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170 K. Morgan

thermal resistance and thermal insulance is shown in equation (1):

R = MIA (1)

where R = thermal resistance [K W-‘I, M = thermal insulance [m’ K W-‘I, A = surface area [m’].

Thermal resistance [K W-‘1 acts as the insulation of the whole animal. When the thermal insulation is expressed for a layer it is called thermal insulance [m’ K W-‘I. The thermal insulance defines the thermal property of a material and is not related to the size of the horse. Therefore, the thermal insulance can be used to compare the thermal properties between horses independent of the size of the horse. Furthermore, it is preferred to determine the thermal insulance since the surface area of a horse is difficult to measure.

Heat flow and thermal insulance

Heat transfer through the tissue and coat layer can be treated as flow of heat through an insulated layer to which Fourier’s law may be applied (Holman, 1986). Thermal insulance, M [m’ K W-‘I, is defined by equation (2) (IS0 3l/IV 4-11.1 (ISO, 1982)):

A4 = (7Y - Q/q (2)

where M = thermal insulance [m’ K W-l], T,= temperature [K], q = density of heat flow rate [W mm’].

The temperature difference 7;-T, will be the difference between the core and the temperature of the outer surface of the skin, used to calculate the tissue thermal insulance. The temperature difference between the skin and the coat surface temperature will be used when calculating the thermal insulance of the coat. It can be considered to be steady state when the metabolic rate is constant and there is no heat

storage.

Factors affecting thermal resistance

The magnitude of the thermal resistance will be affected by the surface area of the horse, the thermal insulance of the tissue and the thermal insulance of the coat. The effect of surface area is exemplified in Table 1, where twice the surface area will result in

half the thermal resistance when the horses have the same thermal insulance. The thermal insulance of the tissue is regulated by vasomotor control of the blood flow to the peripheral tissue. The tissue thermal insulance will be maximal when the blood vessels are fully vasoconstricted in cold, ambient temperatures, In ambient air temperatures close to the body core temperature the thermal insulance will be minimal and the peripheral blood vessels will be fully vasodilated. Fat in the peripheral tissue can increase the thermal insulance, since fat is three times more insulating the other tissues (Guyton, 1991). Accord- ing to McArthur (1991) shivering can reduce the thermal insulance to 65% of maximum. The thermal insulance of the coat varies between individuals due to several factors like differences in coat length, depth and density. In cold environments the individual can increase coat thermal insulance by piloerection (Cymbaluk and Christison, 1990). A moist or wet coat, caused by sweating, bathing or rain, will have a lower thermal insulance.

Thermal resistance, LCT and extra feed demand

Within a certain limited range of the ambient temperature, called the zone of least thermo- regulatory response, the horse is able to maintain balance between heat produced and dissipated heat by regulating the thermal resistance of the tissue only to adjust the non-evaporative heat loss (Mount, 1973; Monteith and Unsworth, 1990). The lower limit of the zone of least thermoregulatory response is called the Lower Critical Temperature (LCT). The two main factors that determine LCT are body insulation and food intake (Bianca, 1968). By definition the tissue thermal resistance is maximal at the LCT. Below LCT, non-evaporative heat loss will increase linearly with decreasing ambient temperature, assum- ing that the total thermal resistance will be maximised and remain constant below LCT. The heat loss will then exceed the thermoneutral heat production. The increased energy demand below LCT to maintain heat balance can be met by increased feed intake, shivering and behavioural changes. Thus information about the maximal total thermal resistance is needed to determine LCT and the extra feed energy requirement below LCT.

