Modelling of the effects of heat stress on some feeding behaviour and physiological parameters in cows

A data base has been generated by using results reported in refereed papers that have assessed the effect of heat stress on dairy cows. Studied parameters have been evaluated using variance-covariance analysis to separate between and within variations. Simple and multiple polynomial or quadratic regressions were carried out to quantify the heat stress effects on nutritional parameters. The main results indicated that heat stress effects vary according to the physiological and dietary parameters. Heat stress effects on animal behaviour, physiological parameters and production appeared to occur at thermo hygrometric index (THI) ranging from 80 to 90. Respiratory rate and rectal temperature have the highest correlations with maximal ambient temperature and THI. Pronounced effect of heat stress was observed on dry matter intake (DMI). An average DMI reduction of 4.88 g/kg and a decrease of 11.5 g/kg MBW were noted when THI and NDF increase. These reductions were found to be moderate in diets with high level of concentrate and low fiber amount. The limited results with digestibility measurements show that digestibility of dry and organic matter based diets decreases during thermal stress conditions.