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In grazing dairy systems, the CP content of perennial ryegrass-white clover pastures often exceeds cows' nutritional requirements. Excess dietary nitrogen (N) is excreted in urine, increasing the risk of N leaching into soil and waterways or being released as nitrous oxide, a greenhouse gas. The relationship between milk urea concentration and excess dietary N may provide an indicator to support managing the risk of higher urinary N losses. The first objective of this observational study was to compare milk urea concentration within and between lactation seasons, in bulk milk collected from dairy herds using pasture-based systems in New Zealand. The second objective was to identify farm management factors that affect bulk milk urea (BMU) concentration by comparing farms with divergent BMU. Milk urea was measured in tank milk (n = approximately 2.2 million collections per lactation season; approximately 230 collections per farm across approximately 9,600 farms, with milk collected daily or every 2 d) over the 2016-2017, 2017-2018, 2018-2019, and 2019-2020 lactation seasons. Divergent BMU farms (n = 50/group), with annual mean BMU consistently low (L50) or high (H50) across New Zealand over 4 seasons, were selected and their farm management metrics (farm-reported, annual) and milk production were compared. Additionally, divergent BMU groups within 7 different New Zealand regions were compared (n = 50/group per region). Bulk milk urea concentration from all farms across all 4 lactation seasons followed a normal distribution (range 0 to 69.5 mg/dL), increased through lactation (25.5, 27.5, and 31.6 mg/dL for spring, summer, and autumn, typically corresponding to early, mid, and late lactation, respectively), and varied between lactation seasons. Monthly BMU ranged from 13 to 26 mg/dL for L50 and from 33.2 to 45.8 mg/dL for H50. Distributions of milking frequency, breed, and geographical region were different for L50 and H50. Farm area, herd size, and milk solids production per cow were similar between the 2 groups, whereas milk production per hectare and stocking rate (cows/ha) were greater for H50. Estimated total DM eaten per hectare, pasture and crops eaten per cow and pasture and crops eaten per hectare were all greater for H50. The H50 group applied substantially more annual N fertilizer. Observations for L50 and H50 groups were mirrored by regional divergent BMU groups, with some exceptions that likely reflect differences in farm management systems between regions. Understanding drivers of BMU concentration may provide tools for farmers to improve N use efficiency and reduce risk of excess urinary N loss from dairy herds.