Table 1. Examples of maximal and minimal total thermal resistances [K W - ‘1 for horses with different size but with the same maximal and minimal total thermal insulance

[m’ K W-‘1

Surface area 3 m2 Min Max

6 m’ Min Max

Thermal insulance [m’ K W-‘1 0.120 0.360 0.120 0.360 Thermal resistance [K W -‘I 0.040 0.120 0.020 0.060

Thermal insulance in sport horses 171

MATERIALS AND METHODS

Horses and climatic chamber

Five horses were used in the experiment; four Standardbred trotters (three geldings weighing 540, 530 and 465 kg, and one mare weighing 475 kg) and one Shetland pony stallion weighing 135 kg. All the horses were fed individually for maintenance and light work, 0.71 MJ kg -“” metabolisable energy per day, in agreement with NRC (1973). The amount of energy given to each horse was calculated from their individual daily rations and feed tables. The horses were acclimatised to an indoor temperature of 1552O’C. They were let outside in a paddock during daytime, except for experiments in the climatic chamber. The experiments were performed in a climatic chamber with one horse capacity. There were six different levels of ambient air temperature; - 3, 6, 15, 20, 30 and 37 C. Consequently, each horse participated six times in the experiment. The ambient air temperature had stabilised before the horse entered the climatic chamber. At each experiment a horse was kept in the climatic chamber for about 1.5 h. This duration was sufficient since the measured parameters stabihsed after 0.5 h or less. Inside the climatic chamber, before and after the measuring period, oxygen uptake and carbon dioxide production was estimated with an open face mask connected to a gas analyser (Servomex, Sussex, UK. integrated into an Oximeter 3200, lsler Bio- engineering AC, Switzerland). Since there was no change in gas exchange before and after the measuring period the conditions were considered to be steady state. During the measuring period the horse was given a small amount of hay to keep it occupied and to distract it from interfering with the measuring equipment.

Cakulations

The thermal insulance of the peripheral tissue. M ,,$(W, was calculated with equation (3). The thermal insulance of the coat, M,,,,,, was calculated with equation (4). Both equations (3) and (4) derive from equation (2):

jM,,,\,“, = (fb<,d! - &&qevap + Ynon & (3)

MC<,,, = (f&1” - t,“,f)/%,,” s\ap (4)

The rate of non-evaporative heat loss, ~.~,,l_.~,,~~, was calculated from equation (5) (Holman, 1986; Monteith and Unsworth, 1990):

4 ““Xl -sr.lp = c*a*(% - %) + %,“,*(IrTurf - T,,,) (5)

The coefficient of convection, tl,,,,, depends on the thermal conductivity of the air, the characteristic

dimension of the object and on Nusselt’s number.

The evaporative heat loss was measured with a hygrometric tent (Yeck and Kibler, 1956). The evaporative heat loss was divided by the estimated surface area to calculate the rate of evaporative heat loss, qsrap. The detailed equations of the rate of non-evaporative and evaporative heat loss are presented in Morgan et al. (1997).

Measurements cf temperature

In order to determine the insulance of the peripheral tissue and coat the following temperatures were measured: body core temperature, skin tempera- ture, temperature of the surface of the coat, ambient air temperature and radiant temperature of the enclosure. The readings of the coat temperature as measured with the thermistors were compared with readings by the infrared thermometer with an accuracy of + 0.5”C (Everest Interscience Inc., Tustin, CA, U.S.A.), so as to check the accuracy of using thermistors for measurement of coat temperature.

The deep body temperature was registered with a thermistortip catheter (7F, 1.25 m, Swan-Ganz; Edwards Lab, Santa Ana, CA, U.S.A.). The catheter was placed in the jugular vein with an introducer technique and the thermistor tip was located close to the heart. The catheter was then connected to a 9520A Cardiac Output Computer (Edwards Lab), where the blood temperature was displayed. The deep body temperature was recorded every 3 min.

