A mathematical model developed from heat transfer principles to predict the
thermal status of a homeotherm was applied to sheep and cattle outdoors an
d pigs and broiler chickens indoors. The climatological variables considere
d in the model include air temperature, wind speed, vapour pressure and sol
ar radiation. For sheep, the fleece depth varied seasonally and thermal bal
ance was achieved by a metabolic response, vasodilation and panting. For ca
ttle, the thermal responses included sweating and piloerection of the coat.
The insulation provided by the pig's sparse hair coat was neglected, but t
he increase in its body insulation with age and environmental conditions wa
s included as a major determinant of heat loss. For chickens, the insulatio
n provided by the body tissue and feathers was described by a single therma
l resistance. Their thermal responses included feather fluffing,, vasomotor
action in the combs and feet, and changes in respiration rate and body tem
perature.
The models were tested successfully for each species by simulating the expe
rimental conditions used by previous workers and comparing the predictions
with measured values of heat loss, skin and body temperature. The intercept
ion of solar radiation by animals outdoors was also tested successfully for
solar elevations up to 45 degrees.
For sheep, the predicted heat loss agreed with measurements to within 10%.
The onset of vasodilation for a shorn sheep on maintenance food intake was
predicted successfully to occur at an air temperature of 25 degrees C, and
the variation of skin temperature on the legs with air temperature was pred
icted to within the uncertainty of the measurements. The model predicted th
e heat loss from cattle in the cold with acceptable accuracy when the wind
speed was low, but overestimated heat loss from calves by up to 30% in wind
. in warm conditions, the evaporative heat loss from cattle as a consequenc
e of sweating was predicted with acceptable accuracy. The errors incurred b
y ignoring solar radiation penetration into the coat were acceptably small,
given the associated reduction in model complexity. Sensitivity analysis s
howed that the predictions of heat loss from sheep and cattle were sensitiv
e to wind speed and coat length, especially when the coat is short. For bot
h species, the level of stress was sensitive to ambient vapour pressure at
high air temperatures.
For a single new-born pig, the model underestimated heat loss at 30 degrees
C with an overall error of -9% over the range of wind speeds likely to be
experienced indoors. The model over-predicted heat loss by an average of 20
% at 20 degrees C, probably due to the absence in the model of a temperatur
e-dependent huddling response. However, far a 25 kg pig exposed to air temp
eratures from -5 to 35 degrees C, the model predicted the skin temperature
on the trunk - a good indication of its thermal status - to within the Limi
ts of the experimental uncertainty. The total heat loss from chickens expos
ed to temperatures in the range 0-38 degrees C was predicted with an overal
l error of 6%. In a separate test, the body core temperature of hens was pr
edicted to within 0.3 degrees C on average for the same range of air temper
ature, again within the limits of experimental uncertainty. Sensitivity ana
lysis showed that the prediction of body temperature for chickens was most
sensitive to ambient humidity at high air temperatures, and to body resista
nce. The paper discusses the limitations of the models and the need for mor
e measurements of heat losses from current breeds of livestock. (C) 2000 El
sevier Science B.V. All rights reserved.