L. De Wet

 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

 Requirements:  All environments, physiological factors and coverings  One value to describe environment and body  But not possible – so need the following:  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.

 Many indices for humans  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 o C Mechanism to control Ta: Heat  blood pressure  perspiration  evaporation (if air is not saturated)

 If moisture level is high,  discomfort  DI equation:  DI = (2*Ta)+[Ta*(RH/100)] +24  Where Ta = T mx  Rh = Rh mn  DI < 80 IS FAVOURABLE

DI value(no units) Level of discomfort Relatively uncomfortable Very uncomfortable Extremely uncomfortable 110 and higher Dangerous

 FOR ANIMALS (E.g. Dairy cattle)  Animals cannot cool off as easily as humans, so provision must be made for them  Equation:  THI = Ta+0.36Tdp+41.2  Where Ta = T mx or Ta  And Tdp = dew point temperature THI < 70 IS FAVOURABLE

 Dairy cattle  Normal THI for dairy cattle is < 70  Body Ta ↑ when Ta ↑  THI = critical  Milk production and feeding ↓ when THI > 72  ↓ ↓ between 76 and 78

 Types of shade:  Food & H20 always available  Trees  Grass roofs  White painted corrugated roof  Management  Example  Holstein  Thermo neutral zone for milk production  -5 o C – 21 o C

 If cattle eat less  ↓ milk production & reproduction  If environmental T > 21 o C – 25 o C, production ↓ (because food intake ↓)  Critical Ta is 21 o C for milk production for Holsteins, and Jerseys & Brown Swiss cows = 24 o C and lower  Critical body temperature (Tc) = 37.5 o C – 38.5 o C  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