Temperature and Humidity Index (THI) during the 2021/2022 summer and its impact on dairy cattle in Rio Grande do Sul, Brazil

Abstract

This study aimed to calculate the temperature and humidity indexes (THIs) derived from extremely high air temperatures during the months of December 2021, January, and February 2022 in Rio Grande do Sul. The main goal was to characterize and regionalize potential impacts on dairy production. We used hourly measurements of temperature and relative humidity from 28 meteorological stations of the INMET/SIMAGRO/SEAPI network. These data cover ten ecoclimatic regions of the state. THI and Estimated Milk Loss (EML) were calculated, with the daily number of hours classified under each THI category during the trimester by municipality and region. The effects of region and month on THI and EML were evaluated through analysis of variance at a 5% significance level. Differences between means were compared using the Tukey HSD test (P<0.05). The probability of each daily hour having a THI in thermal discomfort (THI>70) was determined through analysis of variance for binomial variables by region, with the observed effect of time (P<0.05), and means were compared using the non-parametric Bonferroni test at 5%. To group these data, we applied the ScottKnott test. The Baixo Vale do Uruguai stood out in the trimester with the highest THI values, indicating thermal discomfort. Conversely, no heat stress was indicated in the Serra do Nordeste. In all regions, the most frequent daily classification of thermal discomfort ranged from attention to alert, with January being particularly significant. The highest EML estimates for most of the eight production levels were recorded in January. High estimates of productivity loss occurred in cows with higher milk production potential.