The eff ect of selected factors on urea concentration in the milk of Polish Holstein-Friesian cows

The objective of the study was to determine the relationships between milk urea concentration andfactors such as lactation number, stage of lactation, month and season of the test day, age at calving, milkyield and protein percentage. Data for the calculations consisted of 7,731 test-day records from 1,078Polish Holstein-Friesian cows. Test-day milking was performed for first, second and third lactationsduring the period from December 2010 to December 2011. Calculations were performed using theMIXED procedure in SAS/STAT. A mixed linear model using was applied in which parameters wereestimated by the restricted maximum likelihood (REML) method. Least squares means for fixedeff 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 significantdiff erences between the second and third lactations. For primiparous cows the milk urea concentrationincreased 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. thefirst and second or second and third. Statistically significant diff erences were noted between the firstand third lactations only in months 9 and 10 of lactation. Seasonal changes in milk urea content varieddepending on the lactation number. In the first lactation the milk urea concentration was lowest inspring and highest in autumn. This tendency was not observed in the second and third lactation. Milkurea concentration was positively associated with both milk yield and protein percentage