The present experiment was conducted to evaluate the effect of simulated heat stress on digestibility and methane (CH4) emission. Four non-lactating crossbred cattle were exposed to 25°C, 30°C, 35°C, and 40°C temperature with a relative humidity of 40% to 50% in a climatic chamber from 10:00 hours to 15:00 hours every day for 27 days. The physiological responses were recorded at 15:00 hours every day. The blood samples were collected at 15:00 hours on 1st, 6th, 11th, 16th, and 21st days and serum was collected for biochemical analysis. After 21 days, fecal and feed samples were collected continuously for six days for the estimation of digestibility. In the last 48 hours gas samples were collected continuously to estimate CH4 emission. Heat stress in experimental animals at 35°C and 40°C was evident from an alteration (p<0.05) in rectal temperature, respiratory rate, pulse rate, water intake and serum thyroxin levels. The serum lactate dehydrogenase, aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase activity and protein, urea, creatinine and triglyceride concentration changed (p<0.05), and body weight of the animals decreased (p<0.05) after temperature exposure at 40°C. The dry matter intake (DMI) was lower (p<0.05) at 40°C exposure. The dry matter and neutral detergent fibre digestibilities were higher (p<0.05) at 35°C compared to 25°C and 30°C exposure whereas, organic matter (OM) and acid detergent fibre digestibilities were higher (p<0.05) at 35°C than 40°C thermal exposure. The CH4 emission/kg DMI and organic matter intake (OMI) declined (p<0.05) with increase in exposure temperature and reached its lowest levels at 40°C. It can be concluded from the present study that the digestibility and CH4 emission were affected by intensity of heat stress. Further studies are necessary with respect to ruminal microbial changes to justify the variation in the digestibility and CH4 emission during differential heat stress.

The study was planned to evaluate the indigenous animal adaptive capabilities during optimum temperature versus heat stress (HS).Materials and Methods: Four adult buffaloes were exposed at 25°C, 30°C, 35°C, and 40°C for 21 days at every treatment in environmentally controlled chamber and physio-biochemical variation and animal behavior was observed. Results:The study revealed significantly increased rectal temperature, respiration rate, water intake, sodium, reactive oxygen metabolites, cortisol, aspartate aminotransferase, and alanine aminotransferase while, pulse rate and thyroid hormones decreased during thermal stress. Panting, restlessness, salivation, and sweating were higher during HS while, rumination and urination contrastingly lowered. Conclusion:The results reflect the impact of hyperthermia both acute and chronic, on the animals forcing various physiobiochemical, endocrinal, and behavioral changes for acclimatization during a stressful period aimed at maintaining homeothermy.

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