Intercomparison of seven Temperature-Humidity Index (THI) equations for assessing the impact of heat stress in relation to milk production to find out best THI for central zone of Kerala

Heat stress experienced by cattle can be quantified using Temperature Humidity Index (THI). Seven different THI developed by different scientists all over the world were compared in this study to find out best THI for assessing the influence of heat stress in relation to milk productionfor central zone of Kerala. The dry bulb, wet bulb, average relative humidity and temperature were used to estimate the THI. The daily data of wet bulb, dry bulb, maximum and minimum temperature, forenoon and afternoon relative humidity and average relative humidity during the period of 2013-2017 were collected from principal agro meteorological observatory, Vellanikkara and daily values were converted into monthly average values. Using these data THI was calculated using seven equations for four seasons. The standard method of IMD was followed to group months into seasons, i.e., winter (January-February), pre monsoon (March-May), south west monsoon (Jun-Sep) and post monsoon (October-December). Annual and seasonal mean values of each THI equations were calculated and compared. If seasonal mean value exceeded annual mean value of THI, such seasons were considered as the season of heat stress. Daily milk yield data of seven Holstein- Friesian cows after second calving were collected from university farm, Vellanikkara and monthly average milk yield was calculated. Results showed that seasonal average value exceeded annual average value of THI only during pre monsoon season. Calculated THI values for pre monsoon season were correlated with milk yield and THI having highest R-2 was selected as the best THI. THI developed by NRC, 1971 showed maximum R-2 value hence found to be most suitable equation to assess the heat stress during pre monsoon season. The equation is given as follows:. THI 1 = (1.8 x T-db + 32) - [(0.55 - 0.0055 x RH) x (1.8 x T-db - 26.8)] (R-2 = 0.1711) (NRC, 1971).