Thermoregulation - Heat Stress and Animal Productivity - page 4

Fig. 2 Lower and upper critical
temperatures for dairy animals

Lower critical
temperature

Upper critical
temperature

10 ° C Buffalo

cattle, indigenous

20 ° C

8 ° C

Holstein

Cattle , crossbred

24 ° C

6 ° C

Jersey
cattle, crossbred

4 ° C

28 ° C

2 ° C

Jersey

32 ° C

0 ° C

Cattle, indigenous

-2 ° C

36 ° C Buffalo

-4 ° C

-10 ° C

38 ° C

Holstein

-20 ° C

-30 ° C

2.3 Factors Affecting TNZ

As the thermal load exceeds the evaporative heat
loss capacity of an animal, core temperature rises
and may lead to hyperthermia. Continuous rise
and unabated heat may cause animal death due to
hyperthermia.
At high temperatures, the potential for non-
evaporative heat loss is reduced, and animals rely
on the evaporative water loss to dissipate any
excess heat generated by metabolism (McArthur
and Clark 1988 ). The UCT is 25-26°C for dairy
cows and most likely remain unaltered irrespec-
tive of previous acclimatisation or of their milk
production (Berman et al. 1985 ) . This is in con-
tradiction with Yousef ( 1985 ) indicating that the
TNZ varies with physiological state and other
environmental conditions.
The UCT can be decided on the basis of ther-
moregulatory functions like increased sweating
and respiratory water loss and increased body
temperature (Berman et al. 1985 ) . Evaporative
water loss from the skin increases at air tempera-
tures above 20°C (Berman 1968 ) . The upper and
lower critical temperatures for various dairy
breeds have been given in Fig. 2 .

￿ Wind velocity - The movement of air around

an animal affects its ability to maintain body
temperature. Air movement increases evapo-
ration and heat convection. In hot tempera-
tures, wind can increase evaporation, thereby
cooling the animal. Calves and heifers less
than 1 year of age can benefit from some type
of protection or shelter from wind. Newborn
calves are more susceptible to effects of cold
than hot temperature. In very cold tempe-
ratures, wind chill is used to calculate the
effective temperature.

￿ Air humidity - The level of ambient humidity

can affect an animal's ability to dissipate body
heat or thermoregulation, particularly in hot-
humid climates. High humidity levels reduce
the rate of evaporative cooling and efficiency
of cooling mechanisms.

￿ Precipitation - Rain and snow wet the animal

hair coat that reduces thermal insulation.

￿ Radiation - Direct solar exposure increases

net heat gain by an animal. The effective