Biomedical Engineering Reference
In-Depth Information
There are several methods available that can
alleviate animal's heat stress by lowering air
temperature or the enthalpy of the ambient air.
A number of animal cooling options exist based
on combinations of the principles of convec-
tion, conduction, radiation and evaporation. Air
movement (fans), wetting the cow, evaporation
to cool the air and shade to minimise transfer of
solar radiation are used to enhance heat dissipa-
tion from high-producing cows. Animals in
pond lose heat fast to cool water primarily by
conduction and coefficient of heat transfer to
water from skin. Passive solar designs make
use of the sun's energy for the heating and cool-
ing of the animal buildings.
environment to maintain normal body temperature.
Animal can give and/or receive heat energy from
the environment. Solar radiation increases heat
load by increasing the surface temperature of
cattle and buffaloes, and air temperature above
normal body temperature also increases the heat
load. At high temperature, in addition to increas-
ing heat load, heat exchange at the body surface
is reduced. Heat stress increases maintenance
cost and decreases dry matter intake (DMI) and
milk yield on exposure to hot or hot and humid
environments for long with THI above 80 (Huber
et al.
1994
). High-producing cattle and buffaloes
are more vulnerable to heat stress. The problem
of heat stress becomes more acute as the produc-
tion level increases (Armstrong
1994
) .
Two types of radiation, that is, direct radiation
from the sun and diffused radiation from the sky,
influence the microclimatic conditions around the
animal. The diffused solar radiation is scattered at
least once before it reaches the surface of animal
and influences mainly on cloudy days. Even on
clear cloudless day, a certain diffused radiation
appears. Therefore, heat is transferred into the
animal house or building from solar radiation on
cloudy day. The intensity of the direct solar radia-
tion can reach more than 1,000 W/m
2
, while dif-
fused radiation may be about 50-100 W/m
2
(Gustafsson
1988
). The total solar radiation on
animal also sometimes called global radiation is
the sum of the direct radiation and the diffused
radiation (Duffie and Beckman
1974
) . Radiant
energy that strikes a surface may be absorbed,
reflected or transmitted. The relationship between
reflectance and absorption is based on the assump-
tion that a black surface has zero reflectance and
the absorption is one. The factor for reflectance
and absorption varies for different materials. If the
material is non-transparent, the fraction of trans-
mission will be zero, that is, all the radiant energy
is either absorbed or reflected. To achieve mini-
mum heat load on the animal building from solar
radiation, the reflectance factor should be high,
and absorption factor should be low for the
exposed areas of the materials, that is, roof top or
the upper surface of the roof. In direct radiation
when the surface is perpendicular to the sun, the
rate of solar radiation received is maximum
1
Introduction
Thermal stress is a major limiting factor in live-
stock production under tropical climate and also
during summer season in temperate climates.
Heat stress occurs when the ambient temperature
lies above thermoneutral zone. Meteorological
variables which influence the ambient tempera-
ture significantly are dry bulb temperature, wet
bulb temperature, wind velocity and intensity of
solar radiation and radiations from surrounding
structures in the animal byre. Ambient tempera-
ture and humidity are used to assess thermal
comfort, and derivation based on both is termed
the temperature-humidity index (THI). High-
producing cows have been shown to be in heat
stress if the THI exceeds 72 and in severe heat
stress at a THI above 80 (Armstrong
1994
) . Milk
production of dairy cows declines at air tempera-
ture above 25°C and with relative humidity above
50% (Chiappini and Christiaens
1992
) . The
severity increases as the temperature rise beyond
the thermoneutral zone or above the capacity to
handle heat effectively.
Lactating dairy cows exposed to high ambient
temperature and high relative humidity or radiant
energy (direct sunlight) tends to decrease milk
yield and may exhibit other signs of heat stress.
Lactating cows and buffalo produce large amounts
of heat due to digestion and metabolic processes,
and this heat needs to be exchanged with the
