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Objectives: The precise measurement of climate conditions that cows are exposed to inside a dairy barn is crucial, because already small variations in THI can negatively influence dairy cows. Besides the housing conditions also the social hierarchy affects the actual experienced heat stress by individual animals because cows actively seek comfortable climate conditions. The objectives of this study were to examine heat stress conditions dynamically at cow level, to investigate the relationship to the climate conditions at stationary locations inside a dairy barn and to compare the climate conditions at cow level between primiparous and multiparous cows.
Materials and Methods: The study was conducted on a commercial dairy farm in Sachsen-Anhalt, Germany from May 2014 to July 2014. The herd consisted of 1,200 Holstein dairy cows with an average milk production of 10,147 kg. The barn was positioned in a NE-SW orientation with open ventilation and a mechanical fan-system. Ambient temperature and relative humidity were recorded using EL-USB-2+ data loggers (Lascar electronics, Salisbury, UK) and used to calculate the THI according to the equation reported by the NRC (1971): THI = (1.8 x AT + 32) - ((0.55 - 0.0055 x RH) x (1.8 x AT - 26)). Days of heat stress were defined as days with a mean THI ≥ 72. Stationary climate conditions within the barn were recorded on 2 locations within the milking parlor and on 3 locations within the experimental pen. Climate loggers within the milking parlor were located in the holding area and the rotary parlor. Climate loggers within the experimental pen were secured on an alley position, on a central position, and on a window position located nearby a ventilation opening in the outer wall. Climate conditions at cow level were recorded with climate loggers attached to the collar of the cows within an isolated rubber tube. These climate data were recorded on cohorts of 6 to 10 primiparous and multiparous cows, respectively in 7 replicates for one week each. Multiparous cows were housed inside the experimental pen for a minimum of 3 weeks before each replicate and primiparous cows were transferred to the experimental pen at the first day of each replicate to enforce a subordinate rank in the social hierarchy.
Results: Sixtyone primiparous and 62 multiparous cows were enrolled in the study. A total of 34,657 time values (2 minute intervals) from 52 experimental days was collected. The AT and THI differed significantly between all stationary loggers. The lowest AT and THI was measured at the window logger in the experimental pen and the highest AT and THI was measured at the central logger in the experimental pen. The RH measured at the pen loggers was 4.53% (P < 0.05) lower than measured at the milking parlor loggers. The highest RH was measured at the holding area logger and the lowest RH was measured at the window logger in the experimental pen. The AT measured at the mobile cow loggers was 1.56 °C (P < 0.05) higher than measured at the stationary loggers. The THI at the mobile cow loggers was 2.33 THI points (P < 0.05) higher than measured at the stationary loggers. Furthermore, the mean daily THI was higher at the mobile cow loggers than at the stationary loggers on all experimental days. Number of days averaging THI ≥ 72 were 12.86 percentage points higher measured at mobile cow loggers than at stationary loggers (n = 427, P < 0.05). There was no significant difference for the AT, RH, and THI between primiparous and multiparous cows. The THI at the pen loggers was 0.44 THI points (P < 0.05) lower when the experimental cow group was located inside the milking parlor. The THI measured at the mobile cow loggers was 1.63 THI points (P < 0.05) higher when the experimental cow group was located inside the milking parlor.
Conclusions: Our results indicate, that the actual heat stress experienced of dairy cows differs significantly from the heat stress conditions measured at stationary locations inside the barn and heat and humidity are not eliminated effectively from the cows immediate surrounding. Thus, a wide range of microclimates between different locations as well as between individual cows exists inside a dairy barn. Heat stress is underestimated when climate conditions are obtained from one stationary location inside the barn. For the determination of specific THI thresholds, the climate conditions should be obtained in the immediate surrounding of the cows.