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.

With the increasing human population and urbanization, the demand for animal origin products is going to grow, especially in the developing nations till the 2050s and the production needs to be escalated and optimized with the changing climate. Heat stress is known to reduce the animal performance, production, shelf life and meat quality in all animals. The beef cattle are globally reared, following different managemental practices, so the usage of natural resources like land and water, manpower, fodders, production systems and the environmental impact also varies profoundly. Recent changes in the climate, global warming and depletion of resources have severely affected the production and heat stress is now a common constraint all over the world. Due to evolutionary diversification the tropical and temperate breeds are comparatively more thermotolerant, but the beef cattle in the colder regions are vulnerable to high environmental temperatures. Also, the production of beef increases the carbon footprint and is much less eco-friendly than growing plant-based protein. So, we comprehended the environmental temperature variation over the continents and impact of heat stress on beef cattle. Also, other factors like cattle population, land and pasture usage, livestock units in trade, methane emissions and gross beef production value were examined to evaluate the collective impact of all these on the beef sector. Our findings and predictions reveal that, in the advent of climate change, depleting natural resources and rise in the greenhouse gases, beef production will be a constant challenge, which can be only achieved by maintaining a healthy cattle population and optimum usage of natural resources. Only then can the beef sector be efficient, sustainable, and a profitable enterprise in future.

The adaptability of cattle to thermal stress has been extensively studied however the exact dynamics of acclimatization is still less investigated. The present study was conducted to assess the acclimatization dynamics in crossbred cattle. In the present experiment non-pregnant dry crossbred cattle (n = 4) were exposed to 25°C and 40°C temperature with a relative humidity of 40–50% in a psychrometric chamber from 10:00 to 15:00 h/day for 21 days. The physiological parameters were recorded and blood samples were collected at 15:00 (after exposure) hours on day 1, 6, 11, 16 and 21. All the studied parameters except T₃ altered significantly (P < 0.05) at 40°C temperature as compared to 25°C. At 40°C exposure a significant (P < 0.05) acclimatization response in respiratory rate was first observed on day 11. The dry matter intake increased significantly (P < 0.05) from day 6 onwards whereas water intake decreased at day 21 only. The LDH, AST, ALT activity, urea concentration and redox status did not reach to pre-stress levels even at day 21. Acclimatization in T₄ and cortisol levels was observed on day 11 and 16, respectively. It can be concluded that in extreme heat stress conditions an acclimatization period of 6 to 21 days was required in crossbred cattle.

The study was conducted on four adult murrah buffaloes to evaluate the acclimatization process during simulated thermal stress. The animals were exposed to 25, 30, 35 and 40°C in climatic chamber, daily for five hours for 21 days at each treatment. Physiological variables i.e. respiration rate (RR), rectal temperature (RT) and pulse rate (PR) were recorded daily, while blood collection was done every fifth day for biochemical and hormonal estimation. Digestibility trial (apparent) and methane collection was performed during last four days at all the treatments. The physiological variables increased significantly at 35°C and 40°C as compared to other treatments (P < 0.05). An increase in feed intake, dry matter and ether extract digestibility was confirmed while, the average methane emission decreased during heat stress than at optimum temperatures. Thyroid hormones declined and glucocorticoids increased at 35°C and 40°C. A decrease was also recorded for urea nitrogen and creatinine but, total protein and albumin initially decreased and then increased back to pre stress levels at 40°C. The physiological responses are immediately activated as acclimatory process followed by metabolic, digestive and endocrinal changes aimed at systemic acclimatization. It was observed that, for successful homeorhetic and homeostatic response to multi-stressors the buffaloes require more than 21 days acclimatization period.