Abstract The objective of this study was to investigate the difference of ruminal cellulytic bacteria quantities in lactating dairy cows under heat stress using real-time PCR. Forty eight cows were assigned to mid-stage or late-stage, high producing or medium producing, primiparous or multiparous groups with a factorial design. Rumen fluid was sampled 5h in the morning postfeeding via oral cavity with stomach-tube when the THI of barn was continuously high than 72 for one week. The real-time PCR was employed with the specific primers targeting 16s rDNA genes of the fermentation bacteria to test the abundance of six cellulytic bacteria. The quantities of Ruminococcus flavefaciens, Prevotella ruminicola and Genus-level ruminicola was 13.9%, 5.3% and 3.7% higher for late-stage than mid-stage cow, respectively. The quantities of Prevotella ruminicola and Ruminococcus flavefaciens was 14.4% and 12.6% higher for high producing cow than medium producing cow, respectively. The quantities of Ruminococcus flavefaciens, Prevotella ruminicola, Fibrobacter succinogene and Ruminococcus albus was 12.6%, 3.2%, 2.4% and 2.1% higher for multiparuos than primiparous cow, respectively. In summary, there is a variance for Ruminococcus flavefaciens and Prevotella ruminicola in lactating dairy cows under heat stress.