The skin temperature, the surface temperature of the coat and the air temperature were measured with thermistors with a rated accuracy of _t 0.2”C (Fenwal Electronics/APD, Milford, MA, U.S.A.). The skin temperatures and the surface temperature of the coat were measured at five different locations; neck, forearm, barrel, rump and gaskin (Fig. 1). The thermistors for the skin and coat measurements were each placed on a purpose-made holder (Fig. 2) taped onto the horse. The holder for the coat thermistor was attached to a thin aluminium net to improve heat transfer to the sensor and to keep it on the surface of the coat. The thermistors were calibrated in a water bath before use. The radiant temperatures of the surrounding surfaces facing the horse were measured with an infrared thermometer with an accuracy of _t 0.5”C (Everest Interscience Inc., Tustin, CA, U.S.A.). Radiant temperatures of the walls, the ceiling and floor were measured before and after the measuring period with the horse present. A mean value for the period was calculated using the coefficients for each surface; the ceiling 0.165, the floor 0.165 and the walls 0.67.

K. Morgan

Fig. I. The locations on the horse where skin and coat temperature were measured. The relative surface areas are

also presented.

Relative influence cf an error in temperature readings

An error in temperature readings (skin, coat and air) will affect the accuracy of the calculations of thermal insulance. The relative error for the results of thermal insulance was calculated assuming a measured error of k 0.2‘C. The relative errors are presented in Table 2.

The large maximum error term for thermal coat insulance, + 38% at 30°C can be explain by a small temperature difference compared to the assumed measurement error. Thus, there is a considerable uncertainty in the measured insulance of the coat. In warm conditions, however, the influence of this uncertainty on the total heat balance is small, since coat insulance is small compared to the total insulance. We did not calculate the thermal

Fig. 2. The figure shows the special made holders for the thermistors; A, for the coat surface, viewed from above; B, for the skin, viewed from the side. The holder for the coat thermistor has an aluminium net attached to the thermistor

to keep it on the surface of the coat.

‘Table 2. The maximum error of the calcu- latrons of thermal insulance when the rated

accuracy of the thermistors = rt 0.2 C

Thermal insulance Ambient temperature oc 30 ‘C

Tissue ) 2.9% * 10% Coat + 6.9% i_ 38%

insulances when the ambient air temperature was 37-C. The skin and coat temperatures were then very close to the body core temperature, 38°C and the ambient air temperature. The temperature differences were too close to the rated accuracy of the thermistor to conduct a reliable calculation.

Mathematical and .rtati.rtical analysis

The calculations of the thermal insulance were based on mean values of the temperature readings (body core, skin, coat and air) for the last 30 min of the experiment. The thermal insulances were calculated for each of the five measured locations. A weighed average of the thermal insulance was calculated according to equation (6) to estimate an overall value for the whole horse. The coefficients in equation (6) were estimated with relative surface areas for each measured location on the horse. The relative surface areas were estimated to neck 25%, forearm 6%, barrel 37%, rump 20% and gaskin 12% (Hornberger, 1972).

M irelphed = 0.25~,,,,, + 0.06~~,,,,,,, + 0.37~~,,,,,

+ 0.20~,,,“,, + 0.1 =fg,,k,,

It was difficult to prevent the thermistor placed on the neck from moving slightly when the horse moved its head. This might have affected the measured temperature and consequently the calculated thermal insulance. However, an error of 10% in the measurement from the neck only affected the weighted average with 2.5%.

The weighted averages for each of the calculated parameters, tissue thermal insulance and coat thermal insulance were plotted versus the ambient air temperature in two separate diagrams. A regression line for each parameter was fitted according to the principle of least squares in order to look for trends in the results, within the range of ambient air temperature used in the experiment. McArthur (198 1) suggested that the tissue thermal insulance of the trunk and the extremities differ. An analysis of variance of the results of tissue thermal insulance was conducted to test for significant differences between the trunk and the extremities. The statistical analyses

Thermal insulance in sport horses

5 10 15 20

Ambient air temperature PCj

Fig. 3. The results of the tissue thermal insulance (.) plotted versus the ambient temperature. The regression line (-----) shows the trends of the result.

were done using the statistical computer program Statistica 4.5 and 5.0 (StatSoft, 1994). The maximum value for each thermal insulance was determined at the maximum of the regression line where the derivative of line is zero.

