1. Introduction
A minimal amount of energy is needed to ensure homeostasis. This amount of energy is described as the basal metabolic rate and indicates the energy needed for metabolic processes in a lying position, awake state, after food digestion is complete, in a thermoneutral ambient temperature without further activity [
1]. The so-called maintenance energy requirement (MER) goes beyond the basal metabolic rate. MER is defined as the energy required for thermoregulation, spontaneous activity, and moderate exercise, and can be influenced by factors, such as breed, age, husbandry, body weight, activity level, and thermoregulation [
1]. Strenuous exercise and environmental temperature have an effect on energy requirements to varying extents [
1]. Appropriate energy supply adjusted to the requirements of military working dogs during pre-training is a basic prerequisite for working dogs, so they are fully able to exercise. For adult dogs at maintenance (laboratory kennel dogs or active dogs), the daily metabolizable energy requirements are generally estimated to be approximately 130 kcal/kg body weight (BW)
0.75 [
1]. When kept in a domestic environment, with little stimulus and opportunity to exercise, daily metabolizable energy requirements are lower, from 95 kcal/kg BW
0.75 [
1] up to 0.410 ± 0.121 MJ/kg BW
0.75, which corresponds to about 98 ± 29 kcal/kg BW
0.75 [
2]. Values above the average requirements of 130 kcal/kg BW
0.75 are found for young adult laboratory and active pet dogs, Great Danes, and terriers (140 to 200 kcal/kg BW
0.75) [
1]. Requirements described for hunting dogs amount to 240 kcal/kg BW
0.75 or for sled dogs pulling heavy loads to 270 kcal/kg BW
0.75 [
1]. Exceptional cases are sled dogs during races, showing kcal requirements of about 1050 kcal/kg BW
0.75 [
1] or 8995 kcal/dog/day up to 13,779 kcal/dog/day [
3]. Further studies have demonstrated as well increased energy requirements for working dogs in general (167 kcal/kg BW
0.75) compared to conventionally kept pet dogs (124 kcal/kg BW
0.75) [
4]. This has also been shown for, similarly, highly active sporting and functional dogs, such as sled dogs and hunting dogs [
4]. While energy consumption of agility dogs in North America do not greatly exceed daily metabolizable energy requirements of pet dogs, being about 106 kcal/kg BW
0.75 per day [
5]. Average energy consumption of working dogs trained in odour detection, explosive detection, and human detection, amounted to 136 ± 38 kcal/kg BW
0.75 [
6]. With regard to military working dogs in Germany, however, no specific representative data could be obtained with regard to their specific energy requirements. Metabolic processes can be influenced and altered negatively because of inappropriate energy supply. Under energy deficiency, the ability to perform work is impaired and muscle protein will be catabolized for energy, while on the contrary, an excess of energy leads to overweight and obesity, which is linked to a number of diseases [
1]. In order to ensure and optimize physical performance, there is a need for concrete knowledge about appropriate energy supply adjusted to the requirements of each military working dog. One method to measure the daily energy requirements of dogs is by food intake [
1].
As the conditions of keeping and training military working dogs are uniform, there is a unique opportunity to acquire further data on daily energy consumption of dogs, with elevated activity levels, under standardized methods. The aim of the present study was to accompany 20 intact Belgian Shepherds var. Malinois for four weeks during their pre-training as military working dogs to draw conclusions about an adequate energy supply of military working dogs during this particular period.
4. Discussion
Twenty intact military working dog candidates of the same breed and age (Belgian Shepherd Dogs var. Malinois, 12 months old) were kept and trained under standardized methods in central German winter temperatures. Energy intake, in connection with changes of different body condition parameters, were evaluated over 28 days.
The initial assessment of body measurements on day 1 as basis for comparative presentation showed a physically homogeneous group. During the course of the study, the average body weight increased from 22.1 kg to 23.4 kg. Hawthorne et al. published data on dogs in similar expected weight classes in adulthood (Labrador Retrievers) describing that these dogs gained body weight more slowly in advanced adolescence and had reached 99% of expected adult weight by 52 (SD ± 0.79) weeks of age [
11]. It is concluded that the body weight gain of the dogs observed in the present study is only, to a small extent, due to age-related body growth, but rather to other influences, such as adaptive responses to physical activity. The increase in shoulder height of 0.5 cm corresponds to the growth pattern described by Hawthorne et al. and, in contrast to body weight, is due solely to age-related body growth.
