This experiment was conducted to investigate the effects of heat stress on microbial number in rumen fluid of dairy cows during different lactating periods by real-time fluorescence quantitative PCR. A 2?3 crossover design was used and eighteen healthy Chinese Holstein cows were selected. The cows were allocated into three groups based on their parity, average days in milk and milk yield: early lactation (postpartum 15 to 100 days) group, mid-lactation (postpartum 112 to 175 days) group and late lactation (postpartum 200 to 230 days) group, and six cows were in each group. A self-control trial between heat stress period and non-heat stress period was used based on temperature and humidity index (THI) measurement on site. The results showed as follows: compared with non-heat stress period, heat stress significantly increased the microbial number in early lactation group, the number of Ruminococcus flavefaciens in late lactation group and the number of Bacteroides succinogenes in mid-lactation group (P<0.01). The number of total bacteria and Ruminococcus albus in mid- and late lactation groups and the number of Bacteroides succinogenes in late lactation group were significantly decreased by heat stress (P<0.01). There were strong correlations between THI and the number of total bacteria (P<0.01), anaerobic fungi (P<0.05), Ruminococcus albus (P<0.01)and Ruminococcus flavefaciens (P<0.05) in rumen fluid of dairy cows in early lactation group, but the correlations between THI and these indexes in mid- and late lactation groups were poor. Conclusions:1)heat stress significantly increases the microbial number in early lactation group compared with non-heat stress period, but there are no regularity effects of heat stress on microbial number in mid- and late lactation groups; 2) it is indicated that the rumen microbial number in early lactation group is more sensitive to heat stress.
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