Elevated vaginal and reticular temperature, and heart rate indicate early heat stress in dairy cows

Summary

Even in moderate climate zones, as Switzerland, increasing frequencies of heat periods or extreme weather events are recorded nowadays. Particularly dairy cows on pasture are directly exposed to climate conditions, what emphasizes the importance of a reliable to detection of signs of heat stress signs reliably. The objective of the present study was therefore to investigate the suitability of physiological parameters as early indicators of heat stress in grazing dairy cows in Switzerland. Therefore, the vaginal (VT) and the reticular (RT) temperatures of 24 lactating dairy cows were recorded during two summer periods (2018 and 2019). Additionally, the heart rate (HR) for each cow was measured between 9 am and 2.30 pm. On pasture the ambient climatic conditions were recorded continuously to calculate the temperature-humidity-index (THI) as well as the comprehensive-climate-index (CCI). For the analysis, 12 periods of up to 6 consecutive days with increasing THI and
CCI were selected to display conditions of increasing heat stress. The mean THI amounted 63±5.7 and ranged from 44.8 to a maximum of 77.8. In total, VT was on average 38.5±0.3 °C and RT 39.0±0.9 °C. The measured HR averaged 74.6±13.7 bpm and ranged from 50 to 180 bpm. Both, the body temperatures as well as the HR were affected by climatic conditions and increased with rising ambient THI (P<0.001). The RT showed higher variations than VT, which might be caused by increasing water intake during heat load. In conclusion, this study showed that dairy cows even in moderate climate zones experience heat load and the measurement of physiological parameters could indicate early signs of heat stress.