Milk urea testing as a tool to monitor reproductive performance in Ontariodairy herds

Dairy herd improvement test-day data, including milk urea concentrations me asured using infrared test method, were collected from 60 commercial Ontari o Holstein dairy herds for a 13-mo period between December 1, 1995, and Dec ember 31, 1996. The objective of the study was to describe, at the cow and the group level, the relationship between DHI milk urea concentrations and reproductive performance in commercial dairy herds. When interpreted at the cow level, there was no association between milk ur ea and the risk for pregnancy from an insemination occurring within the 45- d period preceding test day. However, a negative curvilinear relationship e xisted between milk urea and the risk for pregnancy from a first, second, o r third insemination event occurring within the 45-d period following test day, with the odds for pregnancy being highest when the milk urea on the te st day preceding the insemination was either below 4.5 mmol/L or greater th an 6.49 mmol/L, compared with a concentration between 4.5 and 6.49 mmol/L. When interpreted at the group level, there was no association between group mean milk urea for cows between 50 and 180 DIM, and the group conception r ate for cows receiving a first, second, or third insemination event in the 45-d period either preceding or following test day. Thus, while DHI milk ur ea measurements may be useful as a management tool to improve the efficienc y of production or reduce nitrogen excretion, through helping to optimize t he efficiency of protein utilization, they may have limited utility as a mo nitoring or diagnostic tool for reproductive performance. The results of th is study suggest that good fertility may be achieved across a broad range o f milk urea concentrations.