At the time of data acquisition, the participating dogs were at the stage of their pre-training for their following specialized work. The specific service-oriented activities, such as odour detection, obedience training, and protection work should be assigned as strenuous activity. Every dog spent in average about 10 min per day (during the whole study in average 280 min) at this activity level (here categorized as moderate and high). As expected, the dogs experienced an adjustment of their physical constitution through continuously increased training in the areas of odour detection, obedience training, and protection work, with medium and high activity levels. This was recorded by documenting the development of SCF and MSCDL. The results show a significant decrease in SCF as well as a significant increase in MSCDL thickness. Physical exertion leads to muscle fibre hypertrophy in the muscle area, which is in turn physiologically associated with increased energy requirements [
1] and an increase in body weight [
1]. In addition, the physiological body growth at the age of 12 months, as written above, has only little influence on the expression of body weight. It could be concluded that the increased thickness of the MSCDL is due to adaptive reactions to recurrent physical training. Computer tomography (CT), magnetic resonance imaging (MRI), and dual-energy X-ray absorptiometry (DEXA) are described as methods for objective and reliable measurement of muscle mass, but with limitations, such as technique used in dogs, like repeated ionizing radiation exposure, high costs, limited access to equipment, and requirement for pharmacological immobilization with CT and MRI [
12,
13]. A clinically feasible ultrasound as a method to detect changes in muscle mass was chosen as an alternative in the present study to measure muscle thickness, because it was already performed in dogs with promising results and had good correlations between results of MRI- and CT-based measurements [
13,
14,
15]. Factors limiting the reproducibility of the data obtained via ultrasonographic muscle measurements were reduced to a minimum. A single veterinarian, experienced in ultrasound examinations (standardized pressure), examined and collected the data. The chosen location (lumbar epaxial) was evaluated before in ultrasonographic muscle thickness measurements in the canine [
15,
16]. Fixed anatomical reference points ensured measuring at the same location. Additionally, the shaved area of 4 × 2 cm
2 at the described location was still visible in all dogs after 28 days.
During the weekly general examinations, the dogs considered were a healthy group of 20 young dogs of the same breed. At the beginning of the study, all 20 animals were free of endoparasitic infections. At the end of the study, infection with Giardia spp. was detected in three dogs, although the time of infection after day 1 cannot be dated further. The regularly collected documentation of the individual body parameters did not show any outstanding deviations from the comparative values of the other study participants in the 3 affected dogs, and with
n = 3 out of 20, the incidence with Giardia spp. infected animals within the population under consideration is 15%, which is below the incidence of 18.1% that is recorded for the German total dog population [
17]. Statistical analysis revealed that no particular influence of the factor Giardia spp. infection on physical condition was found.
Data available in literature regarding the average energy requirement of dogs differ in some cases considerably (
Table 5). These differences are based on the influence of various factors, such as breed, age, activity level, and ambient temperature [
1]. For example, the National Research Council describes the energy requirement for dogs as 95 to 200 kcal/kg BW
0.75 [
1]. The lower value describes the energy requirements of inactive pet dogs without special physical activity and the higher value describes, for example, the energy requirements of active pet Great Danes kept under laboratory conditions. Under laboratory conditions in kennels or active pet dogs (environmental conditions: dogs in a domestic environment with strong stimulation and ample opportunity for exercise or multi-dog household, or with large exercise areas) have an average energy requirement of 130 kcal/kg BW
0.75. Bermingham et al. described in a meta-analysis after evaluation of 29 publications an average energy requirement of 87.5 to 198.1 kcal/kg BW
0.75 [
4]. Included in this analysis were 713 dogs of various breeds, sizes, weight classes, and activity levels. The authors were able to demonstrate differences in energy requirements between dogs kept at home and the total population under consideration, which also included racing, hunting, and working dogs. Pet dogs had the lowest energy requirement, 124.1 kcal/kg BW
0.75. For working dogs, an average energy requirement of 157.1 to 220.1 kcal/kg BW
0.75 was determined, for hunting dogs 127.1 to 202.5 kcal/kg BW
0.75 and for racing dogs 172.3 to 233.5 kcal/kg BW
0.75. For hunting dogs, the NRC indicates a requirement of 240 kcal/kg BW
0.75 [
1]. There was no indication of ambient temperature during this data collection. Mullis et al. examined a group of 20 adult odour detection, explosive detection, and human detection dogs with regard to their energy consumption during their training on duty and found a requirement of 136 kcal/kg BW
0.75 at an average ambient temperatures of 16 to 27 °C [
6]. In addition, sled dogs were studied at an ambient temperature of −20 degrees Celsius, whose energy requirement at rest is given as 215 kcal/kg BW
0.75 [
18].