Measurement of tissue thermal insulance by local chilling

A separate set of experiments was conducted to determine the maximal tissue thermal insulance. Five Standardbred trotters (four geldings weighing 480 k 22 kg and one mare weighing 492 kg) were used. The horse was placed in an examination stall at room temperature (1 5P20”C). Four locations on the horse, forearm, barrel (both sides) and rump, were chilled with ice for 90 min. Density of heat flow rate (W rn- ‘) was measured at the skin surface with a heat flow transducer (Radiation Energy Balance Systems, Seattle, Washington, U.S.A.). Skin temperature was measured with a copparxonstantan thermocouple. Body temperature was measured in the rectum using an electronic rectal thermometer. The tissue thermal insulance was calculated according to equation (7), which derives from equation (2):

M ,Ib,“e = (L,, - tsk,“)lqmrilruied (7)

RESULTS AND DISCUSSION

Tissue thermal insulance

The analysis of variances showed no significant difference between the tissue thermal insulance of the trunk (side and barrel) and the extremities (forearm and gaskin). The thermal insulance of the peripheral tissue increased as the ambient air temperature decreased (Fig. 3). The best fitted regression line and correlation coefficients are presented in Table 3. The data show that the horse regulated the peripheral tissue thermal insulance when the ambient temperature changed. The minimal tissue thermal insulance was 0.015 rn’ K W-‘. The maximal tissue thermal insulance was 0.082 mZ K W-’ for the regression line within the range of ambient tempera- ture in Fig. 3. The maximum of the regression line was 0.098 m’ K W-’ when the derivative was zero. According to Guyton (1991) the fundamental ratio of maximal to minimal insulance by vasomotor control is considered to be 8: I. The observed maximal tissue thermal insulance of 0.082 m2 K W- 1 in Fig. 3 seems too low in comparison. Using the method of local chilling the average maximal tissue thermal insulance was estimated to 0.100 m’ K W -‘. An explanation for the lower value that was observed in the main

Table 3. The best fit of regressions and correlation coefficients for tissue thermal insulance and coat thermal insulance valid for the actual temperature range in the

present experiment

M,,,,, = 0.079 - O.O0123*t,,, - 3.51. lo-‘*& R’ = 0.72

MC,,, = 0.155 - O.O029*t,,, for 1,,, 2 14.5’C R’ = 0.33 M,,,, = 0.119 for 1,,, < 14.5X

174 K. Morgan

experiment could be that a couple of the horses shivered at ambient air temperatures of - 5 and 5°C which might have lowered the thermal insulance of the tissue. Another explanation could be that tissue insulance is calculated as a weighted value for five different locations of the body, and this could affect

the magnitude. McArthur (1980) showed in a study on sheep that values of tissue thermal insulance expressed as an average for the whole body conceal large variations. Another factor that affects the magnitude of the result is the body temperature. The rectal temperature is usually used as body tempera- ture in calculations of tissue thermal insulance. Data from the measurements in the climatic chamber showed that the central venous blood temperature was on average 0.5’C ( + 0.2”C) lower than the rectal temperature.

Coat thermal insulance

The coat thermal insulances for the four horses in the study with winter coat were plotted versus the ambient air temperature in Fig. 4. The results do not show any effect of piloerection, but a steady level when the ambient air temperature was below 14.5”C. The coat thermal insulance decreased when the ambient air temperature difference was higher than 14.5”C. This decreasing thermal insulance can be explained by an increasing cutaneous evaporative rate, which makes the coat damp. The moisture will enhance the heat transfer. According to Cena and Monteith (1975) greater rates of heat transfer in coats can be accounted for by radiative heat transfer between hairs and free convection induced by temperature gradients. However these explainations

do not seem to apply on the results in the present study.

