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The objective of this research was to determine the differences in nitrogen related compounds (daily urea content in milk, milk urea nitrogen and ammonia emission) in milk samples of Holstein first parity cows reared in Eastern Croatia depending on season of milk recording. Test-day records of first parity Holstein cows reared in Eastern Croatia were used for the statistical analysis. The results of analyses of variance of daily urea content in milk, milk urea nitrogen and ammonia emission of Holstein first parity cows regarding the season (spring, summer, autumn and winter) are presented in the paper. All analysed traits differed statistically highly significant (p < 0.001) regarding the seasons. The highest content of daily milk urea was recorded in summer (27.195 mg/dl), also, milk produced in summer had the highest MUN (mg/dl) as well as the highest ammonia emission (g/cow daily). The lowest content of daily milk urea (20.647 mg/dl) was determined in winter, along with the lowest milk urea nitrogen (9.498 mg/dl) and ammonia emission (72.773 g/cow daily). The research results indicate that the variability of daily milk urea content, milk urea nitrogen and consequently ammonia emission in Holstein first parity cows depends of daily milk production, stage of lactation, age at first calving, and season of milk recording. Therefore, those effects should be taken into account in the statistical model for estimation of the ammonia emission from dairy cattle farms based on milk recording data. |