The data of the 20 Belgian Shepherd dogs in the present study showed an average energy supply of 244 kcal/kg BW
0.75, which is comparable to that of hunting dogs and higher than described for working dogs investigated so far. Influencing factors, such as physiological growth and activity level, have already been considered, and in the present study the effect of growth was found to be less than that of activity level. In addition to the above-mentioned influences, the influence of being kept under ambient temperatures below the thermoneutral zone (TNZ) should also be discussed in connection with the available data. The effect of ambient temperatures below the TNZ during different activity levels on energy requirement is a frequent complication in scientific studies, as they are competing factors that increase energy requirement, which cannot be assessed separately [
1]. The data acquisition of the present study took place in ambient temperatures between 3 °C and 13.6 °C and averaged about 8 °C. As homoiothermal animals, dogs increase their heat production when staying in temperatures outside their TNZ to ensure the maintenance of their internal body temperature. The lower limit of the TNZ is called lower critical temperature, and is described by the National Research Council for adult dogs as 20 °C to 25 °C. Breed-specific deviations may occur, the lower critical temperature is lower for breeds with dense fur than for breeds with less dense fur. The habituation is also a strong influencing factor. For example, dogs accustomed to outdoor kennel housing show less differences in energy requirements after being moved to colder ambient temperatures compared to dogs kept indoors [
1]. To compare the data obtained in this study, studies with similar environmental factors were regarded. An increased energy requirement due to low ambient temperatures was demonstrated for Great Danes kept in kennels. In winter, compared to summer temperatures at 25 °C temperature difference, they required 70 kcal ME/kg BW
0.75 per day more, which corresponds to an increase in energy requirement of 3 kcal/kg BW
0.75/°C [
19]. Durrer and Hannon came to similar conclusions when they looked at Beagles and Huskies at ambient temperatures of 14 and −20 °C respectively [
20]. Blaza et al. described a 25% increase in daily feed intake in Labrador Retrievers when the animals were moved from 15 °C to 8 °C ambient temperature [
21].
According to the data of NRC and Zentek et al. [
1,
19], in the present study, 378 kcal per day and per dog are arithmetically allotted to the increased energy requirement of the dogs due to their stay in ambient temperatures below the thermoneutral zone at 8 °C (12 °C less than TNZ of 20 °C, 3 kcal/kg BW
0.75/°C increase in ambient temperature). The daily energy intake of 2546 kcal per dog or 244 kcal/kg BW
0.75 during a stay in ambient temperatures of 8 °C would thus correspond to a daily energy requirement of 2168 kcal or 206 kcal/kg BW
0.75 during a stay in ambient temperatures in the area of the TNZ at 20 °C.
This would correspond to the energy requirement of Great Danes kept under laboratory conditions and active or in outdoor kennels in summer [
1,
19]. For the dogs considered in this study, an energy requirement of 244 kcal/kg BW
0.75 was evaluated, but this increase should not be attributed solely to the effect of ambient temperature. The factors mentioned, such as activity levels and, even if only to a minor extent, age-related growth, should also be mentioned here. The dogs gained in average 1.3 kg weight during the experiment. According to [
22], the energy contained in 1.3 kg weight gain (added tissue) of young dogs can be estimated as being composed of 26.9% crude fat and 22.5% crude protein; therefore, containing 19.8 MJ (1.524 MJ/100 g gain; 24 kJ/g protein and 39 kJ/g fat). This mean that about 0.7 MJ per day might be attributed to weight gain and should be subtracted from MER. It should also be noted that the dogs observed developed a temperature-adapted winter coat due to habituation. This is more dense and longer than in the breeds considered in the comparative regarded studies (Great Danes and Beagles), so that the energy expenditure for heat retention of the dogs considered in the present study can be estimated to be less than in the studies indicated. This in turn suggests a stronger influence by the activity level.
In order to secure the data for MWDs in Germany, it would be desirable that the study design carried out here be repeated with changes in single factors, such as climate, age of the dogs, or, also, activity levels. The standardized husbandry and training of the dogs allows, by changing individual factors during future studies, to determine the influence of the focused individual factor, which usually competes with other accompanying factors.