Abstract
This study determined the effects of increasing ambient temperature (T) at different relative humidity (RH) and air velocity (AV) levels on the physiological and productive responses of dairy cows. Twenty Holstein dairy cows were housed inside climate-controlled respiration chambers, in which the climate was programmed to follow a daily pattern of lower night and higher day T with a 9°C difference, excluding effects from sun radiation. Within our 8-d data collection period, T was gradually increased from 7 to 21°C during the night (12 h) and 16 to 30°C during the day (12 h), with an incremental change of 2°C per day for both nighttime and daytime T. During each research period, RH and AV were kept constant at 1 of 5 treatment levels. A diurnal pattern for RH was created, with lower levels during the day and higher levels during the night: low (RH_l: 30-50%), medium (RH_m: 45-70%), and high (RH_h: 60-90%). The effects of AV were studied during the day at 3 levels: no fan (AV_l: 0.1 m/s), fan at medium speed (AV_m: 1.0 m/s), and fan at high speed (AV_h: 1.5 m/s). Effects of short and long exposure time to increasing T were evaluated by collecting data 2 times a day: in the morning [short: 1 h (or less) - exposure time] and afternoon (long: 8 h - exposure time). The animals had free access to feed and water and both were ad libitum. Respiration rate (RR), rectal temperature (RT), skin temperature (ST), dry matter intake, water intake, milk yield, and composition were measured. The inflection point temperatures (IPt) at which a certain variable started to change were determined for the different RH and AV levels and different exposure times. Results showed that IPt under long exposure time for RR (first indicator) varied between 18.9 and 25.5°C but was between 20.1 and 25.9°C for RT (a delayed indicator). The IPt for both RR and RT decreased with higher RH levels, whereas IPt increased with higher AV for RR but gave a minor change for RT. The ST was positively correlated with ambient T and ST was not affected by RH but significantly affected by AV. For RR, all IPt was lower under long exposure time than under short exposure time. The combination of higher RH levels and low AV level negatively affected dry matter intake. Water intake increased under all treatments except RH_l-AV_l. Treatment RH_h-AV_l negatively affected milk protein and fat yield, whereas treatments RH_m-AV_m and RH_m-AV_h reduced milk fat yield. We concluded that RH and AV significantly affected the responses of RR, RT, ST, and productive performance of high-producing Holstein cows. These responses already occurred at moderate ambient T of 19 to 26°C.
Highlights
Once thought to be limited totropical areas, the effects of high ambient temperatures on dairy cows have become relevant in temperate climate areas due to rising global temperatures (T; Polsky and von Keyserlingk, 2017; Pinto et al, 2020)
The intensive genetic selection of milk production has resulted in dairy cows that are more susceptible to heat stress than they were in the past (Ravagnolo et al, 2000), rendering the problem of heat stress during the summer increasingly prominent, along with all the accompanying negative effects on dairy cows’ health (Kadzere et al, 2002; de Andrade Ferrazza et al, 2017), production (Hill and Wall, 2015), and reproduction (García-Ispierto et al, 2007; Schüller et al, 2014) as well as increased mortality risk (Vitali et al, 2009)
A combination of higher relative humidity (RH) levels with higher air velocity (AV) levels could be helpful for ascertaining the significant effect of AV, because at high RH, increasing AV could help to raise the potential evaporation rate and further increase cutaneous latent heat dissipation
Summary
Once thought to be limited to (sub)tropical areas, the effects of high ambient temperatures on dairy cows have become relevant in temperate climate areas due to rising global temperatures (T; Polsky and von Keyserlingk, 2017; Pinto et al, 2020). Dairy cows are sensitive to high ambient T (Kadzere et al, 2002). In the thermal comfort zone, cows can balance heat loss with heat production independent from ambient T (Mount, 1979). When ambient T rises above this (thermal comfort) zone, the cows must recruit extra physiological responses to get rid of the produced heat.
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