L. De Wet
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Compare Durban and Bloemfontein weather
Indices quantify effects of environment on
human beings
T or solar radiation = absolute measure of heat
or energy
Standard measurements = describe
environment and can be compared
But what is the effect on humans?
So various factors can be combined to provide
an index to measure the effect
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Requirements:
All environments, physiological factors and coverings
 One value to describe environment and body
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But not possible – so need the following:
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Simple and accurate
3 parameters: environment, reaction and effect
Environment – Ta, Rh, wind speed & radiation
Reaction – dependant on species, metabolic rates, cover
type & length, age, sex, nutrition, diseases, etc.
Effects – production, growth, reproduction, physiology,
pathology, etc.
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Many indices for humans
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Example: windchill index – necessary when
temperatures are low  unpleasant and/harmful to
humans
At higher temperatures the wind has a cooling effect
by removing hot air parcels near skin surface and by
promoting evaporation of perspiration
•Temperature
(Ta) and relative humidity (Rh)
•Body Ta = 37 oC
•Mechanism to control Ta:
•Heatblood
pressureperspiration
•evaporation (if air is not saturated)
Vapour pressure
Ta
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If moisture level is high, discomfort
DI equation:
DI = (2*Ta)+[Ta*(RH/100)] +24
 Where Ta = Tmx
 Rh = Rhmn
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DI < 80 IS FAVOURABLE
DI val
ue(no units)
80 - 90
90 - 100
100- 110
110 and higher
Level of discomfort
Relatively uncomfortable
Very uncomfortable
Extremely uncomfortable
Dangerous
Ta
RH
DI
o
( C)
(%)
36
10
100
33
30
100
27
80
100
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FOR ANIMALS (E.g. Dairy cattle)
Animals cannot cool off as easily as humans, so
provision must be made for them
Equation:
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THI = Ta+0.36Tdp+41.2
Where Ta = Tmx or Ta
And Tdp = dew point temperature
THI < 70 IS FAVOURABLE
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Dairy cattle
Normal THI for dairy cattle is < 70
 Body Ta ↑ when Ta ↑
 THI 70-72 = critical
 Milk production and feeding ↓ when THI > 72
 ↓ ↓ between 76 and 78
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THI
Livestock Weather
Recommended minimum precautions
safety Index
<70
Normal
Nature or artificial shade
70-72
Critical index for milk
Above + well ventilated, open sided barn,
production
freely available water in shade
Alert critical level for
Above + overhead sprinklers & fans in
milk production
kraal / milk parlour; diet changes,
72-78
consider breed of cow, change
management
78-82
Danger
Above + shaded feed bunks, sprinklers &
fans at feed
82 or above
Emergency
All above, but not in mean values - only
on individual days during Nov-March
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Types of shade:
Food & H20 always available
 Trees
 Grass roofs
 White painted corrugated roof
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Management
Example
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Holstein
 Thermo neutral zone for milk production
 -5oC – 21 oC
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If cattle eat less ↓ milk production & reproduction
If environmental T > 21 oC – 25 oC, production ↓
(because food intake ↓)
Critical Ta is 21 oC for milk production for Holsteins,
and Jerseys & Brown Swiss cows = 24 oC and lower
Critical body temperature (Tc) = 37.5 oC – 38.5 oC
 Recovery time = 7 – 8 days
 Milk production ↓ 15% if THI > 72
 SA is safe in winter
 Summer – large areas have THI> 72, so farmers must
provide additional cooling for cattle for ↑ production
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