Abstract
The aim of this study was to use milk urea to evaluate production parameters, efficiency of nitrogen utilization, and urinary nitrogen excretion during lactation in Holstein dairy cows. Cows in the 1st lactation phase compared to the 2nd and 3rd lactation phases showed: increase (P < 0.05) in daily milk yield; decrease (P < 0.05) in fat and protein content in milk; decrease in milk urea content (P < 0.05); with a lower individual number of animals (31% of dairy cows in the 1st stage of lactation, 39 % of dairy cows in the 2nd stage of lactation, and 44 % of dairy cows in the 3rd phase of lactation) with an above-limit milk urea content (35.0 ± 4.7; 34.9 ± 4.1, 34.8 ± 4.1 mg.dl-1); decreased urinary nitrogen excretion (P < 0.05); and efficiency of nitrogen utilization from feed to milk (P < 0.05). According to the phase of lactation, the efficiency of nitrogen utilization decreased and urinary nitrogen excretion increased with increasing milk urea content.