The objective of this study was to evaluate the association of non-dietary factors on the concentration of milk urea nitrogen (MUN), as an indicator of efficiency of nutrient utilization by lactating dairy cows. A dataset containing 7,006 observations from 855 lactating Holstein cows were used. Milk yield, body weight, days in milk and number of lactation were collected on the milk sampling days. Monthly samples of milk were collected to determine contents of milk fat, protein, lactose, total solids, MUN and somatic cells count (SCC). Yield of milk components (fat, protein, lactose and total solids) as well as 4% fat-corrected milk (FCM) were also used in the study. Regression analyses of MUN, as the dependent variable, on FCM, SCC and content and yield of milk components were done to analyze the data. Mean MUN and milk yield averaged 13.3 mg/dL and 35.8 kg/day, respectively. Milk contents of fat, protein, lactose and total solids as well as SCC (natural logarithm transformed) were negatively associated with MUN while the opposite was observed for yields of milk lactose and total solids and milk production. Yields of milk fat and milk protein (kg), FCM, days in milk and number of lactation all were correlated with MUN, however there was lack of fit for the linear equation. No significant correlations between MUN and body weight and MUN and fat:protein ratio were observed. Among the non-dietary factors, milk yield and milk protein concentration correlated most with MUN.
dairy cow; milk yield; MUN; milk protein