ABSTRACT The objective with this study was to estimate the voluntary feed intake (VFI) of sows during the occurrence of heat waves in the Triângulo Mineiro and Alto Paranaíba mesoregion, Minas Gerais, Brazil. The Instituto Nacional de Meteorologia (INMET) database was used to calculate the temperature-humidity index (THI) from 2008 to 2017 for Araxá, Frutal, Ituiutaba, Patos de Minas, Patrocínio, Uberaba, and Uberlândia, and the estimated VFI with and without effective occurrence of heat waves (EOH). An economic estimation of the revenue based on the VFI was performed. The data were analyzed by Friedman's non-parametric test (two-way non parametric analysis), using the significance level of 5%. The sows raised in Patrocínio showed a higher average prediction of VFI (6.611 kg without EOH and 5.743 kg with EOH) than Ituiutaba (6.424 kg without EOH and 3.969 kg with EOH) and Frutal (5.868 kg without EOH and 4.648 kg with EOH). In the classification “without EOH”, the microregions of Araxá (6.502 kg), Uberaba (6.452 kg), and Uberlândia (6.478 kg) did not differ from the other cities, but “with EOH”, Araxá (4.864 kg) differed from Patrocínio (5.743 kg). The reduction of VFI due to thermal stress represented a revenue reduction of R$ 74.60 per litter. The averages for predicted VFI were lower in the EOH category than in the probable occurrences of heat waves category for all cities. The predominance of heat waves reduces the VFI of sows, particularly in Ituiutaba and Frutal.

RESUMO: Objetivou-se determinar dentre nove equações do ITU a que melhor representa os efeitos do estresse por calor para bezerros leiteiros mestiços criados em ambiente tropical. Foram avaliados 12 bezerros, machos e fêmeas, com idade de 20 a 60 dias, criados no bezerreiro tropical. As frequências respiratória (FR) e cardíaca (FC), as temperaturas retal (TR), corporal superficial (TCS), de bulbo seco (Tbs) e de bulbo úmido (Tbu) foram quantificadas pela manhã e tarde. Calcularam-se nove equações do ITU. Utilizou-se como critério de determinação da melhor equação do ITU, a maior correlação entre as variáveis fisiológicas com os ITUs. As médias das nove equações de ITU, Tbs, Tbu, Pv, Tpo, FR, FC, TR e TCS foram superiores pela tarde em comparação ao turno da manhã, enquanto que a UR teve comportamento inverso. Os maiores valores de temperatura retal foram observados em temperaturas acima de 26,4 °C e umidade inferior a 55,5%. As correlações entre Tbs e Pv com a FR (0,697 e 0,707), TR (0,703 e 0,706) e TCS (0,818 e 0,817) foram significativas e positivas, enquanto que as correlações da UR, com essas mesmas variáveis fisiológicas, foram significativas e negativas (-0,622, -0,590 e -0,638, respectivamente). O melhor índice foi o ITU calor sensível ( I T U = 3,43 + 1,058 x T d b - 0,293 x R H + 0,0164 x T d b x R H + 35,7), que se correlacionou significativamente com a frequência respiratória (r=0,668 e r²=0,446), frequência cardíaca (r=0,259 e r²=0,067), temperatura retal (r=0,693 e r²=0,479) e temperatura corporal superficial (r=0,807 e r²=0,650). Conclui-se que a equação do ITU calor sensível é a que melhor representa os efeitos do estresse por calor dos bezerros leiteiros mestiços em ambiente tropical.

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.