Bulk milk urea as an indicator of herd dietary nitrogen surplus and nitrogen use efficiency on Canterbury dairy farms.

Managing herd dietary nitrogen surplus (DNS) remains a core challenge on pasture-based dairy farms to reduce the risk of nitrogen loss to the environment. To manage their herd’s DNS, farmers need readily available, practical indicators. In an observational longitudinal study of five Canterbury dairy farms over five seasons (2014/15 to 2018/19), we explored the usefulness of bulk milk urea concentration (BMU; mg/dL) and bulk milk urea nitrogen to milk protein nitrogen ratio (BMU-N:milk-N) to assess estimated herd DNS and dietary nitrogen use efficiency (DNUE). The analyses included correlations, linear and quadratic regression, and multivariate modelling to determine relationships of herd DNS and DNUE with BMU and BMU-N:milk-N and factors affecting BMU and BMU-N:milk-N. Herd DNS was moderately positively correlated with BMU and BMU-N:milk-N (r = 0.48-0.56, and r = 0.46-0.61, respectively). In contrast, herd DNUE was moderately negatively correlated with BMU and weakly to moderately correlated with BMU-N:milk-N (r = -0.48 to -0.56, and r = -0.38 to -0.49, respectively). Final multivariate models accounted for 51.6-52.8% of the variation in BMU, and 43.6-46.2% of the variation in BMU-N:milk-N. The results suggest that BMU can be used as a near real-time indicator of herd DNS and DNUE in current New Zealand dairy farm systems.