Rodrigo de Andrade Ferrazza, Henry David Mogollón Garcia, Viviana Helena Vallejo Aristizábala, Camilla de Souza Nogueira, Cecília José Veríssimo, José Roberto Sartori, Roberto Sartori, João Carlos Pinheiro Ferreira
stress (HS) adversely influences productivity and welfare of dairy
cattle. We hypothesized that the thermoregulatory mechanisms vary
depending on the exposure time to HS, with a cumulative effect on the
adaptive responses and thermal strain of the cow. To identify the effect
of HS on adaptive thermoregulatory mechanisms and predictors of caloric
balance, Holstein cows were housed in climate chambers and randomly
distributed into thermoneutral (TN; n=12) or HS (n=12) treatments for 16
days. Vaginal temperature (VT), rectal temperature (Tre), respiratory
rate (RR), heart rate (HR) and dry matter intake (DMI) were measured.
The temperature and humidity under TN were 25.9±0.2 °C and 73.0±0.8%,
respectively, and under HS were 36.3±0.3 °C and 60.9±0.9%, respectively.
The RR of the HS cows increased immediately after exposure to heat and
was higher (76.02±1.70bpm, p<0.001) than in the TN (39.70±0.71bpm).
An increase in Tre (39.87±0.07 °C in the HS vs. 38.56±0.03 °C in the TN,
p<0.001) and in VT (39.82±0.10 °C in the HS vs. 38.26±0.03 °C in the
TN, p<0.001) followed the increase in RR. A decrease (p<0.05) in
HR occurred in the HS (62.13±0.99bpm) compared with the TN
(66.23±0.79bpm); however, the magnitude of the differences was not the
same over time. The DMI was lower in HS cows from the third day
(8.27±0.33 kg d−1 in the HS vs. 14.03±0.29 kg d−1
in the TN, p<0.001), and the reduction of DMI was strongly affected
(r=−0.65) by changes in the temperature humidity index. The effect of
environmental variables from the previous day on physiological
parameters and DMI was more important than the immediate effect, and
ambient temperature represented the most determinant factor for heat
exchange. The difference in the responses to acute and chronic exposure
to HS suggests an adaptive response. Thus, intense thermal stress
strongly influence thermoregulatory mechanisms and the acclimation
process depend critically on heat exposure time.
Keyword: Hyperthermia; Thermoregulation; Thermal index; Acclimation; Climate chamber; Bos taurus.
Fonte: Journal of Thermal Biology, v. 2017
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