[Consequences of high ambient temperatures for lactating dairy cows - an issue under practical conditions in Northern...

The aim of the presented study was to gather quantitative data regarding the impact of high ambient temperatures on dairy cows under practical conditions in southern Lower Saxony.

In summer 2018 (from mid of July to end of September), the climatic conditions in the dairy barn as well as the temperatures of feeds (silages and partial mixed ration, PMR) and feed leftovers were recorded on the Farm for Education and Research of the University of Veterinary Medicine Hannover in Ruthe. Furthermore, daily dry matter intake (of PMR) and milk yield on herd basis as well as respiration rate and rectal temperature of 10 selected cows were measured as indicators for heat stress.

During a heat period (daily average temperature up to 27.9° C) dry matter intake (PMR) and milk yield were reduced by about 15 % and 8 %, respectively although ambient temperatures had already been relatively high at the beginning of the observation period (daily average temperature between 19.6 and 23.0° C). Daily dry matter intake of PMR was NEGATIVELY: correlated with daily average temperature-humidity index in the barn (Spearman's correlation coefficient: -0.774, p < 0.0001) and POSITIVELY: correlated with daily milk yield (Spearman's correlation coefficient: 0.766, p < 0.0001). Correlation coefficients were slightly higher when a 1-day time lag was considered (Spearman's correlation coefficient: -0.796 and 0.800, respectively; p < 0.0001). During the heat period hyperthermia (> 39° C) was frequently recorded in lactating cows (up to 100 %).

The presented results indicate that heat stress in dairy cows may occur to a considerable extent under extreme climatic conditions also in the region of Northern Germany. In consequence, this is associated with a significant decrease in performance. As severe and long-lasting effects of heat stress on health and performance of dairy cows are well described in the literature, especially the temporarily high prevalence of hyperthermia should be taken seriously. Under these circumstances, differentiating between hyperthermia and fever is of special importance for veterinary practitioners.