Biomedical Engineering Reference
In-Depth Information
increased. Due to the hyperventilation-induced
decrease in blood CO
2
, the kidney secretes HCO
3
−
to maintain this ratio. This reduces the amount of
HCO
3
−
that can be used (via saliva) to buffer and
maintain a healthy rumen pH. In addition, pant-
ing cows drool, and drooling reduces the quantity
of saliva that would have normally been deposi-
ted in the rumen. Furthermore, heat-stressed cows
ruminate less due to reduced feed intake and
therefore generate less saliva. The reductions in
the amount of saliva produced and salivary HCO
3
−
content and the decreased amount of saliva enter-
ing the rumen make the heat-stressed cow much
more susceptible to subclinical and acute rumen
acidosis (Kadzere et al.
2002
) . When cows begin
to accumulate heat, there is a redistribution of
blood to the extremities in an attempt to dissipate
internal energy. As a consequence, there is
reduced blood flow to the gastrointestinal tract,
and as a result nutrient uptake may be compro-
mised (McGuire et al.
1989
) . Therefore, accumu-
lation of end products of fermentation (VFAs)
contributes to the reduced pH and indirectly
enhances the risk of negative side effects of an
unhealthy rumen (i.e. laminitis, milk fat depres-
sion, etc.) (Baumgard and Rhoads
2007
) .
A prerequisite of understanding the metabolic
adaptations which occur with heat stress is an
appreciation of the physiological and metabolic
adaptations to thermal-neutral negative energy
balance (i.e. underfeeding or during the transition
period). Cows in early lactation are classic exam-
ples of when nutrient intake is less than necessary
to meet maintenance and milk production costs,
and cows typically enter negative energy balance
(Moore et al.
2005a
). Negative energy balance is
associated with various metabolic changes that
are implemented to support the dominant physio-
logical condition of lactation (Bauman and Currie
1980
). Marked alterations in both carbohydrate
and lipid metabolism ensure partitioning of
dietary-derived and tissue-originating nutrients
towards the mammary gland, and many of these
changes are mediated by endogenous soma-
totropin which is increased during periods of
negative energy balance (Bauman and Currie
1980
) . There is reduction in circulating insulin
coupled with a reduction in systemic insulin
sensitivity. The reduction in insulin action
allows for adipose lipolysis and mobilisation of
nonesterified fatty acids (NEFA; Bauman and
Currie
1980
). Increased circulating NEFA is typi-
cal in 'transition' cows and is an important source
of energy (and precursor for milk fat synthesis)
for cows during negative energy balance. Post-
absorptive carbohydrate metabolism is also
altered by the reduced insulin action during nega-
tive energy balance with the net effect of reduced
glucose uptake by systemic tissues (i.e. muscle and
adipose). Reduction in nutrient uptake coupled
with the net release of nutrients (i.e. amino acids
and NEFA) by systemic tissues are key homeor-
hetic (an acclimated response vs. an acute/homeo-
static response) mechanisms implemented by
cows in negative energy balance in order to sup-
port lactation (Bauman and Currie
1980
) .
8
Mechanism by Which Heat
Stress Reduces Milk Yield
Heat stress reduces both the feed intake and milk
yield of cows. The decline in nutrient intake has
been identified as a major cause of reduced milk
synthesis (Fuquay
1981
) . A reduction in energy
intake combined with increased energy expendi-
ture for maintenance lowers energy balance and
partially explains why lactating cattle lose
significant amounts of body weight during severe
heat stress (Rhoads et al.
2009
; Shwartz et al.
2009
). Heat-stressed cows have been observed to
respond immediately and an immediate reduction
(~5 kg/day) in dry matter intake (DMI) with the
decrease reaching a peak at ~day 4 and remaining
stable thereafter. Thermal-neutral pair-fed cows
had a feed intake pattern similar to heat-stressed
cows. Heat stress reduced milk yield by ~14 kg/
day with production steadily declining for the
first 7 days and then reach a plateau. Thermal-
neutral pair-fed cows also had a reduction in milk
yield of approximately 6 kg/day, but milk pro-
duction reached its nadir at day 2 and remained
relatively stable thereafter. This indicates the
reduction in DMI can only account for ~40-50%
of the decrease in production when cows are heat
stressed and that ~50-60% can be explained by
