Dissipation of heat from standing horses exposed to ambient temperatures between −3°C and 37°C

Pergamon

J. therm. Bid. Vol. 22, No. 3. pp. 177-186. 1997 t“ 1997 Elsevier Science Ltd All rights reserved

Printed in Great Britain PII: SO306-4565(97)00007-7 0306-4565/97 $17.00 + 0.00

DISSIPATION OF HEAT FROM STANDING HORSES EXPOSED TO AMBIENT TEMPERATURES BETWEEN

-3°C AND 37°C

KARIN MORGAN, ANDERS EHRLEMARK and KRISTER S,&LLVlK

Swedish University of Agricultural Sciences, Department of Agricultural Engineering, Building Design Section, P.O. Box 7032, S-750 07. Uppsala, Sweden

(Received 23 March 1996: accepted in revised form 22 February 1997)

Abstract-l. The aim of this experiment was to study how the rates of evaporative and the rate of non-evaporative heat loss from horses were influenced by short-term exposure to different ambient temperatures between - 3°C and 37°C.

2. The measurements were made in a climatic chamber on five horses at six different temperatures - 3 C, 6’ C, I5 ‘C, 2o’C, 3O’C and 37’C.

3. In ambient air temperatures below 20°C the rate of evaporative heat loss was almost constant. The rate of evaporative heat flow showed a pronounced increase above ambient temperatures of 2O’.C.

4. The rate of non-evaporative heat loss was found to increase by 2.78 W m-’ per degree Celcius as the ambient air temperature decreased, as expected from the laws of physics. c” 1997 Elsevier Science Ltd

NOMENCLATURE INTRODUCTION

A A< d dT e,,,, e,,, edlR

Fir k, m M,“, Nu P Ilrn

;I:, %d, qnun-cirp qrrd %0.\ RR t ‘$11 t LO”< T Ilr T L”dl T,., X <>“I

surface area [ml] surface area of enclosing environment [m’] characteristic dimension [m] temperature difference vapbur pressure at the skin surface [kPa] atmospheric vapour pressure [kPa] difference in vapour pressure [kPa] acceleration due to gravity, 9.81 [m ss’] Grashof number (free convection) thermal conductivity of air [W m ’ K - ‘1 body weight [kg] total thermal insulance [K m’ W ‘1 Nusselt number atmospheric air pressure [kPa] rate of total heat production [W mm’] volumetric air flow [m’ se ‘1 rate of evaporative heat loss [W mm’] rate of non-evaporative heat loss [W mm’] rate of radiant heat loss [W mm21 rate of convective heat loss [W m ‘1 respiratory rate [cycles min ‘1 ambient air temperature [‘Cl coat surface temperature [“Cl ambient air temperature [K] coat urface temperature [K] radiative temperature of environment [K] water content in the outlet air [g kg-‘] water content in the inlet air [g kg-‘] coefficient of thermal expansion’ (1 /T,,, for ideal gases) emissivity of the body, here used 0.95 emissivity of the enclosure latent heat of vaporisation of water [J g- ‘1 air density [kg mm’] Stefan-Boltzmann constant, 5.67. lo-’ [W mm” K ?

The normal use of horses results in their being exposed to short - term changes in the thermal environment. Consequently there is a great challenge to the thermoregulatory system to maintain an almost constant body temperature. One example of a changing thermal situation is when a horse is kept in the cold season in an insulated and ventilated stable providing an even thermal environment, and the horse is let outside in a paddock for part of the day. Another example is when a horse exercises and thus aquires an extra heat load, which it has to dissipate to the environment during and after the work. Horses are also exposed to quite different climates when they travel to different geographical locations.

To maintain an almost constant body core temperature in a homeothermic animal, like the horse, there has to be a thermal balance. The simplest form of the thermal balance is expressed as (Bianca, 1968):

Heat production = heat loss k heat storage

animal, where it is dissipated to the environment as

(1)

177

A change in heat storage results in a changed body temperature. To maintain a constant body temperature without any change in heat storage, the rate of heat production must be met by an equal rate of heat

loss. Heat is continually produced by metabolism.

The heat has to be transferred to the periphery of the kinehatic viscosity of air

178 K. Morgan et al.

non-evaporative and evaporative heat loss. The heat transfer from the body core to the periphery is regulated by physiological processes. The ability to dissipate heat from the periphery and the methods by which heat can be dissipated are governed by the physical environment. The rate of non-evaporative heat loss through convection, conduction and radiation depends on the temperature difference between the surface of the animal and its environment. Evaporation takes place from the mucous membrane in the respiratory system and at the skin, actively by sweating or passively by insensible evaporation. The rate of evaporative heat loss depends on the difference in vapour pressure between the skin and the environment and on the vapour transfer through the coat (McArthur, 1987).

