Biomedical Engineering Reference
In-Depth Information
from conduction) so that the distance between
the blood vessels and the surface is much
greater (Kadzere et al.
2002
) . Furthermore, it is
uncomfortable or even more painful for the
cow to be lying with a filled udder, since
there is an external pressure and heat from the
floor on the udder when lying (Oszterman and
Redbo
2001
) .
strain was not due solely to increased milk yield
(Cole and Hansen
1993
) . Either greater heat
production or interference with heat loss could
explain greater strain in non-lactating cattle and
that although bST use is efficacious in hot cli-
mates, its use should be coupled with methods
to reduce the magnitude of heat stress during
summer months.
4
Effect of Heat Stress
on Bovine Somatotropin
(bST)-Administered Cows
5
Metabolic Heat Production
Heat production of metabolic functions accounts
for approximately 31% of intake energy by a
600-kg cow producing 40 kg of milk containing
4% fat (Coppock
1985
) . Physical activity
increases the amount of heat produced by skeletal
muscles and body tissues. Maintenance expendi-
tures at 35°C increase by 20% over thermoneu-
tral conditions ( NRC
1981
), thus increasing the
cow's energy expenditure, often at the expense of
milk yield. Body heat production associated with
milk yield increases as metabolic processes, feed
intake and digestive requirements increase. The
heat load accumulated by the cow subjected to
heat stress is the sum of heat accumulated from
the environment and the failure to dissipate heat
associated with metabolic processes. Therefore,
with similar body size and surface area, the lac-
tating cow has significantly more heat to dissi-
pate than a non-lactating cow and have greater
difficulty dissipating the heat during hot and hot-
humid conditions. A comparison of low (18.5 kg/
day) or high (31.6 kg/day) milk-producing cows
indicated that low and high-yielding cows gene-
rated 27 and 48% more heat than non-lactating
cows despite having lower body weight (Purwanto
et al.
1990
) . Berman et al. (
1985
) reported that
rectal temperature of cows increased by 0.02°C/
kg FCM for cows producing >24 kg/day, and
greater heat production can explain the increas-
ing rate of decline in milk yield for cows as pro-
duction increased from 13.6-18.1 to 22.7 kg of
milk per day and THI increased from 72 to 81
(Johnson et al.
1963
) . High production greatly
accentuates heat stress in the lactating cow par-
ticularly unable to dissipate extra heat.
The feeding and management recommendations
for a cow treated with bovine somatotropin (bST)
are considered similar to that of high-producing
cow. Nutrient intake should be increased with
special attention to energy and water needs. The
cows should be provided with cooling systems.
Israeli research indicated that the response to bST
under high environmental temperatures may have
been slightly reduced. Work conducted at the
University of Missouri showed that heat-stressed
injected cows responded to bST with increased
milk production and dry matter intake. Hot tem-
peratures did not reduce the effects of bST. The
effects of high milk yield have been demonstrated
by West et al. (
1990,
1991
) who reported that
milk temperature was greater for cows adminis-
tered with bST compared with controls in a hot
and humid climate; low yielding cows were more
responsive to bST than high-yielding cows, pos-
sibly because of the higher body temperature
associated with greater milk yield. Cows admin-
istered with bST exhibited significantly greater
heat production in both thermoneutral and hot
environments, though cows were apparently able
to dissipate the greater heat produced, evidenced
by greater total evaporative heat losses and coo-
ling heat loss for the bST-treated cows which
enabled cows to maintain normal body tempera-
tures (Manalu et al.
1991
) . Administration of bST
to both lactating and non-lactating cows in a hot,
humid climate (Florida) resulted in elevated
body temperature and respiratory rate for both
groups of cattle, suggesting that the greater heat