Total thermal insulame

The total thermal insulance is the summarised value of the thermal insulances of the peripheral tissue, the coat and the boundary layer of the air acting in series. The maximal total thermal insulance will occur in cold, dry and calm conditions. Bruce and Clark (1979) and Bruce (1986) derived an equation to estimate the thermal insulance of the boundary layer in a cold environment:

M,,, = [5.3 + 15.7*(a”h/m0”)] -’ (8)

where M,,, = thermal insulance of the boundary layer [m’ K W-‘I, 1‘ = the velocity of the air [m s ‘1, nr = body mass [kg].

Assuming a 500 kg horse and the velocity of the air is 0.1 m s’ then the thermal insulance of the boundary layer will be estimated to 0.140 m’ K W-‘. A maximal total thermal insulance for the horses in the present study can then be estimated to 0.360 mz K W ’ by adding a tissue thermal insulance of 0.100 m’ K W- ‘, a coat thermal insulance of 0.120m’K W ’ and a thermal insulance of the boundary layer of 0.140 m2 K W-‘. This maximal total thermal insulance of 0.360 rn’ K W -’ can be compared to the values presented in studies by other authors. Ousey et al. (1992) found the total thermal insulance in pony foals to be 250450 s mm’ (0.20-0.35 mz K W ‘). The total thermal insulance of a 500 kg horse can be estimated as 0.29 m’ K We ‘, from the climatic energy demand in a study on feeding and metabolic rate by McBride et al. (1985).

0,16.- ; - - ;. I_ -. I. I

0,0,3 ~.....,......,._____~ ______.__._________._______~ !f! . . . . . . . . . . . . . . .

0,06 .................... j ........................... i ........................... j ........................... t ........................... i ...........................

._

_. .................. ~........................... { ........................... i.. ........................ . ........................... b.. ........................

.................... . ........................... i_ .......................... . .......................... i ........................... I ..........................

Ambient air temperature pcj

Fig. 4. The results of the coat thermal insulance (.) plotted versus the ambient temperature. The regression line (- ) is a step function and shows the trends of the result.

Thermal insulance in sport horses 175

Young and Coote (1973) reported total thermal resistance as high as 0.432-0.558 m’ K W-’ for a

brood mare and a yearling with a hair coat depth of 18.1 and 22.8 mm respectively, when they were kept outside during the winter and were not groomed.

SUMMARY AND CONCLUSIONS

Data for the thermal properties and how they are regulated and physiological status of the horse are needed to estimate the span in ambient temperature within which a horse is thermally comfortable. The aim of the study was to measure the thermal properties; maximal and minimal thermal insulance of the peripheral tissues and thermal insulance of the coat of sport horses.

The measurements were done in a controlled environment in a climatic chamber with one horse capacity on five different horses in six different temperatures between -3 and 37’C. The thermal insulances were calculated from the temperature difference between the body core and the skin for the tissue thermal insulance and between the skin and the coat surface for the coat thermal insulance. The temperatures were measured with thermistors.

In this study the thermal insulance of the peripheral tissue varied between a minimum of 0.015 m2 K W~‘andamaximumof0.100m’K We’. The coat thermal insulance for a dry winter coat was estimated to 0.120 mz K W-’ in a cold and calm environment. The maximal total thermal insulance of sport horses was estimated to 0.360 m’ K W -’ with an estimated thermal insulance of the boundary layer of air to 0.140 m’ K W-‘.

Acknowledgements-l should like to express my special thanks to the following. My supervisor Anders Ehrlemark and Krister S%llvik for counselling and valuable comments on the manuscript. Stiftelsen Lantbruksforskning, LRF, for funding the experiment. Assistant Gunnar Ohlsson helped with the set-up of the measuring equipment. The technical staff of the Department of Medicine and Surgery at the Swedish University of Agricultural Sciences co-operated in an excellent way with the experiments. The veterinary assistance and help with measurements was mainly performed by DVM Pia Funkquist but was also performed by DVM Stina Marntell.

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