Our hypothesis was that adult horses at rest maintain an almost constant body core temperature despite short-term changes in the ambient temperature due to changes in the means of heat loss.

The aim of this experiment was to measure the rates of non-evaporative and evaporative heat loss from horses and to study how these rates were influenced by brief exposure to different ambient temperatures.

THEORY

Heat loss

The thermal balance of a standing horse involves total heat production (Plot) and non-evaporative heat loss by convection (so,,) and radiation (qrad) and evaporative heat loss (qevap). The heat production due to work and the heat loss by conduction can be neglected for a standing horse. The heat balance equation (Equation 2) for a standing horse can be written (Mitchell, 1974; Clark and McArthur,

1994):

P,“, = 4C0”V + qrod + 4<~.7Jl (2)

The distribution of non-evaporative and evaporative heat for the animal as a whole in relation to air temperature is presented diagrammatically in Fig. 1 (McLean, 1973; Monteith and Unsworth, 1990; Clark and McArthur, 1994).

The non-evaporative heat loss is a function of the temperature difference between the body core and the environment and the total insulation of the animal, Equation 3:

4.“n PIO/? = (Gore - &JM,O, (3)

Also, in a cold environment there will be a minimal evaporative rate due to conditioning of the air in the

respiratory tract and a minimal cutaneous evaporation. The total insulation between an animal’s body core and its environment is provided by three thermal insulating layers acting in series: the peripheral body tissue, the coat and the boundary layer of air (McArthur, 1981, 1991). Within a limited range of ambient temperature, called the zone of least thermoregulatory response, the non-evaporative heat loss can be physiological controlled by a change in the thermal insulance of the tissue layer, through vasomotor control of peripheral blood flow. The lower limit of the zone of least regulatory response is defined as the lower critical temperature, LCT. At and below LCT the thermal insulance of the tissue is maximal due to maximal vasoconstriction. The non-evaporative heat loss will then increase linearly as the ambient temperature decreases, provided that the insulance remains constant. In a warm or hot environment the thermal insulance of the tissue layer is minimal, to facilitate heat transfer. The non-evaporative heat loss will then be restricted by a small temperature difference between the environment and the horse, which has to rely on respiratory and cutaneous evaporative heat loss to dissipate heat in order to maintain its body temperature. The evaporative heat loss will be a function of both respiration rate and the difference in vapour pressure between the skin and the ambient air.

The total heat loss must equal the total heat production for the system to be balanced. The total heat production, P,,,, is a function of the feed intake, assuming a certain feed intake that at least

Heat (W)

Tcdre Ambient temperature

Fig. 1. Diagrammatic presentation of heat production, P,,,, evaporative heat loss, Q., and non-evaporative heat loss, Q.., in relation to ambient temperature (after McLean, 1973; Monteith and Unsworth, 1990; Clark and McArthur, 1994). The the level of heat production, P,,,, depend on the feed intake. The evaporative heat loss, Q<, depend on the respiration rate, RR, and the difference in vapour pressure, e-diff. The non-evaporative heat loss, Q.., is a function of the temperature difference, dT, between the surface of the horse and the environment but also the total thermal

insulance of the horse, M,,,.

Dissipation of heat from standing horses 179

covers maintenance. Within a limited range of

ambient temperatures heat production is constant and independent of air temperature. This range of ambient temperatures is usually called the thermoneutral zone. The thermoneutral zone for horses has been estimated as ranging from - 15‘C to + 1O‘C (McBride et al., 1983) and from 5 to 27°C

(Sainsbury, 1984). Season, region, breed, age and acclimatisation will alter the absolute values of thermoneutrality (Cymbaluk and Christison, 1990). The two main factors determining the lower critical temperature are body insulation and food intake (Bianca, 1968). Below LCT, total heat loss will be greater than the thermoneutral heat production and the animal has to increase heat production to maintain the body core temperature. The increased energy demand below LCT can be met by increased feed intake, shivering or behavioural changes.

