Heat stress in dairy cows : Measuring and modelling the effects of environmental conditions on thermoregulatory responses

Abstract

This study determined the effects of increasing the 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 temperatures 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 changes of 2°C per day for both nighttime and daytime temperatures. During each research period, RH and AV were kept constant at one of five 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 three levels: no fan (AV_l: 0.1m/s), fan at medium speed (AV_m: 1.0m/s), and fan at high speed (AV_h:1.5m/s). Effects of short and long exposure time to increasing T were evaluated by collecting data two 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 lib.Respiration rate (RR), rectal temperature (RT), skin temperature (ST), DMI, 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, while 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 levels negatively affected DMI. Water intake increased under all treatments except RH_l*AV_l. Treatment (RH_h*AV_l) negatively affected milk protein and fat yield while treatments (RH_m*AV_m and RH_m*AV_h) reduced milk fat yield. We concluded that RH and AV levels affected significantly the responses of RR, RT, ST and productive performance of high-producing Holstein cows. These responses already occurred at moderate ambient temperatures of 19 to 26°C.