Analysing the implications of increased nitrogen application on greenhouse gas emissions and productivity of New Zealand sheep and beef farms

New Zealand’s goal to double agricultural exports as well as meet Kyoto protocol commitments is a challenge. New Zealand sheep and beef farmers have been advised that applying more nitrogen fertiliser is the cheapest way to boost productivity. However, an increase in nitrogen fertiliser application results in greenhouse gas emissions increasing. This study analysed the implications of increased nitrogen fertiliser application on productivity and greenhouse gas emissions for New Zealand sheep and beef farms. Three scenarios were modelled to use the additional pasture production achieved from the different rates of nitrogen fertiliser and compared with a base model sheep and beef farm. These scenarios were (1) better feeding livestock to increase end live weight, (2) increasing stocking rate and (3) better feeding livestock to reduce the number of grazing days. The model farm was based on weighted average of all sheep and beef farms in New Zealand (Class 9). Simulations were then run with the different rates of nitrogen fertiliser ranging from 20 to 100kgN/ha/yr through each scenario. Consistent with the hypotheses, the efficiency of utilisation of extra grass production is an important determinant of the ratio of product output to GHG emissions. For scenario 1 and 2, productivity increased with the ratio of profit to kg of GHG emissions increasing. In scenario 1, the profit per kg of GHG emissions increased 27% in simulation 1 from the base model farm. This occurred when nitrogen fertiliser was increased to 20 kg/N/ha/yr from 5.6 kg/N/ha/yr. The ratio increased 0.6% for scenario 2 for the same change in nitrogen fertiliser. In scenario 3, the ratio of profit to kg of GHG emissions decreased 9% for the same change in nitrogen fertiliser. In all scenarios, GHG emissions increased. When N fertiliser is increased, productivity increased, greenhouse gas emissions could not be reduced and the proposed emissions trading scheme will have little impact on profitability. Strategic use of N could improve hill-country resilience. With an increase in strategic N fertiliser application, livestock can be better fed; thus, increased live weight and reducing the number of grazing days.