AGRICULTURAL | LANDBOUKUNDIG
AGRICULTURAL
What is the reason for concern?
Heat stress (HS) or hyperthermia is a global problem that significantly decreases
the ability of animals, like ruminants, to produce sufficient animal protein for human
consumption. Distinct indirect (mediated by decreased nutrient intake) and direct
effects of heat stress are responsible for reduced animal productivity during the warm
summer months. Heat stress directly affects multiple systems, and the summation of
these altered physiological systems contributes to reduced animal productivity. There
is an urgent need to better understand how HS alters nutrient utilisation and ultimately
reduces animal productivity and how to prevent or reduce HS as far as possible.
By dr Kobus Swart
Animal Nutrition Specialist
S
outh Africa is a semi-arid coun-
try and traditionally experiences
high summer temperatures,
especially in the western regions
of the country. Climate change and/
or fluctuations have also been a reality
for centuries and animal production has
to adapt to high temperatures from time
to time. This adaptation is both annual-
ly on a seasonal basis during summer
months or over a longer cyclic period of
climate change when exceptional high
or extreme temperatures, usually as heat
waves, are experienced. In regard to heat
waves (i.e. occurrences with maximum
daily temperatures Tmxd> 30°C on 3 or
more consecutive days) and extreme heat
waves (occurrences with Tmxd≥ 35°C
on 3 or more consecutive days). The
median number of heat waves per annum
from the suggested five is projected to
increase in the future. The recent summer
of 2018/19 South Africa had to face such
extreme high temperatures.
Environmentally-induced HS compromises
efficient animal production and jeopardis-
es animal welfare. Defining the biology
and mechanisms of how HS jeopardises
animal performance is critical in devel-
oping approaches to ameliorate current
production issues. It is a prerequisite
for generating future mitigating strate-
gies (genetic, managerial, nutritional,
and pharmaceutical) to improve animal
well-being and performance.
TEMPERATURES THAT CAUSE HEAT
STRESS
An ambient temperature of 30°C seems to
be the critical point at which Bos Taurus
and Bos Indicus begin to differ in ability
to maintain near normal rectal tempera-
tures and respiratory rates. The accepted
comfort threshold for most cattle breeds
is 32°C.
In general, HS-induced production losses
for beef cattle are not as severe as those
experienced by the dairy industry. It is not
entirely clear why growing cattle tolerate
higher Temperature Humidity Index (THI)
conditions and exhibit a greater heat-
strain threshold than lactating dairy cows.
Feedlot cattle are adversely affected
by high temperatures, relative humidity,
solar radiation and low wind speeds. With
these temperatures in mind, tolerance
thresholds are reached regularly in the
Northwest, Northern Cape and Free State
during the summer months.
METABOLIC ADAPTATIONS TO
REDUCED PLANE OF NUTRITION
A prerequisite for understanding meta-
bolic adaptations that occur during HS
is an appreciation for the physiological
and metabolic adjustments that grow-
ing animals initiate during malnutrition.
Collectively, changes in post-absorptive
nutrient partitioning occur to support a
dominant physiological state of skele-
tal-muscle synthesis. One well-described
strategy is the glucose-sparing effect that
growing animals utilise when on a lowered
plane of nutrition or in a negative energy
balance.
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