INFLUENCE OF ENVIRONMENTAL AND GENETICAL FACTORS ON UREA CONTENT IN HOLSTEIN BREED COWS MILK

The aims of this paper were to determine to which extent the environmental factors (order and stadium of lactation, age with the first calving, calving season, region and herd) have the influence on the content of urea in milk, and connection of urea content in milk with other features of milk production with cows. The largest share of urea content in milk was recorded in the first lactation, in the period between 110 and 140 days, when it was around 23.6 mg/100 ml. In the end of the first lactation the average urea content in milk was around 21.6 mg/100 ml. The second lactation is chara¬cterized by somewhat bigger urea content, in the period immediately after calving when the highest values from all tracked lactations was recorded. The age of cows with their first calving also had an impact on urea con¬tent in milk. Cows that calved in the age from 24th to 26th month had the highest value of urea content, being 23.2 mg/100 ml for the stated period. The lowest value of urea content was recorded with cows that calved in the age of 18 months and it was below 20 mg/100 ml. Season of calving also influenced the urea content in milk. In winter season 2004 the lowest values of urea content in milk were recorded while in the autumn sea¬son of the same year the highest urea content in milk was measured (24 mg/100 ml). In the following calving season increase of urea content in milk followed. The differences in urea content in milk were determined between the counties. The highest value of heritabi-lity (0.08) was estimated by the model where, as a comparison group, the interaction between the herds and control day was used. In the research the share of variability was explained by the interaction herd-control day and it was 67%, while 25% of variability of urea con¬tent in milk remained unexplained. This model was used when estimating the breeding values. A model was also tested where the influence of herd was used as a comparison group, and by this model the lowest value of heritability was estimated (0.03). In models where interaction between the herd and years of testing was used as comparison group, higher values of heritability (0.04 and 0.05) were measured with respect to the previous model. Calculated phenotypical correlations between the urea content and milk production features: daily amount of milk, daily amount and content of fats and proteins were statistically significant (P <0.0001). Positive and low phenotypical correlation (0.15) was determined between the content of urea and daily amo¬unt of milk, between the urea content and daily amount of fat (0.10), and between urea content and amount of proteins (0.16). The coefficient of correlations was low and negative (-0.05) between the urea content and content of milk fat, while between the urea content and content of proteins it was low and positive (-0.03).