Biomedical Engineering Reference
In-Depth Information
due to positive heat gain transmitted to the
appetite centre cause a decrease in feed intake;
therefore, less substrates become available for
enzymatic activities, hormone synthesis and
heat production, which help in cooling the
body. Among the genetic adaptations that have
developed in zebu cattle during its evolution
have been the acquisition of genes for ther-
motolerance. Zebu breeds are better able to
regulate body temperature in response to heat
stress than are cattle from a variety of
Bos
taurus
breeds of European origin.
subcellular to the whole-animal level to help the
cows avoid physiological dysfunction and to suit
their environment. The endeavour by thermoregu-
latory processes to stabilise body core tempe-
rature within a set narrow limit is essential to
control biochemical reactions and physiological
processes associated with normal metabolism
(Shearer and Beede
1990
) .
2
The Thermoneutral Zone
The range of environmental temperature within
which the animals use no additional energy to
maintain their body temperature is called the
thermoneutral zone (Fig.
1
). Within the ther-
moneutral zone (TNZ), the physiological costs
are minimal, and productivity is maximum
(Du Prezz et al.
1990
) . Lactating dairy cows
have a high metabolic rate and heat increment;
therefore, they require an effective thermoregu-
latory mechanism to maintain their body tempera-
ture. The metabolic heat production rises due
to the metabolism of nutrients, and because of
this reason, the heat load increases (Kadzere
et al.
2001
). Both above and below TNZ, the
changes in behaviour and diurnal pattern of
animals are observed. This range is bounded
by the lower critical temperature (LCT) and
upper critical temperature (UCT) given in
Fig.
1
for different species. 'Effective ambient
temperature (EAT)' is the actual temperature
felt by the animal and may be very different
from the air temperature. The EAT of a calf
housed in a clean, dry hutch bedded with straw
may be 8-10°C warmer than the air temperature,
and the EAT of a heifer exposed to wind and
rain may be considerably lower than the ambient
temperature. The upper critical temperature of
dairy cattle is lower than other livestock species
(Wathes et al.
1983
) . Generally, the TNZ range
(from lower critical temperature (LCT) to
upper critical temperature (UCT)) is influenced
by animal age, species, breed, feed intake, diet
composition, previous state of temperature accli-
mation or acclimatisation, production, speci fi c
housing and pen conditions, tissue insulation
(fat, skin), external insulation (coat) and animal
behaviour (Yousef
1985
) .
1
Introduction
All livestock species maintain their body tem-
perature within a close defined thermoregulatory
limit regardless of the external environment
variations. The most comfortable environmental
temperature range for temperate dairy cattle is
between 5 and 25°C, which is believed to be the
thermal comfort zone (McDowell
1972
) with
minimal physiological cost and maximum pro-
ductivity (Folk
1974
). Body temperature in all
homeotherms is maintained by the thermoregu-
latory mechanisms within ±1°C of its normal set
temperature under ambient conditions and that
do not impose heat stress (Bligh and Harthoorn
1965
). Above this temperature (25°C), dairy cow
increases respiratory frequency and rectal tem-
perature that cause a decline in milk yield and
reproductive performance (Bitman et al.
1984
) .
A rise in the core temperature also increases heat
loss by panting and sweating (Spain and Spiers
1998
). These responses are considered normal
thermoregulatory physiological mechanisms to
transfer heat from the body to the surrounding
environment. An increase in respiratory frequency
with or without change in tidal volume or panting
expends energy that increases the daily main-
tenance requirement of an animal by 7-25%
(NRC
1981
) .
The stress is defined as the magnitude of forces
external to the body which tend to displace its
systems from their resting or ground state (Yousef
1985
). In this light, heat stress for the dairy
cows indicate all high temperature-related forces
that induce adjustments occurring from the
