculated by HPnocturnal grass min, which excludes the component of physical activity and reflects only metabolic variations. In contrast, the mean daily HP of the straw feeding day (HPstraw) comprises the energy requirement for normal physical activities, mainly standing, and activities related to the ingestion of feed. As shown by Susenbeth et al. (2004), the contribution of the latter component to HP mainly results from the activity of eating and chewing, other factors being of only minor importance. Susenbeth et al. (2004) also demonstrated that the energy requirement of ingestion strongly depends on the type of diet with relatively low values (14 J·min–1·kg of BW–1) for long straw compared with other feeds. Requirements of MEm include the energy cost for physical activities, which is limited during respiration chamber confinement. Under such conditions, postural changes and standing are the main contributors. Our data showed considerable among-animal variation for daily standing times during straw feeding, which agrees with other observations (Ortigues et al., 1993; Schrama et al., 1995). Feed quality and quantity can influence the activity pattern and thereby alter HP and MEm requirement (Birkelo et al., 1991). Interestingly, in the present study the observed differences among animals in daily standing time were not correlated with MEm. The increment in HP caused by standing amounted to 6.8% compared with lying HP during the collection period; during the straw feeding period, HP during standing was 5.4% greater than that during lying. These values are less than the range of 8 to 24.9% summarized recently by Susenbeth et al. (2004). As expected, straw feeding resulted in decreased HP (Figure 1). The lower standing energy costs calculated for steers fed straw indicate metabolic changes (e.g., in protein turnover and ion transport), possibly as an energy-sparing effect.
In the present study, a mean MEm of 416 ± 9 kJ·kg of BW–0.75·d–1 was estimated in the balance trial (MEmBT). This value is at the lower end of the published range (370 to 620 kJ·kg of BW–0.75·d–1) for cattle and confirms results of Jentsch et al. (1995), showing lower MEm values for lower productivity breeds such as Galloway and Scottish Highland bulls than found for Holstein Friesians. Generally, Hereford cattle are considered adapted to extensive production systems. Therefore, our data agree with others, showing that MEm is influenced by the genetic potential for performance traits (Montaño-Bermudez et al., 1990; Laurenz et al., 1991). One reason for this relationship may be a larger mass of metabolically active organs, such as the gastrointestinal tract, in animals with a greater potential for productivity. Jenkins et al. (1986) scaled Brown Swiss, Angus, and Here-ford for BW at slaughter. They found that Brown Swiss had the greatest amount of internal tissues, with Angus intermediate, and Hereford at the lower end. Moreover, it has been shown that breeds with lower production potential are more efficient in adjusting their MEm to restricted feeding levels than those with larger mature size and lactation yield (Taylor et al., 1986). Another reason for the low MEm of the steers may be the restricted feeding during the adaptation period that led to a decreased MEm (Webster, 1978). Furthermore, the observed decrease in MEm at increased stages of maturity (Freetly et al., 2003) could play a role because at the time of the experiment, the steers were almost 2 yr of age.
We report considerable differences in MEmBT among animals, with the greatest difference of 22.8%. This value is in the reported range of 5 to 35% (Johnson et al., 2003). Maintenance energy requirement determined in the balance trial was negatively correlated (r = –0.81; P = 0.013) with BW at the end of the experiment. During the 5 mo before the start of the balance trial, the animals were fed at level of 1.6 x MEm based on the average BW of all steers. Hence, the animals with the heaviest BW received relatively less feed than the lightest animals. Despite this difference, heavier animals gained more BW and the BW ranking of the animals remained constant (r = 0.882; P = 0.0037) over the whole adaptation period (Table 4). Therefore, the BW development seems to reflect the individual animal differences in MEm, which also are reflected in the MEm estimates based on HP measurements (Table 3). The digestibility of energy and CH4 energy losses, as well as the ME and metabolizability were not correlated to MEmBT values, indicating differences in metabolism among animals. Because the calculated energy cost of standing did not differ among animals (P = 0.30; Table 3), it can be concluded that the differences in MEm are related to metabolic, not to behavioral differences.
Rectal temperature can be used as an approximate estimate of body core temperature. If HP differs among animals and heat loss is approximately equal, it would follow that differences in core (i.e., rectal temperature) are responsible. We measured rectal temperature under identical conditions for each animal after an overnight feed withdrawal. The results indicated significant differences among animals (Table 3). The correlation with MEmBT values was 0.7, signifying a relationship to metabolism as mentioned previously. Thus, it seems that differences in basal metabolic rate among individual animals could be detected by rectal temperature measurements during the feed withdrawal state, provided that animals are healthy and adapted.
Heart rate is widely used as an estimate of energy expenditure in animals, as well as in humans, because of the high correlation (up to 0.95) to continuous HP measurements in individuals (Schutz and Deurenberg, 1996; Derno et al., 1998; Brosh et al., 2002). Nonetheless, these reports demonstrate a need for individual calibration because basal heart rate differs among individuals. Our heart rate data, taken after a 17-h feed withdrawal, did not correlate with MEm values. This result could possibly be explained by the fact that MEm includes energy expenditure caused by feeding and physical activity, whereas our heart rate values were collected when the animals were in the feed withdrawal state and the standing position.
Mean 24-h HP of animals fed straw after a period of near maintenance feeding predicted MEm rather well. The mean MEm calculated by this procedure differs by only 4.5% from the mean MEm determined by the conventional balance trail. Among-animal variation in MEmBT is mirrored by similar differences among individual animals in MEm calculated from HP data. Therefore, it was concluded that measurement of HP during straw feeding estimates MEm with appropriate accuracy and at much lower cost than current methods.
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Footnotes
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1 The study was funded by the core budget to the Research Institute for the Biology of Farm Animals. 
2 The authors thank K. Pilz, H. Pröhl, G. Bittner, L. Strehlow, and H. Scholze for excellent technical assistance.
3 Correspondence: Wilhelm-Stahl-Allee 2 (phone: 0493820868684; fax: 0493820868652; e-mail: derno{at}fbn-dummerstorf.de).
Received for publication January 27, 2005.
Accepted for publication June 15, 2005.
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Abstract
Introduction
