Background:Heat stress (HS) affects the ruminal microbiota and decreases the lactation performance of dairy cows. Because HS decreases feed intake, the results of previous studies were confounded by the effect of HS on feed intake. This study examined the direct effect of HS on the ruminal microbiota using lactating Holstein cows that were pair-fed and housed in environmental chambers in a 2 × 2 crossover design. The cows were pair-fed the same amount of identical total mixed ration to eliminate the effect of feed or feed intake. The composition and structure of the microbiota of prokaryotes, fungi, and protozoa were analyzed using metataxonomics and compared between two thermal conditions:pair-fed thermoneutrality (PFTN, thermal humidity index:65.5) and HS (87.2 for daytime and 81.8 for nighttime). Results:The HS conditions altered the structure of the prokaryotic microbiota and the protozoal microbiota, but not the fungal microbiota. Heat stress significantly increased the relative abundance of Bacteroidetes (primarily Gram-negative bacteria) while decreasing that of Firmicutes (primarily Gram-positive bacteria) and the Firmicutes-to-Bacteroidetes ratio. Some genera were exclusively found in the heat-stressed cows and thermal control cows. Some co-occurrence and mutual exclusion between some genera were also found exclusively for each thermal condition. Heat stress did not significantly affect the overall functional features predicted using the 16S rRNA gene sequences and ITS1 sequences, but some enzyme-coding genes altered their relative abundance in response to HS. Conclusions:Overall, HS affected the prokaryotes, fungi, and protozoa of the ruminal microbiota in lactating Holstein cows to a different extent, but the effect on the structure of ruminal microbiota and functional profiles was limited when not confounded by the effect on feed intake. However, some genera and co-occurrence were exclusively found in the rumen of heat-stressed cows. These effects should be attributed to the direct effect of heat stress on the host metabolism, physiology, and behavior. Some of the"heat-stress resistant"microbes may be useful as potential probiotics for cows under heat stress.

Tansol Park;Lu Ma;Shengtao Gao;Dengpan Bu;Zhongtang Yu

Department of Animal Sciences,The Ohio State University,Columbus,OH,USA;Department of Animal Science and Technology,Chung-Ang University,Anseong-si,Gyeonggi-do,Republic of Korea;State Key Laboratory of Animal Nutrition,Institute of Animal Science,Chinese Academy of Agricultural Sciences,Beijing 100193,People's Republic of China;CAAS-ICRAF Joint Lab on Agroforestry and Sustainable Animal Husbandry,Beijing 100193,People's Republic of China

畜牧与生物技术杂志（英文版）

[1]Tansol Park;Lu Ma;Shengtao Gao;Dengpan Bu;Zhongtang Yu-.Heat stress impacts the multi-domain ruminal microbiota and some of the functional features independent of its effect on feed intake in lactating dairy cows)[J].畜牧与生物技术杂志（英文版）,2022(06):1902-1916

confounded,metataxonomics,thermoneutrality,PFTN,protozoal

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