Use of the milk composition stress-comfort ratio to monitor heat stress in cows in tropical regions

Abstract

This study aimed to use the daily maximum temperature and humidity index (THImax) to calculate the ratio between average milk composition in the three hottest (stress) and three mildest months (comfort) of the year in an important milk-producing region in Brazil, with potential practical application in other tropical regions. The study was conducted in the municipalities of Araxá, Sacramento, and Uberaba in Minas Gerais State, from 2017 to 2020. THImax was calculated using daily maximum temperature and minimum relative humidity data. The three months with the largest number of days with THImax ≥ 76 were classified as the stress period, and the three months with THImax < 76 as the comfort period. Average fat, protein, and lactose content, total solids, and non-fat solids were calculated for both periods using 10,063 milk composition analyses, and the stress-to-comfort (S:C) ratio of the milk components was determined. Fat content declined during the stress period, with an S:C ratio below 1.0. Most ratios were also below 1.0 for protein and total solids, while values for lactose and non-fat solids showed minimal deviation from 1.0. THImax values ≥76 significantly alter the milk composition of crossbred dairy cows in tropical regions, largely by decreasing the fat content, resulting in an S:C ratio below 1. Determining the milk composition ratio between the hottest and mildest months enables the identification of thermal discomfort in crossbred cows in tropical regions and provides a practical tool for dairy producers to monitor heat stress.