A Review: Growth and Physiological Adaptability of Sheep to Heat Stress under Semi –Arid Environment

Asian countries are characterized with humid subtropical climate, Heat stress is the most important climatic
stress which adversely affects the livestock and sometimes even threatens the survival of animals. Among
climatic components that may impose stress on many different traits to sheep are ambient temperature,
humidity, air movement, photoperiod, solar radiation, wind speed, etc, of which the ambient temperature is
the most important variable. Heat stress affects performance and productivity of small ruminants in all phases
of production. The degree to which these stress impacts on productivity will differ between the agro-ecological
regions and between production systems. The level of production achieved by a particular genotype in harsh
environment depends on the contribution and expression of many different traits which may be partitioned into
those directly involved with production and adaptation. Growth, such as body weight and physiological
performance of animal is affected due to hyperthermia. As the primary non-evaporative means of cooling for
the small ruminants (radiation, conduction, convection) become less effective with rising ambient temperature,
the animals become increasingly reliant upon evaporative cooling in the form of sweating and panting.
Increasing air temperature, temperature-humidity index and rising rectal temperature above the critical
threshold levels are related to decreased dry matter intake (DMI) and milk yield. Stress depresses growth rate
primarily through the depression of food intake, but also by affecting digestion and metabolism. Alleviation of
heat stressed animals by physical, physiological and/or nutritional techniques, in addition to carrying out
proper routine management practices at the suitable times, can help the heat stressed animals to express their
genetic potentials in tropical and subtropical areas.