Nitrogen (N) leached into groundwater from urine patches of cattle grazing in situ is an environmental problem in pasture-based dairy industries. One potential mitigation is to breed cattle for lower urinary nitrogen (UN) excretion. Urinary nitrogen is difficult to measure, while milk urea nitrogen concentration (MUN) is relatively easy to measure. For animals fed diets of differing N content in confinement, MUN is moderately heritable and is positively related to UN. However, there is little information on the heritability of MUN, and its relationship with other traits such as milk yield and composition, for animals grazing fresh pasture. Milk urea nitrogen concentration data together with milk yield, fat, protein and lactose composition and somatic cell count was collected from 133 624 Holstein-Friesian (HF), Jersey (J) and HF×J (XBd) cows fed predominantly pasture over three full lactations and one part lactation. Mean MUN was 14.0; and 14.4, 13.2 and 13.9 mg/dl for HF, J and XBd cows, respectively. Estimates of heritability of MUN were 0.22 using a repeatability model that fitted year-of-lactation by month-of-lactation by cow-age with days-in-milk within month-of-lactation and cow-age, and 0.28 using a test-day model analysis with Gibbs sampling methods. Sire breeding values (BVs) ranged from −2.8 to +3.2 indicating that MUN could be changed by selection. The genetic correlation between MUN and percent true protein in milk was −0.22; −0.29 for J cows and −0.16 for HF cows. Should the relationship between MUN and UN observed in dietary manipulation studies hold similarly when MUN is manipulated by genetic selection, UN excretion could be reduced by 6.6 kg/cow per year in one generation of selection using sires with low MUN BVs. Although J cows had lower MUN than HF, total herd UN excretion may be similar for the same fixed feed supply because more J cows are required to utilise the available feed. The close relationship between blood plasma urea N concentration and MUN may enable early selection of bulls to breed progeny that excrete less UN.

With the advent of confined feeding systems and associated herd size increases, dairy farms have had to import more feed from off the farm, leading to on farm nutrient surpluses. Management-intensive grazing (MIG) is an alternative to confined feeding. Under MIG, the herd is rotated among small paddocks every 12–24 h for efficient conversion of forage into milk using grass pasture to provide most of the herd's nutritional needs. Previous research on MIG reported high concentrations of nitrate under grassed monolith lysimeters and grazed pastures. However, because of the high levels of nitrogen (N) fertilization and ponding of cow urine in lysimeters, these previous studies may have overestimated nitrate leaching losses from MIG systems as practiced in the mid-Atlantic region. To determine the extent of nitrate losses to groundwater under MIG in the mid-Atlantic, we sampled shallow groundwater biweekly for 3 yrs on six watersheds in three Maryland dairy farms, one confined and two MIG-based farms. Transects of nested piezometers and ceramic tipped suction lysimeters were installed in each watershed. Seasonal mean nitrate-N concentrations under the four grazed watersheds were generally lower than under the confined feeding farm watersheds and were generally below the widely used drinking water standard of 10 mg L−1. Average nitrate-N concentrations for all four grazed watersheds were between 4 and 7 mg L−1, while the two confined feeding farm watersheds averaged 7 and 11 mg L−1, indicating that the MIG system did not cause excessive nitrate leaching.