Parameters for the determination and evaluation of heat stress in dairy cattle in South Africa

Not all parameters are trustworthy and practical to use as parameters to de termine heat stress in dairy cattle. The temperature-humidity index (THI) i s still the best, simplest and most practical index (parameter) for measure ment of environmental warmth which cause heat stress in dairy cattle. It is practical, easy to determine and relatively trustworthy to use body temper ature and respiratory rate as parameters to determine heat stress in dairy cattle. These physiological parameters must always be used together with TH I values to determine and evaluate heat stress in dairy cattle. For practical purposes, plasma cortisol concentration and milk composition cannot be used as parameters to determine heat stress in dairy cattle altho ugh good indications of acute or chronic heat stress can be obtained. Vanil lic acid is a break-down product of adrenalin found in milk, but before its concentration in milk can be used as an indicator/parameter of heat stress in dairy cows, more about the pharmacodynamics of adrenaline in the milk h as to be known. Selection and breeding of dairy cows on the basis of their adaptibility to heat stress using the most practical heat stress parameters will ensure that their offspring will have superior performance in the pre vailing environmental conditions.