Dietary Nitrate for Methane Mitigation Leads to Nitrous Oxide Emissions from Dairy Cows

Nitrate supplements to cattle diets can reduce enteric CH 4 emissions. However, if NO 3 − metabolism stimulates N 2 O emissions, the effectiveness of dietary NO 3 − for CH 4 mitigation will be reduced. We quantified N 2 O emissions as part of a dairy cow feeding experiment in which urea was substituted in nearly iso‐N diets with 0, 5, 14 or 21 g NO 3 − kg −1 dry matter (DM). The feeding experiment was a Latin square with repetition of Period 1. Each period lasted 4 wk, with CH 4 emission measurements in Week 4 using respiration chambers. During Period 3, N 2 O concentrations in chamber outlet air were monitored semicontinuously during 48 h. High, but fluctuating, N 2 O concentrations were seen at the two highest NO 3 − levels (up to between 2 and 5 μL L −1 ), and dynamics were linked with recent feed intake. In Periods 4 and 5, N 2 O concentrations and feed intake were determined from all four respiration chambers during two 7‐h periods. Emissions of N 2 O coincided with feed intake, again with N 2 O concentrations in the microliter per liter range at the two highest NO 3 − intake levels. Neither feed nor excretion of NO 3 − via urine were significant sources of N 2 O, indicating that emissions came from the animals. Leakages due to rumen fistulation could also not account for N 2 O emissions. The possibility that N 2 O is produced in the oral cavity is discussed. Nitrous oxide emission factors ranged between 0.7 and 1.0% except in one case at 21 g NO 3 − kg −1 DM, where it was 3.4%. When accounting for N 2 O emissions at the highest NO 3 − intake level, the overall GHG mitigation effect in two different animal‐diet combinations changed from −47 to −40%, and from −19 to −17%, respectively, due to N 2 O emissions.