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

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Nitrate supplements to cattle diets can reduce enteric CH4 emissions. However, if NO3- metabolism stimulates N2O emissions, this will reduce the effectiveness of dietary NO3- for CH4 mitigation. We quantified N2O emissions as part of a dairy cow feeding experiment where urea was substituted in nearly iso-N diets with 0, 5, 14 or 21 g NO3- kg-1 DM. The feeding experiment was a Latin square with repetition of period 1. Each period lasted 4 weeks, with CH4 emission measurements in week 4 using respiration chambers. During period 3, N2O concentrations in chamber outlet air were monitored semi-continuously during 48 h. High, but fluctuating N2O concentrations were seen at the two highest NO3- levels (up to between 2 and 5 µL L-1), and dynamics were linked with recent feed intake. In periods 4 and 5, N2O concentrations and feed intake were determined from all four respiration chambers during two 7-h periods. Emissions of N2O coincided with feed intake, again with N2O concentrations in the µL L-1 range at the two highest NO3- intake levels. Neither feed nor excretion of NO3- via urine were significant sources of N2O, indicating that emissions came from the animals. Leakages due to rumen fistulation could also not account for N2O emissions. Nitrous oxide emission factors ranged between 0.7 and 1.0% except in one case at 21 g NO3- kg-1 DM, where it was 3.4%. When accounting for N2O emissions at the highest NO3- 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 N2O emissions.
TidsskriftJournal of Environmental Quality (Online)
Sider (fra-til)1063-1070
StatusUdgivet - 2015

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