Biomedical Engineering Reference
In-Depth Information
and shiny hair coats reflect a greater proportion
of incident solar radiation than hair coats that are
dark in colour or more dense and woolly (Stewart
1953
; Hutchinson and Brown
1969
; Finch
1986
;
Hansen
1990
). One physiological response to
heat stress is a reduction in heat production
(Kibler and Brody
1952
; Seif et al.
1979
) , which
in turn is caused in large part by a reduction in
feed intake (Kibler and Brody
1952
; Johnston
et al.
1958
; Seif et al.
1979
; Lough et al.
1990
) ,
milk yield (Johnson
1965
; Lough et al.
1990
;
Elvinger et al.
1992
; Aggarwal
2004
) and thy-
roid hormone secretion (Magdub et al.
1982
;
Al-Haidary et al.
2001
; Aggarwal
2004
) . Heat
stress also leads to activation of heat loss mecha-
nisms. Blood flow to the periphery increases so
that heat loss via conduction and convection is
enhanced (Choshniak et al.
1982
) . Cattle change
posture and orientation to the sun to reduce gain
of heat from solar radiation. Moreover, chronic
exposure to elevated environmental temperatures
results in a lightening of the hair coat (Stewart
and Brody
1954
). Heat stress also leads to
activation of evaporative heat loss mechanisms
involving an increase in sweating rate and respi-
ratory minute volume (Kibler and Brody
1952
;
Choshniak et al.
1982
; Gaughan et al.
1999
;
Al-Haidary et al.
2001
) . About 70-85% of maxi-
mal heat loss via evaporation is due to sweating
with the remainder by respiration (Kibler and
Brody
1952
; Finch
1986
). Heat loss via skin
is more in cows, and heat loss by respiration
is higher in buffaloes. This is due to less number
of sweat glands in buffaloes (Aggarwal and
Upadhyay
1997,
1998
) . As air temperatures
approach those of skin temperature, evaporation
becomes the major route for heat exchange with
the environment.
Reactions of homeotherms to moderate cli-
matic changes, generally, are compensatory and
are directed at maintaining or restoring thermal
balance (West
1999
) . When the environmental
temperature reaches near the cow's body tem-
perature, high ambient relative humidity percent-
age (RH%) reduces evaporation and affects the
cow's cooling capability, and core temperature
increases. This occurs due to the negative effects
of high humidity on dissipation of body heat
because of the decline in effectiveness of radia-
tion, conduction and convection and the efficiency
of evaporative cooling (West
1993
) . Vaporisation
from the respiratory tract and the outer body
surface is negatively affected by levels of the
temperature and percentage of relative humidity
of the air. It has been observed that buffaloes can
acclimate more to high than low temperatures
(Zicarelli et al.
2005
) . Through natural selection,
buffaloes have acquired several morphological
features that allow them to adapt to hot-humid
areas. For instance, melanin-pigmented skin is
useful for protection against ultraviolet rays,
and low hair density facilitates heat dissipation
by convection and radiation. In hot-dry cli-
mates, low humidity determines intense evapo-
rative heat loss, which in buffaloes is limited by
the low number of sweat glands. In addition,
respiratory evaporation is less effective than in
cattle due to induced alkalosis as a consequence
of a rapid increase of blood pH (Koga
1991
) . In
hot-humid climates, evaporative heat loss is not
as effective in body heat dissipation. Thus, buf-
faloes rely on wallowing for efficient thermoreg-
ulation, and high secretion of sebum protects the
skin in the water or mud (Hafez et al.
1955
) . In
particular, buffaloes in hot conditions increase
blood volume and flow to the skin surface to
maintain a high skin temperature and facilitate
heat dissipation while in the mud or in the water
(Koga
1999
) .
6
Response to Heat Stress
The change in the environmental conditions, as is
the case during heat stress, affects the normal
metabolic balance and produces a positive feed-
back when the temperature is above the thermal
comfort or upper range of the tolerance. In dairy
cattle, as milk production increases, metabolic
heat production rises with the metabolism of
large amounts of nutrients, which makes the
high-producing cow more vulnerable to high
ambient temperatures and humidity than animals
that are less active metabolically. 'Metabolism
and productivity run parallel' (Brody
1945
) , and
therefore, high-producing cows are affected more
