Estimating greenhouse gas emissions from New Zealand dairy systems using a mechanistic whole farm model and inventory methodology

The strategy for New Zealand dairy farming (DairyNZ, 2009) formulates targets for increased national milk production and a reduction in greenhouse gas (GHG) emissions, but acknowledges these two targets conflict because GHG typically increase with increased milk output. Our objective was to determine if both targets could be achieved by implementing combinations of five mitigations. A farm scale computer model, which includes a mechanistic cow model, was used to model a typical pasture based New Zealand dairy farm as the baseline farm. The five mitigations were: (1) improved reproductive performance of the herd resulting in lower replacement rates, (2) increased genetic merit of the cows combined with lower stocking rate and longer lactations, (3) keeping lactating cows on a loafing pad for 12 h/day for 2 mo during autumn, (4) growing low protein crops of grains and/or silages of maize, barley and oats on a portion of the farm and feeding this to lactating cows, (5) reducing fertilizer N use and replacing some of this with nitrification inhibitors and the plant growth stimulant gibberellins. No single mitigation strategy achieved both targets of increasing production by 10–15% and reducing GHG emissions by 20%, but when all were simultaneously implemented in the baseline farm, milk production increased by 15–20% to 1200 kg milk fat + protein/ha, and absolute GHG emissions decreased by 15–20% to 0.8 kg CO2-equivalents (CO2-e)/kg fat and protein corrected milk (FPCM), which is equivalent to a decrease from 11.7 to 8.2 kg CO2-e/kg fat + protein. The synergies of the mitigations resulted in reduced dry matter intake and enteric CH4 emissions, a reduction in N input and N dilution in feed, and, therefore, reduced urinary N excretion onto pastures, and an increase in feed conversion efficiency (i.e., more feed was used for production and less for maintenance). Mechanistic CH4 models as part of farm scale models are important because current GHG inventory methodology cannot properly evaluate CH4 emissions for a range of potential mitigation strategies. There is also a need to develop capabilities in farm scale models to accurately simulate urine patches and N2O emissions from these patches. aper is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors: K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson.