This paper presents a new thermal stress index for dairy cows in inter-tropical regions, with special mention to the semi-arid ones. Holstein cows were measured for rectal temperature (T-R), respiratory rate (F-R) and rates of heat exchange by convection (C), radiation (R), skin surface evaporation (E-S) and respiratory evaporation (E-R) in the north eastern region of Brazil, after exposure to sun for several hours. Average environmental measurements during the observations were air temperature (TA) 32.4 degrees C (24.4-38.9 degrees), wind speed (U) 1.8 m.s(-1) (0.01-11.0), relative humidity 63.6 % (36.8-81.5) and short-wave solar radiation 701.3 W m(-2) (116-1,295). The effective radiant heat load (ERHL) was 838.5 +/- 4.9 W m(-2). Values for the atmospheric transmittance (tau) were also determined for tropical regions, in order to permit adequate estimates of the solar radiation. The average value was tau=0.611 +/- 0.004 for clear days with some small moving clouds, with a range of 0.32 to 0.91 in the day period from 1000 to 1300 hours. Observed t values were higher (0.62-0.66) for locations near the seacoast and in those regions well-provided with green fields. Effects of month, location and time of the day were all statistically significant (P<0.01). A total of 1,092 data were obtained for cows exposed for 1 to 8 h to sun during the day; in 7 months (February, March, April, July, August, September and November), 4 days per month on the average. A principal component analysis summarised the T-R, F-R, C, R, E-S and E-R measurements into just one synthetic variable (y(1)); several indexes were then obtained by multiple regression of y(1) on the four environmental variables and its combinations, by using Origin 8.1 software (OriginLab Corp.). The chosen equation was the index of thermal stress for cows, ITSC=77.1747+ 4.8327 T-A-34.8189 U+1.111 U-2+118.6981 P-V-14.7956 P-V(2)-0.1059 ERHL with r(2)=0.812. The correlations of ITSC with T-R, F-R, C, E-S, R and E-R were 0.275, 0.255, -0.493, -0.647, -0.818 and 0.075, respectively. Correlations of the index with the physiological variables are presented, and ITSC is compared to three other indexes.
