The effects of system changes in grazed dairy farmlet trials on greenhouse gas emissions

An important challenge facing the NZ dairy industry is development of production systems that can maintain or increase production and profitability, while reducing impacts on receiving environments including water and air. Using research ‘farmlets’ in Waikato, Canterbury and Otago (32-200 animals per herd), we assessed whether system changes aimed at reducing nitrate leaching can also reduce total greenhouse gas (GHG) emissions (methane and nitrous oxide) and emissions intensity (kg GHG per unit of product) by comparing current and future dairy systems. The system changes, termed ‘future’ systems, included (i) using fewer, higher producing cows, (ii) reduced N fertiliser inputs, (iii) reduced herd replacement rate, (iv) greater use of high energy/low N feed and (v) using off-paddock facilities to reduce the time cows spend on pasture or forage crops. Annual average GHG emissions for each system were estimated for three or four years using calculations based on the New Zealand Agricultural Inventory Methodology, but including key farmlet-specific emission factors that were determined from regional experiments. Total annual GHG footprints range between 10,800 kg and 20,615 kg CO2e/ha, with emissions strongly related to the amount of feed eaten. Methane (CH4) represented 79 to 83% of the total GHG footprint across all modelled systems, with enteric CH4 from lactating cows grazing pasture being the major source. Excreta deposition onto paddocks was the largest source of nitrous oxide (N2O) emissions, representing 7-12% of the total GHG footprint for all systems. When total emissions were represented on an intensity basis, emissions ranged from 9.6 kg to 12.3 kg CO2e/kg milksolids (fat + protein), with the future systems generally resulting in lower emissions intensity. The ‘future’ systems had lower GHG footprints than the ‘current’ system, except for one of the future systems in Canterbury, which had a higher stocking rate. The reduced feed supplies and associated lower stocking rates of the future systems were the key drivers of lower total GHG emissions in all three regions. The main effects of these future system attributes were reduced total enteric CH4 emissions and reduced nitrogen intake, which lowered N excretion and, thus, N2O losses. Future systems designed to reduced N leaching generally also reduced GHG emissions.