Potential use of milk urea nitrogen to abate atmospheric nitrogen emissions from Wisconsin dairy farms

Urinary urea N (UUN) is the principal nitrogen (N) source controlling emissions of ammonia (NH₃) and nitrous oxide (N₂O) from dairy manure. The objectives of this study were (i) to study the integrative nature of dietary crude protein (CP) management, secretion of milk urea N (MUN), excretion of UUN, and N emissions from dairy production systems; (ii) to evaluate how associative changes in dietary CP, MUN, and UUN affect atmospheric N emissions from dairy farms; and (iii) to discuss some of the challenges and opportunities to an expanded use of MUN to enhance dietary CP use and decrease UUN excretion and N emissions from dairy farms. Milk urea N records of 37,889 cows in 197 herds in Wisconsin revealed that approximately one half of tested cows were likely consuming dietary CP in excess of requirement. Farm simulations were used to quantify the effect of dietary CP on whole-farm N emissions. At a statewide average MUN of 12.5 mg dL^-1, 48 to 87% of UUN was emitted as NH₃, with the lowest loss from pasture-based farms and the greatest loss from tie-stall farms. Each 1 mg dL^-1 decrease of MUN (range, 16-10 mg dL^-1) provided an associated daily decrease in UUN of 16.6 g per cow, which decreased NH₃ and N₂O emissions from manure by 7 to 12%. Although more site-specific information is required on herd MUN-UUN relationships and more a reliable interpretation of MUN assay results is needed, monitoring of MUN may be used to enhance dietary CP use and to reduce UUN excretion and N emissions from Wisconsin dairy farms.