Abstract
ABSTRACT: The aim of this study was to determine among nine temperature-humidity index (THI) equations, the one that best represents the effects of heat stress on crossbred dairy calves reared in a tropical environment. Twelve male and female calves, aged 20 to 60 days, and raised in a tropical pen were evaluated. Respiratory (RR) and heart rates (HR), rectal (RT), body surface (BST), dry bulb (Tdb) and wet bulb (Tbw) temperatures, partial vapor pressure (Pv), relative humidity (RH) and dew point temperature (Tpo) were quantified in the morning and afternoon. Nine THI equations were calculated. The highest correlation between physiological variables and this was used to select the best THI equation. Averages for nine THI equations, Tdb, Twb, Pv, Tdp, RR, HR, RT, and BST were higher in the afternoon than in the morning, whereas that for RH was the opposite. The highest values for RT occurred at temperatures above 26.4°C and when humidity was below 55.5%. The Tdb and Pv correlations with RR (0.697 and 0.707), RT (0.703 and 0.706) and BST (0.818 and 0.817) were significant and positive, whereas the RH correlations with the same physiological variables were significant and negative (-0.622, -0.590 and -0.638, respectively). The best index was the THI sensible heat-based ( T H I = 3.43 + 1.058 x T d b - 0.293 x R H + 0.0164 x T d b x R H + 35.7), which was significantly correlated with RR (r=0.668 and r²=0.446), HR (r=0.259 and r²=0.067), RT (r=0.693 and r²=0.479) and BST (r=0.807 and r²=0.650). In conclusion, the THI sensible heat-based equation best represents the effects of heat stress on crossbred dairy calves reared in a tropical environment.
Highlights
The most commonly used thermal stress index is the temperature-humidity index (THI), which evaluates the combined effects of air temperature and humidity
The main advantage of calculating a THI is that the data needed can be obtained on the farm or from a nearby meteorological station, whereas the thermal radiation data received from the animal and the wind speed are more difficult to record because they depend on specific equipment and the data needed are often not publicly available (BOHMANOVA et al, 2007)
The THI has previously been investigated as an indicator of milk production losses from lactating cows (BOHMANOVA et al, 2007) or as a way of assessing sperm quality damage to breeding cattle caused by heat stress (MENEGASSI et al, 2016)
Summary
The most commonly used thermal stress index is the temperature-humidity index (THI), which evaluates the combined effects of air temperature and humidity. It was developed by THOM (1959). Different equations were proposed that could be used to evaluate production animals (NRC, 1971; YOUSEF, 1985; MADER et al, 2006; BERMAN et al, 2016). The different THI equations can take into account dry and wet bulb temperatures (Tdb and Twb), relative. The THI has previously been investigated as an indicator of milk production losses from lactating cows (BOHMANOVA et al, 2007) or as a way of assessing sperm quality damage to breeding cattle caused by heat stress (MENEGASSI et al, 2016). The efficiency of this index is still not clear for dairy crossbred calves, because most of the previous research was conducted using young purebred or adult animals (BROUCEK et al, 2009; BERNABUCCI et al, 2014; MENEGASSI et al, 2016 ), which means that it is difficult to find results for young and crossbred cattle
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