EN
The objectives of this study were to describe the influence of lactation month, production season, age of cows, production level, milk protein content and udder health status on black-and-white cow milk urea concentrations. Dairy herd improvement of 61928 test-day data were collected over the period of 2000-2003 from 304 south Podlasie dairy herds. The average milk urea concentrations (mg/l), protein content (%) and milk day yield (kg) were as follows: 199 mg/l, 3.42% protein content and 18.4 kg milk day yield while 50% of the total analyzed tested milk had optimal milk urea concentrations from the nutritional point of view (150-300 mg/l). The lactation month, production season, age of cows, production level, protein content groups and udder health status effects were high and statistically significant in every factor at P ≤ 0.05. The highest level of milk urea concentrations were distinguished in the summer season of the experiment (VI-VIII), for cows with the lowest milk protein content (< 3.2%), and lowest somatic cell counts (< 25 thous./ml), and for milk produced at fourth month of lactations. The average urea milk concentrations for those factors were respectively: 225, 213, 217 and 214 mg/1 l.