Carbon footprint of dairy goat milk production in New Zealand

摘要

The aim of this study was to assess the cradle-to-farm gate carbon footprint of indoor and outdoor dairy goat farming systems in New Zealand, identifying hotspots and discussing variability and methodology. Our study was based on the International Organization for Standardization standards for life cycle assessment, although only results for greenhouse gas emissions are presented. Two functional units were included: tonnes of CO_2-equivalents (CO_2e) per hectare (ha) and kilograms of CO_2e per kilogram of fat- and protein-corrected milk (FPCM). The study covered 5 farms, 2 farming systems, and 3 yr. Two methods for the calculation of enteric methane emissions were assessed. The Lassey method, as used in the New Zealand greenhouse gas inventory, provided a more robust estimate of emissions from enteric fermentation and was used in the final calculations. The alternative dry matter intake method was shown to overestimate emissions due to use of anecdotal assumptions around actual consumption of feed. Economic allocation was applied to milk and co-products. Scenario analysis was performed on the allocation method, nitrogen content of manure, manure management, and supplementary feed choice. The average carbon footprint for the indoor farms (n = 3) was 11.05 t of CO_2e/ha and 0.81 kg of CO_2e/kg of FPCM. For the outdoor farms (n = 2), the average was 5.38 t of CO_2e/ha and 1.03 kg of CO_2e/kg of FPCM. The average for all 5 farms was 8.78 t of CO_2e/ha and 0.90 kg of CO_2e/kg of FPCM. The results showed relatively high variability due to differences in management practices between farms. The 5 farms covered 10% of the total dairy goat farms but may not be representative of an average farm. Methane from enteric fermentation was a major emission source. The use of supplementary feed was highly variable but an important contributor to the carbon footprint. Nitrous oxide can contribute up to 18% of emissions. Indoor goat farming systems produced milk with a significantly higher carbon footprint per area of land farmed compared with outdoor farming systems, although the 2 systems were not significantly different when results were expressed per kilogram of FPCM, at 0.81 kg CO_2e and 1.03 kg CO_2e per kg of FPCM, respectively. Both systems had footprints less than other reported dairy goat carbon footprints and on par with those for New Zealand dairy cows. The methodology used to determine enteric methane is important for an accurate and meaningful assessment. The choice of manure management system and supplementary feed can substantially affect the carbon footprint.