Et>aporatiotz

Heat loss by sweating is considered to be an important thermoregulatory function in horses (Jenkinson, 1972; Reece, 1991). Evaporative heat loss from horses has been measured in different thermal environments. Allen and Bligh (1969) and Johnson and Creed (1982) measured cutaneous evaporative heat loss from horses with a ventilated capsule. The 240 kg horse in the study of Allen and Bligh (1969) had little or no cutaneous water vapour loss at 20-25 C. but at 4O’C the loss was in order of 100 g m 2 h ‘. Johnson and Creed (1982) found the cutaneous evaporative heat loss to be 61- 94gm-‘h-l at 20-25°C increasing to 306- 1210 g m ’ h ’ within 20-30 min when the ambient temperature was increased to 45°C. The pattern of sweating has also been studied. Alien and Bligh (1969) found fluctuating rates of water vapour loss from the horse, though the average level of water vapour loss increased with increasing ambient temperature. Jenkinson (1972) states that the horse shows fluctuations in sweat output as sweating increases with heat exposure. Johnson and Creed (1982) found fluctuations in the sweat output of horses exposed to heat, that were larger than the fluctuations recorded by Allen and Bligh (1969). The pattern in sweating induced by an increase in ambient temperature observed by Johnson and Creed (1982) was that sweating occurred initially as pulses, with a frequency of one pulses per minute, followed by a continuously fluctuating discharge.

Methods to measure and estimate heut production and heut dissipation

Indirect calorimetry estimates the heat production from quantitative measurements of the respiratory

gas exchange. Direct calorimetry measures the rate of heat dissipation of a subject. The general principle of a direct calorimeter is to enclose the subject in a well-insulated chamber. The heat dissipation can then be measured as the increase in temperature and water content in the air together with a well defined air flow (McLean and Tobin, 1987). Another method of measuring the total evaporative heat loss is to use a hygrometric tent, described by Yeck and Kibler (1956), where the increase in water content of the air that passed through the tent is measured. To obtain the total heat loss, the non-evaporative heat loss has to be estimated. The rate of non-evaporative heat loss by radiation and convection can then be calculated from the temperature difference between the animal’s surface and the environment. In the study discussed here we used a hygrometric tent to measure total evaporative heat loss, and used temperature differ- ences to estimated rates of non-evaporative heat loss.

MATERIALS AND METHODS

Horses and climatic chamber

Five horses were used in the experiment; four Standardbred trotters (three geldings weighing 540 kg, 530 kg, 465 kg and one mare of 475 kg) and one Shetland pony stallion weighing 135 kg. All the horses were fed individually for maintenance and light work at 0.71 MJ kg- “” metabolisable energy per day, a level which agreed with the feeding recommendations of National Research Council (1973). The amount of energy supplied to each horse was calculated from the daily rations and feed tables, since the feed was not analysed. The horses were acclimatised to an indoor temperature of 15--20°C. They were let outside ( - 5-C to 5 C) in a paddock for four hours during days when they did not take part in the experiment. The measurements were done in a hygrometric tent which was placed in a climatic chamber. There were six different levels of temperature in the experiment. The temperature levels, with relative humidity in brackets, were - 3 C (50%), 6’ C (55%), 15’.C (55%), 20°C (45%), 3O“C (40%) and 37 C (40%) as measured inside the tent. Each horse participated once at each temperature level and the order was randomised. In each experiment, the horse was kept in the climatic chamber for one and a half hours. The structure of the experiment in relation to time for the measurements is shown in Fig. 2. The measured parameters reached steady state conditions after half an hour or less. Inside the climatic chamber, the horse was given a small amount of hay to keep it occupied and to distract it from interfering with the measuring equipment.


Prqaration of the horse

Radiant temp Radiant temp measurment measmment

I

I F

I _

Measuring period 60 min

Entrance EXit to tent

Fig. 2. The figure shows the time axis for the structure of measurements in the climate chamber.

Measurement of evaporative heat loss

A hygrometric tent (2.4 x 2.4 x 2.2 m) was used to measure evaporative heat loss from the horse. The tent was made of plastic film for constructional use. Several air inlets were placed on one side of the tent. The outlet air was exhausted by a fan through a ventilation duct opposite the air inlets. The airflow rate through the tent was measured in the ventilation duct by an orifice flow meter using a differential pressure sensor with an accuracy of f 0.5 Pa (Alexander Wiegand, Klingenberg, Germany). The flow meter was calibrated with a hot-wire anemome- ter (accuracy f 4%; SWEMA, Farsta, Sweden). To calculate the water content in the air, the dew point temperature of the inlet and the outlet air was measured using a dew point hygrometer (Model 660, EGandG Environmental Equipment Division, Burlington, MA, USA) with a rated accuracy of Ifr 0.3”C and a dew point sensitivity of f 0.06’ C. The atmospheric air pressure was measure with a mercury barometer. The STP (Standard Temperature and Pressure) corrected airflow through the tent and the dew point temperatures were then used to calculate the evaporative heat loss. The rate of evaporative heat loss of the horse was calculated from Equation 4. The surface area was estimated by Equation 5 (Hodgson et al., 1993).

