Influência do ambiente térmico sobre variáveis fisiológicas de termoregulação de novilhas leiteiras mestiças em recria a pasto.

Abstract

The effects of heat stress on high-yielding cows have been studied, but with little emphasis on the heifer category, which also has a damaging effect on environmental challenges, even mestizo animals. The objective of this study was to analyze physiological variables of thermoregulation of crossbred dairy heifers exposed to the sun for 4 hours, to correlate them with the Temperature and Humidity Index (THI) and to verify the possible impact of thermal environment conditions on the body mass gain of these animals, from November 2016 to February 2017. After exposure to the sun, heifers were taken to the trunk for quantification of respiratory rate, rectal temperature and body surface temperature. Simultaneously, dry bulb and wet bulb temperatures, wind speed and black globe temperature were measured. Subsequently, the THI was calculated. The animals were also weighed biweekly, after a 12-hour fast, during the experimental period in order to observe if there was difference in the average monthly gain of the heifer's body mass. This evaluation was made based on the minimum, average and maximum monthly data of ambient temperature and relative humidity, with subsequent THI calculation, from the Meteorological Station of the Experimental Farm Glória. The ambient temperature found was within the recommended standards for crossbred cattle (29.96 ° C). The mean relative humidity was also within the ideal range for cattle (50.51%), but the maximum and minimum values extrapolated these ideal limits. The rectal temperature and the mean respiratory rate of heifers presented the normal intervals (38.85 ° C) and 41.97 (mov.min-1), indicating an adaptation to the heat. There was no correlation between THI and rectal temperature, respiratory rate r: 0.265 and body surface temperature R: 0.603 black flank and r: 0.580 white flank, showed a significant and positive correlation with THI. The mean body mass gain was lower in January (14.19 kg), month of higher ambient temperature and higher THI. Thus, it was concluded that the rectal temperature and respiratory rate remain within the normal range, showing their adaptability to the warm environment. Respiratory rate and body surface temperature correlate with THI, and body mass gain may be compromised as a function of the uncomfortable thermal environment.