AGRICULTURAL
27 | Heat stress
Inadequate nutrient consumption during
thermal-neutral conditions is associat-
ed with a variety of metabolic changes
implemented to support the synthesis of
high-priority tissues like skeletal muscle.
Marked alterations in both carbohydrate
and lipid metabolism ensure partitioning
of dietary- and tissue-derived nutrients
toward muscle, and altered concentra-
tions of anabolic and catabolic signals
mediate many of these changes. One
characteristic response is a reduction
in circulating insulin coupled with a
decrease in adipose insulin sensitivity,
which allows for adipose lipolysis and
fatty acid mobilisation.
Post-absorptive carbohydrate metabo-
lism is also altered by reduced insulin
action during feed restriction, resulting in
reduced glucose uptake by adipose tis-
sue. The reduced nutrient uptake coupled
with the prolonged net release of fatty
acids by adipose tissue is a key mecha-
nism implemented by malnourished ani-
mals which prioritises protein synthesis.
POST-ABSORPTIVE CHANGES DURING
HEAT STRESS
Reduced feed intake may appear to
explain a majority of the performance
decreases in growing animals, but the
direct effects of heat may markedly alter
the hierarchy of tissue synthesis. The
heat-stressed animal initiates a variety of
post-absorptive metabolic changes that
are largely independent of reduced feed
intake and whole-animal energy balance.
These changes in nutrient partitioning
seemingly are adaptive mechanisms
employed to prioritise the maintenance
of the correct body temperature. Thus,
the heat stress response markedly alters
post-absorptive carbohydrate, lipid and
protein metabolism independently of
reduced feed intake.
The most intriguing mechanism of the
heat-stressed animal is the apparent lack
of fat mobilisation from adipose tissue
coupled with a reduced responsiveness
to lipolytic stimuli for additional energy.
Evidence from many species suggests
that carbohydrate metabolism is altered
during HS, evident through changes
such as basal and stimulated circulating
insulin levels. Liver and blood metabo-
lism show clear differences in glucose
production and use during heat stress.
HS also affects post-absorptive protein
metabolism, as illustrated by changes in
the quantity of carcass lean tissue of rumi-
nants. Muscle protein synthesising are
reduced by environmental HS. Owing to
the contribution of skeletal muscle to over-
all animal mass and the fact that it is an
energetically expensive tissue to maintain,
small changes in its fuel efficiency can
have large impacts on feed conversion
and nutrient flow. The increase in skele-
tal-muscle protein catabolism is peculiar
given the role of insulin in stimulating pro-
tein synthesis and preventing proteolysis.
The breaking down of skeletal muscle
is likely a strategy to supply precursors
for gluconeogenesis and acute phase
proteins rather than to supply oxidative
substrates.
CONCLUSIONS
The primary difference between a ther-
mal-neutral and a heat-stressed animal in
a similar energetic state is the inability of
the HS beast to employ glucose-sparing
mechanisms to prioritise meat synthesis.
From an animal agriculture standpoint,
these survival strategies reduce produc-
tivity and seriously jeopardise farm eco-
nomics.
Defining the biology and mechanisms
of how HS threatens animal health and
performance is critical in developing
approaches to ameliorate current produc-
tion issues. It is also a prerequisite for
generating future mitigating strategies to
improve animal well-being, performance
and agriculture economics..
Om die produktiwiteit van produksiestelsels
vir beeste te verbeter.
Adviesdiens vir herkouervoeding
Voerprosessering, voedingsbestuur en formulering
van voere vir volhoubare produksieprestasie
Formuleer en verskaf konsentrate om formulasies te
komplimenteer
Dr. Kobus Swart 083 262 0946 • [email protected]
28
SENWES SCENARIO | AUTUMN 2019