A = 1.09 + O.O08*m (5)

The hygrometric tent was calibrated with a vaporiser placed on a weighing scale. The evaporative rate was calculated from the water mass loss divided by the elapsed time. The known evaporative rate was compared to those measured using the measuring equipment described for the experiment. This was repeated seven times. A correction factor of 1.2 was calculated and used to obtain the final results.

Measurement of temperatures

The thermistors were used to measure the temperature of ambient air, coat, skin and body core.

The radiant temperature of the surronunding surfaces was measured with an infrared thermometer. The equipment was described in the previous paper in this issue “Thermal insulance of peripheral tissue and coat in sport horses” by Morgan.

Calculation qf’ non-evaporative heat loss

The non-evaporative heat loss was calculated indirectly from the measured temperatures as the sum of convective and radiant heat loss (Equation 6). We neglected loss by conduction, since the horse was standing.

y.<>,s ?I 0/J = q,<,<, + 4<,>,V (6)

The radiant heat loss is calculated from (Holman, 1986; Mitchell, 1974; Cena, 1973):

In the case of an animal in an enclosure much larger than itself (A < A,) Equation (7a) can be approximated to (Holman, 1986; Mitchell, 1974):

q, <a, = ~l*~*(c~<,, - cm 1 (7b)

Cena and Clark (1979) pointed out that both animal coats and conventional clothing can be considered black body emitters in the thermal radiation spectrum. Cena and Monteith (1976) the coat could absorb 95%. Consequently the emissivity of the body, c,, was chosen to 0.95 in analouge to the model of Ehrlemark (199 1).

The convective heat loss was calculated according to (Holman, 1986; Bruce and Clark, 1979):

Y< ,I,,/ = k, *Nu*(t,,,,, - r,,,)/d (8)

Equations (9a, 9b, 10, 11) for the dimensionless parameters Nusselt number and Grashof number were chosen according to Monteith and Unsworth (1990). Neck, barrel and rump were considered to be a horizontal cylinder and forearm and gaskin to be vertical cylinders, Fig. 3. Also shown in Fig. 3 the principle of the individual characteristic dimension,


Fig. 3. When calculating the rate of non-evaporative heat loss the locations on the neck, the barrel and the rump were estimated as a horizontal cylinder and the location on the forearm and gaskin were estimated as vertical cylinders. The respective characteristic dimensions, d, are indicated in the

picture.

which is the surface the a vertical air flow pass. We assumed free convection and laminar flow, since there was little or no wind speed in the climatic chamber.

Nu = 0.48*Gr025 ,for neck, barrel and rump (9a)

Nu = 0.58*Gr”25 ,for ,forearm and gaskin (9b)

To calculate the Grashof number Monteith and Unsworth (1990) argue that the temperature difference between the surface and the air temperature should be replaced by the difference of virtual

temperature, TV,,,,,,, defined by Equation II. The virtual temperature takes into consideration the fact that the moisture of the air affects its density (Monteith and Unsworth, 1990).

Gr = (g*fl*T,7,na*d’)/v’ (10)

7;i”,,,,, = (L - L) + 0.38*(e,,,,* rA,,, - eurr* r,,,)/Pt,,,P,

(11)

Mathematical and statistical treatments

The purpose of the statistical analyses was to investigate whether or not there was a trend for each parameter, rate of non-evaporative heat loss, rate of evaporative heat loss and rate of total heat loss, that followed the theory of heat loss. We calculated a mean value for each parameter and experiment. The results of the rate of evaporative heat loss, the rate of the non-evaporative heat loss and the rate of total heat loss were each plotted in relation to the ambient air temperature, in separate diagrams. The regression line for each parameter was fitted with the principle

of least squares using the statistical computer package Statistica 4.5 and 5.0 (StatSoft, 1995). The equation of the regression line was considered best fitted according to the criteria: large regression coefficient, minimal final loss and a term with a p-value less than 0.05.

The rate of non-evaporative heat loss was calculated for each of the five measured locations. The mean value of the respective temperatures was calculated for the last half hour of the experimental period and used to calculate the rate of non-evaporative heat loss. In order to compute a weighted average value of the rate of non-evaporative heat loss for the whole horse from these five locations, the relative surface areas were estimated according to Horn- berger (1972) giving the neck a value of 25%, the forearm 6%, the barrel 37%, the rump 20% and the gaskin 12%.

The mean value for the evaporative heat loss was calculated for the last half hour of the experimental period and divided by the surface area of the horse to get the rate of evaporative heat loss per unit area. The rate of total heat loss was calculated by adding the mean values of the rate of non-evaporative and the rate of evaporative heat loss for each horse and experimental period. The duration of the experimental period is as given by Morgan (1997).

