The main goal of this study was to evaluate the health condition of dairy cows in relationship to milk production and milk composition using continuous monitoring boluses. In total, seven Holstein cows had boluses implemented for monitoring rumen pH and temperature every 15 min with an accuracy of ± 0.1 pH/ °C. Milk production test day records were noted by Breeding Services of Slovakia, s.e. (state-owned enterprise) 5 × per each cow with a bolus over 27 weeks of lactation. Dairy cows were

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... ed into three groups according to their mean daily pH. After that, the test day records with the selected group were paired. Only two cows had pH values within the normal pH intervals during the control days. Consequently, there was found to be a 6.8% (P < 0.05) decrease in daily milk production in cows with lowered pH compared to cows with normal pH. Furthermore, there was found to be a 14.08% (P < 0.05) decrease in daily milk production in cows with an increased pH compared to cows with normal pH. Narrower fat to protein ratio and lactose content was found in cows with decreased and increased ruminal pH. The lowest milk fat concentration (P < 0.05) but the highest somatic cell count and urea content were determined in cows with decreased pH. These results show that continuous monitoring of rumen environment is a suitable method for nutrition and health management in dairy herds and allows the nutritionist to make nutritional interventions for pH stabilization within normal range in order to keep good milk production and high milk quality.