The aim of the experiment was to confirm the hypothesis that the rumen function can improve the plastic brushes (RF). For continuous (every 15 minutes) rumen pH measurement, eCOW and WQL boluses were used in 8 cows. An experiment with 28 cows lasted for 4 months, during which time 4 feed doses (TMR) with different levels of physically efficient fiber (peNDF) were tested. Each of the 14 cows in the experimental group had 3 RF in the second period with TMR1 orally administered to the rumen. In the experimental group of dairy cows, the pH was 0.2 higher than in the control group. Group differences were statistically significant in the amount of feed consumed and milk produced in dairy cows only in the second trial with a feed dose with peNDF 13.1. The other monitored indicators were not significant among the groups.