RESULTS AND DISCUSSION

Evaporative heat loss

In ambient air temperatures below 2OC the rate of evaporative heat loss was almost constant (Fig. 4). The rate of evaporative heat flow showed a pronounced increase above ambient temperatures of 20°C according to the regression line (Table 1, Fig. 4).

The results of the rate of evaporative heat loss were comparable with the studies of Johnson and Creed (1982) and Heilemann et al. (1990). The baseline values of the evaporative heat loss (respiratory and cutaneous added) of 48 W m ’ in Fig. 4 were similar to the cutaneous evaporative heat loss estimated at 41-64 W m ’ with a ventilated capsule at 20°C by Johnson and Creed (1982); and a respiratory loss of 18 W m-’ at 20°C estimated from Heilemann et al. (1990). In the present study the evaporative rate, at an ambient temperature of 37738°C of 250P 450 W m ~’ was within the range of the rate of cutaneous evaporative heat loss 208-823 W mm’ measured at 45°C by Johnson and Creed (1982).

The pronounced increase in evaporative heat loss above 2O’C can be considered to be a short-term

182 K. Morgan er al.

0 5 10 15 20 25 30 35

Ambient air temperature [Oc]

Fig. 4. The rate of evaporative heat loss (0) as a function of the ambient air temperature between _ 4 and 38°C. The results of five horses with R’ = 0.54.

effect. Burton et al. (1940) showed that slow adaptation of man to heat had two phases. The first phase showed an increased evaporative rate as non-evaporative heat loss fell. In the second phase the evaporative rate was gradually reduced and non- evaporative heat loss was increased. Guyton (1991) stated that acclimatization to heat, with a decreased sweat flow and a decrease of salt content in the sweat, takes four to six weeks.

Fluctuating sweating pattern

The results of the rate of evaporative heat loss in Fig. 4 were mean values for the last half hour of the measuring period and did not provide information about a fluctuating sweat rate mentioned by Bligh (1967), Allen and Bligh (1%9) and Johnson and Creed (1982). However, when viewing the results of an entire one-hour measuring period from an individual experiment in the present study, a fluctuating pattern of the rate of evaporative heat loss was noticed. This was exemplified in Fig. 5 with the results from one horse. The pattern was of a fluctuating rate, increasing to a higher rate with time.

Table 1. Best fit of regressions and correlation coefficients for non-evaporative, evaporative and total heat loss valid for the actual temperature range in the present

experiment

q cy”p = 48 + l.02*10-4*t:,, R2 = 0.54

9.. W&WI = 117.8 - 2.78*L R’ = 0.92 q.. “& = 88.6 - 2.02*t,,, R2 = 0.77

qnc h;,rre, = 127.9 - 2.92*t,,, R2 = 0.82 q,, ‘“mp = 128.1 - 3.03*t,,, R2 = 0.87

q.< rorerrm = 1 17.3 - 3.03*t,,, R2 = 0.79 q.e g&k,n = I 18.8 - 2.99*t,,, R’ = 0.80

q,O1 = 142 + 2.99*10-“*(t,,, - 12.6)4 R2 = 0.28

This agreed with the model of Bligh (1967) in which myoepithelial expulsion is super-imposed on controlled secretory activity. The sampling time for each bar in Fig. 5 is 3 min. Johnson and Creed (1982) found the frequency of sweating cycles to be one per minute. In humans a sweating period was found to be 0.74 second (Nilsson et al., 1980). It is notable that the fluctuating sweating pattern could be shown in the present study using a different method with a hygrometric tent, as compared to the ventilated capsule employed by Allen and Bligh (1969) and Johnson and Creed (1982). The hygrometric tent measured both respiratory and cutaneous evaporative heat losses and the respiratory heat loss may mask some of the fluctating pattern of cutaneous losses.

Non-evaporative heat loss

The results of the rate of non-evaporative heat loss are presented with a diagram for each measuring point and for the weighted average for the whole horse. The rate of non-evaporative heat loss increased when the ambient air temperature decreased (Table 1, Fig. 6). The best fit of the regression line for rate of non-evaporative heat loss was linear, according to the theory of physics. In the diagram for the weighted values, the mean values for each horse are connected. A slight change in slope within each horse was then found, which could be due to a change in thermal insulance of the tissue with temperature.

