Biomedical Engineering Reference
In-Depth Information
temperature increases 3-5°C in animals
exposed to direct sunlight. During cold, the
solar exposure may be beneficial to animal
and provide fast body warm up, but during
summer, direct radiation causes heat stroke at
high temperatures.
humans and animals, 75-78 stressful and
values greater than 78 cause extreme distress
to lactating cows which are unable to maintain
their normal body temperatures.
It was observed that rectal temperature
increased at THI >80, the respiration rate
started to increase above a THI of 73 and
increased steeply at THI >80 (Lemerle and
Goddard
1986
) . This suggests that homeo-
static mechanisms in animals, including
increased respiration, can prevent a rise in
rectal temperature until the THI reaches 80.
This is similar to the critical THI level of 78
given by McDowell et al. (
1976
) .
(b) By calculating the air temperature (T) and
relative humidity (RH) by using the following
equation:
Body hair coat
- Hair coat provides insulation
to body from the outside environment and
affect its ability to maintain heat balance. The
hair coat helps trap air to make a layer of 'dead
air' for insulation. The body insulation is hin-
dered by soiling with manure, water, urine,
mud, etc., and insulative value of the hair coat
is markedly reduced. Animal with low insula-
tion or soiled with manure and mud in cold
temperatures is likely to have a higher meta-
bolism to cope with the extra heat loss.
Animal shelter and bedding
- Animal shelter
reduces the exposure to direct solar radiations
and effects of precipitation, wind and other
elements on an animal's ability to maintain
thermal balance. Bedding in animal house
reduces the conduction of heat from the animal
into the floor or ground particularly during
cold/winter.
(
) (
)
THI
=×+−− ×
××−
1.8 T 32
0.55
0.0055 RH
(
)
1.8 T 26
where
T
is the air temperature (°C) and RH the
relative humidity (%).
(Tucker et al.
2008
)
The critical values for THI in dairy cows are
determined as 64 for minimum, 72 as the mean
value and 76 as the maximum. At a THI of 75,
50% of the human population feel uncomfor-
table, and this value even dairy cattle also appears
to be uncomfortable (Johnson et al. 1989). A THI
value of 72 equates to 25°C and 50% relative
humidity. If the THI value exceeds 72, the cow
suffers from heat stress and the milk production
declines (West
2003
). When the value is between
78 and 82, the cow is severely affected and cool-
ing by artificial ways is necessary for animal and
sustains production. If THI is above 82, the heat
stress vulnerable cows may cease to produce or
decline productivity, or even death may occur
from heat stress (Du Prezz et al.
1990
) .
A significant depression in milk production
and in reproduction is noticed at an average daily
THI more than 75. However, some depressions
may also be observed above 70 and particularly
above 75 in dairy cattle producing at high levels
or acclimated to a lower ambient temperature and
THI. The average daily THI should be derived on
3
Heat Stress Indicators
Temperature-humidity index (THI) is an index
for assessment of the potential of an environment
to induce heat stress in humans and farm animals.
THI offers a method of combining two of the
more important and easily measured climatic
elements into a possible measure to compare
temperature and humidity data and animal res-
ponse at different locations. The THI may be
calculated by several equations as given below:
(a) By calculating the dry and wet bulb tempera-
tures using the following equation:
(
)
THI
=
0.72 W D
+ +
40.6
(McDowell et al.
1976
)
where
W
is wet bulb and
D
is dry bulb temperature in °C.
Temperature-humidity index values of
70 or less are considered comfortable for
