The Temperature Humidity Index (THI) is the most used to identify the risk of heat stress for dairy cattle, which is based on the environmental temperature (ºC) and humidity (%), and of which they are currently considered 68 points as the threshold for high producing dairy cows. In the Comarca Lagunera, located in north central Mexico, 21% of the Mexican dairy production is generated annually, despite its environmental conditions characterized by temperatures up to 41.5 ° C in the summer, low relative humidity and high solar radiation. The objective was to identify the average number of days in which a THI ≥68 is reached per month in each season of the year with 1, 8 and 16 h of exposure in dairy farms of the region, for which, the information recorded every 10 min was analyzed daily in five farms for 6 years (2015–2020), using the DiGiTH ™ application (DiGiTH Technologies, Mexico) and through ANOVA (proc GLM; SAS Inst. Inc., Cary, NC, USA), to identify differences seasonal for this variable. The attached table shows that spring and summer were the seasons in which the highest number of average days per month were recorded, and winter in which the fewest days were recorded. Regarding the hours of exposure, it is observed that spring and summer are very similar in 1 and 8 hours of exposure, while for autumn and winter a clearer decrease is observed according to the hours of exposure for these seasons. These results indicate a very marked seasonal variability and in terms of the hours of exposure, which should be considered in order to adopt the measures to reduce the negative effects for the dairy cattle of the region.

The Lagunera region, located in north central Mexico (24° 01′-26° 48 ‘LN and 101° 52′-101° 52′ 104° 40′ LO), produce the 21% of the national Mexican cow’s milk despite its climatic conditions (temperatures that fluctuate between 12.7 °C in January and 28.5 °C in June, with extremes of -5 °C and 41.5 °C, in addition to high solar radiation). The Temperature Humidity Index (THI) is the most useful indicator to define if there are heat stress conditions. To identify whether the days in which THI ≥68 (considering the threshold for signs of heat stress to appear in high-producing lactating cows) are increasingly reached at least one hour a day, the information of this indicator was recorded every 10 min daily, from 5 cow farms for 6 years (2015–2020), using the DiGiTH™ application (DiGiTH Technologies, Mexico), and regressions were run to identify if there is a relationship between the years of study and the THI levels. At any level of THI (≥68), there is a tendency to increase the number of days (10.05 d per year; R2 0.37); Disaggregating the data by THI level, this trend is also observed for levels 68 to 71 and 72 to 76 THI, registering an increase of 11.33 d (R2 0.45) and 9.03 d per year (R2 0.45) respectively. At the 77 - 79 THI level there is no trend, while at the ≥80 THI level, a decrease of 5.66 d per year was registered (R2 0.79). The results suggest that in this region, the days of heat stress are increasing, except for the level ≥80 THI, which indicates that palliative measures for heat stress should be intensified, and it is necessary to identify the reason for the decrease in heat stress days at the most dangerous level for livestock.