In an ambient air temperature equal to the body core temperature, the rate of non-evaporative heat loss should be zero. However, our results showed that the rate of non-evaporative heat loss was approxi-


Time [each bar 3minutesl

Fig. 5. The figure exemplifies the sweating pattern of the horses with the results from a single horse in 30’C. Each bar is the sweat rate during 3 min.

mately 12 W m ~’ at the ambient air temperature close to the body temperature. This can be explained by the fact that when calculating the rate of non-evaporative heat loss, we also used the radiant temperature of the environment as an input parameter as well as the ambient air temperature. In air temperatures of 37’C the radiant temperature of the surronding surfaces was lower than the ambient air temperature, resulting in a radiant heat loss from the horse.

In environments colder than the lower critical temperature, the increased total heat loss has to be met by an increase in metabolic heat production. The horse needs extra energy from the feed to maintain heat balance (MacCormack and Bruce, 1991). The slope in the regression equation for non-evaporative heat loss (Table 1) means that the rate of non-evaporative heat loss decreased with 2.78 W m ~’ per degree Celcius fall in ambient air temperature. This slope can be interpreted as the extra metabolisable feed energy demand below LCT, if the slope remains constant.

Total heat 1os.s

The rate of total heat loss (Fig. 7) showed an increase above ambient air temperatures of 25°C. Below 5‘C there was a slight increase in the rate of total heat loss, indicating that the lower critical temperature for the horses in the study was 5°C. Between ambient temperature of 5°C and 25°C the rate of total heat loss was at a constant level of 142 W m ‘. The estimated individual metabolisable feed energy intake was 170 W m-I. With a thermoneutral level of total heat loss of 142 W m *. 84% of the metabolisable energy was converted to

heat. The rest of the metabolisable energy will be stored in the horse.

Problems in estimating rutes of heat loss

The difficulties in calculating the surface area of the horses can cause errors in calculated evaporative rates, total heat loss and estimation of the size of areas facing each other, that will have impact on the heat balance. The formulae we found for calculating surface area gave figures varying between 4.22 m’ and 6.05 m’ for a 500 kg horse (Yousef and Dill, 1969; Ousey et al., 1992; Hodgson et al., 1993). Mitchell (1973) stated two problems in calculating convective heat transfer of animals: the problem of fur and hair and the problem of surface area. Non-evaporative heat exchange between a solid body and its surroundings takes place at a well-defined surface (McArthur and Monteith, 1979). In contrast the surface of a hair-covered animal’s body is not distinct, and measurement of the area of this interface presents difficulties (McArthur and Monteith, 1979). Nusselt number depends on the shape of the object and the characteristic dimension. The equation for the dimensionless Nusselt number (Nu) has great impact on the results when calculating the rate of non-evaporative heat loss. Wiersma and Nelson (1967) used a non-living model to estimate formulae for dimensionless parameters for non-evaporative convective heat transfer from the surface of a bovine animal, Equations 12 and 13;

Nu = Gr”“ifree convection (12)

Nu = Re”“forced convection (13)

Wiersma and Nelson (1967) measured the heat transfer only from the trunk. We tried to apply their

184 K. Morgan et al

160 _ % 160 z ;; 140

s 120

j 100

4: 80

3 E 60

a 40

0 ; 20 cz

0

Neck

-5 0 5 10 15 20 25 30 35 40

Ambbt G tenoanfirn PC1

160 _ ?yE 160

E. 140

4 120

1 100

a 60

f 6 ;;

0 ; 20 0

0

Barrel

. .

-5 0 5 10 15 20 25 30 35 40

Ambkmt ti twpomttm PC1

Forearm

-5 0 5 10 15 20 25 30 35 40

Ambiatt 3 tv I%]

160 _

r, 160

s 140

4 120

li c 100

0: 60

f ;,” B

Weighted value

-5 0 5 10 15 20 25 30 35 40

Ambient ab tempatun PC]

160 P E 160

& 140 I) 4 120

j 100

$ 80

v f 60

b 40

-5 0 5 10 15 20 25 30 35 40

Ambient Gr tenpanhrs PC]

; 160

z 140

1 120

- e 100

f ,“,”

E 40

‘ii 20

% O -20

.

.

Gaskin

0 5 10 15 20

Fig. 6. The rate of non-evaporative heat loss (0) as a function of the ambient air temperature between _ 4 and 38’C. The results of five horses and the figure show a diagram for the weighted average and

a diagram for each of the measuring sites on the horse.

formulae for our calculations on a live horse. Nelson (1967) since the extremities are vertical. For

However the extremities (foreleg and gaskin) differ the calculations in the present study we chose to

from the horizontal cylinder used by Wiersma and calculate the Nusselt number using the formulae from


0 5 10 15 20 25 30 35

Ambient air temperature [‘Cl

Fig. 7. The rate of total heat loss (0) as a function of the ambient air temperature between - 4 and 38 C. The results of five horses with R’ = 0.28.

