Özet:

The N concentration of milk urea is one of several factors that allow prediction of ammonia emissions from dairy cattle manure. The aim of the study was to investigate the effect of selected environmental factors on urea content in milk in the standard and extended lactation period in Polish Holstein-Friesian cows of the Black-and-White variety. The study has analysed 30 839 full lactations, which were at least one day longer than the standard 305-day lactation, but not longer than 600 days. The highest percentage of milk samples with optimal protein and urea content was found in extended lactation periods (>305 days). The percentage of milk tested during this period, containing urea from 150 to 270 mg/l, was 49.26%. The calculations also indicated that a significant influence on the differences in the milk urea content between the average standard and extended lactation period had an average daily production of milk in the first 305-day lactation. The higher the efficiency of the cow, the greater the difference in the level of urea in milk in the comparable periods of lactation. On the basis of these data it was estimated that the milk of cows producing on average more than 30 kg of milk per day contained approximately 12 mg/l more of urea in the standard lactation period than the milk of cows from the same group acquired during the extension. Overall, the results demonstrate that the urea content is higher in the milk from the second phase of lactation, in the milk of youngest cows (first to fourth parity), and in the milk of high yielding cows. It was found that the amount of urea in milk during extended lactation period was lower than in the first 10 months after calving. Due to the fact that the longer lactations resulted in lower levels of urea in milk and lower ammonia emission in last weeks of lactation, it was concluded that the full extended lactation had no negative impact on the environment.

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