Daily mean meteorological data from 437 meteorological stations in the State of Bahia were used to accomplish the bioclimatical zoning of milk cows and the decline in milk yield of cows with production levels of 10 and 25 kg day-1 in summer thermic condition in this State. The variables temperature (Ta) and relative air humidity (RH) were used to calculate the temperature and humidity index (THI). The milk production decline (MPD) was estimated based on data of the THI and the level of production (LP). With these data the PMD and THI mapping was done for the State of Bahia with the program SURFER 7.0. It can be observed that through the zoning, in despite the lowest temperatures in cold months, a bioclimatical area shows great possibilities for climatical stress of milk cows. During the hottest months there are three bioclimatical zones in the State of Bahia with a great occurrence of climatical stress. With the maps, areas with great probabilities for decline in milk production were observed due to unfavorable weather conditions. Those areas show production losses up to 1 kg for cows with (NP) of 10 kg and zones with losses as much as 4.5 kg for cows with (NP) of 25 kg.
milk yield; temperature and humidity index; thermic comfort