Monteith and Unsworth (1990) for a horizontal cylinder for the neck, the trunk and the rump (Equation 9a) and using the formula for a vertical cylinder for the foreleg and the gaskin (Equation 9b). To be certain about the absolute value of the rates of non-evaporative heat loss due to convection and also the absolute value of the total heat loss, we found that there is need for further studies on separate modelling of dimensionless parameters like Nusselt number.

SUMMARY AND CONCLUSIONS

The environment in which a horse is thermally comfortable depends on how it can maintain its thermal balance due to the thermal properties of the horse and its physiological regulatory mechanisms in relation to the physical environment.

Horses are exposed to different thermal environments. Within a range of different physical environmental conditions, the horse can control the rate of heat loss through physiological responses and therefore remain thermally comfortable. In a short-term perspective this is done mainly by altering the thermal insulance of the tissue to regulate the heat flow from the body core to the surface, and by changing the evaporative heat loss through altering the respiration rate or sweat rate or both. The aim of this experiment was to study how rates of heat loss and the division between non-evaporative and evaporative heat loss from horses are influenced by short-term exposure to different ambient temperatures.

We conclude that, based on the result presented here and in the previous paper in this issue, the horses

in the study maintained heat balance despite the short-term change in ambient temperature. The rate of non-evaporative heat loss increased linearly (2.78 W m -’ K ‘) when the ambient air temperature decreased. The evaporative heat loss (respiratory and cutaneous combined) had a steady state level of 48 W m ’ and increased above 2O’C. The total heat loss had a thermoneutral level of 142 W rn-‘.

Acknow,ledgrmerrfs - We wish to express our special thanks to the following. Stiftelsen Lantbruksforskning. LRF. for funding the experiment. Assistent Gunnar Ohlsson, for help with setting up of the measuring equipment. The technical staff of the Department of Medicine and Surgery at the Swedish University of Agricultural Sciences. for their excellent cooperation with the experiments. Veterinary assistance and help with measurements was provided by Pia Funkquist and Stina Marntell. Mr. Martin Sommer corrected the English.

REFERENCES

Allen T. E. and Bligh J. (1969) A comparative study of the temporal patterns of cutaneous water vapour loss from some domesticated mammals with epitrichial sweat glands. Comp. Biol. Physiol. 31, 347.-363.

Bianca. W. (1968) Thermoregulation. In: Adaptation of

domestic animal.s. ed. E. S. E. Hafez. pp.9771 18. Lea and Febiger. Philadelphia, USA.

Bligh J. (1967) A thesis concerning the processes of secretion and discharge of sweat. Em+. Res. I, 2845.

Bruce J. M. and Clark J. J. (1979) Models of heat production and critical temperature for growing pigs. Anim. Prod. 28, 3533369.

Burton A. C.. Scott J. C., M&love B. and Bazett H. C. (I 940) Slow adaptations in the heat exchanges of man to changed climatic conditions. Amcricrzn Journal of Phl;siology 129, 84101.


Cena, K. (1973) Radiative heat loss from animals and man. In: Heat loss ,from animal and man-assessment and

control, ed. J. L. Monteith, and L. E. Mount, pp. 33-58. Butterworth, London.

Cena K. and Clark J. A. (1979) Transfer of heat through animal coats and clothing. In Znternutional RetGeH, of

Physiology, Enoironmental Physiology III, ed. D. Robertshaw, Vol. 20, pp. l-42. University Park Press, Baltimore.

Cena, K. and Monteith, J. L., (1976) Heat Transfer Through Animal Coats. In: Progress in Biometeorolog?,,

Division B, Progress in Animal Biometeorolgy. Vol. 1, Part 1, pp.3433351. Swets and Zeithnger B. V., Amsterdam.

Clark, J. A. and McArthur, A. J. (1994) Thermal exchanges In: Livestock housing. ed. C. M. Wathes, and D. R. Charles, pp.97-122. CAB International, Oxon, UK.

Cymbaluk N. F. and Christison G. I. (1990) Environmental effects on thermoregulation and nutrition of horses. Veterinary Clinics qf North America: Equine Practice. 6,

2, 000.

Ehrlemark, A. (1991) Heat and moisture dissipation ,fiom

cattle ~ Measurements and simulation model. Disser- tation. Report 77. Department of Farm Buildings. Swedish University of Agricultural Sciences, Uppsala.

Guyton. A. C. (1991) Body temperature, temperature regulation. and fever. In: Textbook ofmedicalphysiolog~,,

8th edn., pp.797-808. W. B. Saunders Company. Philadelphia.

