Variability of milk urea nitrogen traits and their potential use in dairy cattle

Milk urea nitrogen (MUN) is an important biomarker that reflects the efficiency of protein metabolism in cows, the level of consumption and quality of “input” protein, and the energy balance in the diet. MUN fluctuations are closely related to possible discrepancies in the total mixed ration (TMR), physiological and genetic factors. The aim of the study was to determine the influence of genetic (cow’s paternal origin) and paratype (calving year-month) factors on milk yield, quality and biochemical parameters of milk, in particular urea nitrogen concentration, in Holstein cows. The research was based on data from 595 cows kept at the Kolos Agricultural Firm LLC in the Kyiv region (Ukraine). The study was conducted under standard conditions with cows fed “without restrictions” and analysis of productivity indicators and biochemical indicators of milk quality. It was found that the factor “year-month of calving” has the greatest impact on productivity indicators and milk quality parameters, causing seasonal fluctuations associated with minor changes in feeding and microclimate. The influence of the sire (cow’s father) on milk yield per lactation was noted – 12.4%, milk fat content per lactation – 13.4%, milk protein content per lactation – 12.9%. The influence of non-genetic factors was quite high for daily milk yield, fat, protein and lactose content, as well as MUN. Even with total use of standard TMR, MUN values varied widely in cows from a minimum of 2.6 mg/dl to a maximum of 32.9 mg/dl, with average values of 12.31 ± 0.24 mg/dl, and depended on the cows’ ability to separate the diet, minor changes in the diet, and the level of protein and non-structural carbohydrates. A tendency was noted whereby at high MUN values (>16 mg/dl) the absolute level of such important milk components as fat, protein and lactose content decreases, which reduces the energy value of milk – ECM (energy-corrected milk). The results obtained indicated the need to include overall average samples of MUN across the entire herd to optimise feeding programmes and animal welfare management