THERMAL STRESS IN DAIRY CATTLE.

Crosses of temperate cattle reared in tropical ambience lapse in heat tolerance, fertility and disease resistance. However, cross breeding zebu with high yielding exotic cattle seems necessary to meet the need for enhanced composite milk production per unit in view of food security. Thermal stress is considered as the main factor responsible for reduction of milk yield in tropical climate. Any model for the study of thermal stress in dairy cattle should encompass related effect of ambient parameters of temperature, humidity, wind speed and solar radiation as the stressor and behavioural, autonomic, neuroendocrine and immunological endpoints as responses. The responses reach different response states depending on the severity of the stressor. Response measurements are at behavioural, physiological, biochemical and cellular level, which also include hormone, protein and gene expression assays. By fitting these measurements to the described model we can work out the biological and economic cost of thermal stress and the level of adaptation of the dairy animal in question. Such studies taking in to consideration the diverse nature of climatic factors is imperative for finding ameliorative measures to reduce the thermal stress experienced by the existing cattle population and for the possible genetic and management strategies for evolving and maintaining a climatically adapted dairy stock in a state like Kerala. This review analyses suitable model for climatic adaptation studies in the hot and humid climate especially in small holder production systems. [ABSTRACT FROM AUTHOR]