Heilemann M.. Woakes A. J. and Snow D. H. (1990) Imestigation on the respiratory water loss in horses at rest and during exercise. Advances in Animal Physiolog? and Animal Nutrition. 21, 52-59.

Hodgson D. R.. McCutcheon J. L.. Byrd S. K., Brown W. S., Bayly W. M.. Brengelmann G. L. and Gollnick P. D. (1993) Dissipation of metabolic heat in the horse during exercise. J. Appl. Physiol. 74, 3, 1161&l 170.

Holman, J. P., (1986) Heat transjtir. McGraw-Hill. Inc. Singapore.

Hornberger M. (1972) Berechnung der Hautoberflache. Berliner und Miinchener tieriirtzliche Wochenschrift 18,

3477348.

Jenkinson A. and McEwan D. (1972) Evaporative temperature regulation in domestic animals. Symp. zool.

Sot. 31, 345-356. Johnson K. G. and Creed K. E. (1982) Sweating in intact

horse and insolated perfused horse skin. Camp. Biochem.

Physiol. C 73, 259-264. MacCormack J. A. D. and Bruce J. M. (1991) The horse

in winter ~ shelter and feeding. Farm Building Progress

105, 10-13. McArthur A. J. (1981) Thermal resistance and sensible heat

loss from animals. J. therm. Biol. 6, 4347. McArthur A. J. (1987) Thermal interaction between animal

and microclimate: a comprehensive model. J. theor. Biol.

126, 203-238.

McArthur A. J. (1991) Metabolism of homeotherms in the cold and estimation of thermal insulation. J. therm. Biol. 16, 3, 149- 155.

McArthur A. J. and Monteith J. L. (1979) Air move- ment and heat loss from sheep. I. Boundary layer and insulation of a model sheep, with and without fleece. Proc. R. So,. Lond. B 209, 187-208.

McBride G. E.. Christopherson R. J. and Sauer W. (1983) Metabolic rate and thyroid hormone concentrations of mature horses in response to changes in ambient temperature. Can. J. Anim. Sci. 65, 375-382.

McLean, J. A. (1973) Loss of heat by evaporation. In: Heat loss from animal and mun -- assessment and control.

J. L. Monteith and L. E. Mount, pp. 59-75. Butterworth, London.

McLean, J. A. and Tobin, G. (1987) Animal and human

calorimetry. University Press, Cambridge. UK. Mitchell, D. (1973) Convective heat transfer from man

and other animals. In: Heat lossfrom animal and man--

assessment and control, ed. J. L. Monteith, and L. E. Mount. Butterworth, London. 59-75.

Mitchell, D. (1974) Physical basis of thermoregulation. In: Etwironmental Physiology. ed. D. Robertshaw. Butter- worths, London. 2-32.

Monteith, J. L. and Unsworth, M. H. (1990) Principles Q/ en~ironmentalphysics, 2nd edn. Edward Arnold, London, Great Britain.

Morgan, K. (1997) Thermal insulance of peripheral tissue and coat in sport horses. J. therm. Biol. manuscript.

National Research Council (NRC). (1973) Nutrient requirements of domestic animals. In: Number 6: Nutrient

requirements of horses, 3rd edn. Washington, D. C. Nilsson A. L.. Nilsson G. E. and Gberg P. A. (1980) A

note on periodic sweating. Acta Ph,vsiol &and 108,

189. 190.

Ousey J. C.. McArthur A. J., Murgatroyd P. R., Steward J. H. and Rossdale P. D. (1992) Thermoregulation and total body insulation in the neonatal foal. J. therm. Biol.

17, 1, 1~10. Reece, W. 0. (1991) Physiology qf domestic animals. Lea

and Febiger, Philadelphia. Sainsbury, D. W. B. (1984) Housing the horse. In: Horse

management. ed. J. Hickman, 63391. Academic Press, London.

StatSoft (1995) Statistica. StatSoft, Inc. Tulsa, OK, USA. Wiersma F. and Nelson G. L. (1967) Nonevaporative

convective heat transfer from the surface of a bovine. Transactions of the ASAE 10, 6, 7333737.

Yeck R. G. and Kibler H. H. (1956) Moisture vaporiza- tion by Jersey and Holstein cows during diurnal temperature cycles as measured with a hygrometric tent. Missouri Agricultural E.yperiment Station. Res

Bull. 00, 600. Yousef M. K. and Dill D. B. (1969) Energy expenditure

in desert walks: man and burro Equus asinus. J. Appl.

Physiol. 29, 681.-683.

Documents

Dissipation of heat from standing horses exposed to ambient temperatures between −3°C and 37°C