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Abstract
To explore the effect of thermal stress on diurnal rhythms of physiological responses, peripheral temperatures, feed and water intake, blood biochemical parameters, stress enzymes and plasma sodium, potassium levels in Tharparkar (TP) and Karan Fries (KF) heifers, 12 animals, six each from two breeds were taken and kept first in control thermoneutral (21–26°C and 50–60% RH) and in heat exposed (38–39°C temperature and 60–70% RH) conditions thereafter. All the data recording and blood sample collections were done twice (morning and evening) a day for a continuous seven days each for control and thermal exposed conditions. Values of physiological responses, peripheral temperatures, blood biochemical parameters, stress enzymes and plasma sodium, potassium levels in both Tharparkar and Karan Fries heifers showed a significant variation (P < 0.05) in the morning and evening. So a diurnal fluctuation of rectal temperature was observed in this experiment. Significant (P < 0.01) variation in all the above parameters was observed in both Tharparkar and Karan Fries heifers in control and heat exposed conditions. Feed intake decreased and water intake increased in the heat exposed condition in both the breeds. Values of physiological responses were lower in the morning and showed an increasing trend, reaching highest values in the afternoon. The changes in physiological parameters were greater in Karan Fries in comparison to Tharparkar heifers that expressed lower heat tolerance capacity in crossbred than indigenous cattle. The correlation coefficient showed a significant relationship among all the parameters studied. This finding suggests that it is important to standardize the time of sample collection in comparative studies, in which more discrete differences may be identified in physiological parameters. Animals are comfortable in thermoneutral conditions and are in stress in thermal exposed conditions, so shade and cooling facilities should be provided during summer to withstand the heat stress and for better health and productivity.