Association of bulk tank milk urea nitrogen concentration with elevated individual cow values and investigation of sampling frequency for accurate assessment.

Individual milk urea nitrogen (MUN) levels ≥ 19.63 mg/dL have been recently reported to significantly affect fertility. The objectives of the present study were to (a) predict the percentage of cows with elevated MUN within a herd using bulk tank (BTMUN) levels, in the absence of individual MUN records, and (b) establish a sampling frequency protocol for the assessment of actual BTMUN levels. A database of 17,687 monthly individual MUN and concurrent 229 monthly BTMUN records from 24 dairy herds was used. A ROC analysis was performed to determine the BTMUN threshold over which cows in the herd have elevated MUN concentrations that, based on literature, affect fertility. Moreover, a regression was run to predict the percentage of cows with elevated MUN within a herd from BTMUN values. A second database of 10,687 daily BTMUN records from 29 herds was used to identify an appropriate sampling frequency to assess the actual BTMUN levels. Eleven different sampling frequencies ranging from once to 8 times per month were assessed. A BTMUN value of 15.76 mg/dL was the optimum threshold over which cows with elevated MUN concentrations are included in a herd. The percentage of cows with elevated MUN values can be accurately predicted using BTMUN values (R ²  = 0.872; P < 0.001). A bulk tank sampling frequency of once per week seems appropriate for most herds in order to assess the actual BTMUN levels, in case daily BTMUN values are not available from milk processors.