The objective of the study was to determine the relationships between milk urea concentration and
factors such as lactation number, stage of lactation, month and season of the test day, age at calving, milk
yield and protein percentage. Data for the calculations consisted of 7,731 test-day records from 1,078
Polish Holstein-Friesian cows. Test-day milking was performed for first, second and third lactations
during the period from December 2010 to December 2011. Calculations were performed using the
MIXED procedure in SAS/STAT. A mixed linear model using was applied in which parameters were
estimated by the restricted maximum likelihood (REML) method. Least squares means for fixed
eff ects in the model were compared by the Tukey-Kramer test. The first lactation diff ered significantly
(p<0.05) from the second and third in terms of mean urea concentration, but there were no significant
diff erences between the second and third lactations. For primiparous cows the milk urea concentration
increased throughout lactation, but for older cows it increased only up to 7–8 months of lactation.
Urea concentrations did not diff er significantly in the same stages of consecutive lactations, i.e. the
first and second or second and third. Statistically significant diff erences were noted between the first
and third lactations only in months 9 and 10 of lactation. Seasonal changes in milk urea content varied
depending on the lactation number. In the first lactation the milk urea concentration was lowest in
spring and highest in autumn. This tendency was not observed in the second and third lactation. Milk
urea concentration was positively associated with both milk yield and protein percentage