Life cycle assessment of greenhouse gas emissions from irrigated maize and their significance in the value chain

The life cycle assessment component of this multi-institutional project determined greenhouse gas emissions in pre-farm, on-farm and post-farm activities involved in the use of maize for the manufacture of corn chips. When the emissions were expressed in terms of carbon dioxide-equivalents (CO₂-e), pre-farm emissions comprised ~6% of the life cycle emissions, on-farm activities comprised ~36% and post-farm activities accounted for ~58% of life cycle greenhouse gas emissions. We used one 400 g packet of corn chips as the functional unit. The single largest source of greenhouse emissions was the emission of nitrous oxide on the farm as a result of fertiliser application (0.126 kg CO₂-e per packet). The next largest was electricity used during the manufacture of the corn chips (0.086 kg CO₂-e per packet). The manufacture of the packaging (box plus packet, being 0.06 kg CO₂-e) was the next largest source and then the oil for frying the corn chips (0.048 kg CO₂-e per packet). Greenhouse gas emissions from fertiliser application were primarily nitrous oxide (N₂O), which has a global warming potential of 310 kg CO₂-e/kg N₂O. In typical irrigated farm systems, these emissions, when converted to CO₂-e, are almost three times more than the greenhouse gas emissions that result from energy used to pump water. However, pumping irrigation water from deep bores currently produces greenhouse gas emissions that are almost three times those from irrigation using surface waters. Greenhouse gas emissions from the use of tractors on typical farms are about one-third of the emissions from pumping water. Farm management techniques can be used to increase soil carbon and reduce greenhouse gas emissions. If farms that currently burn stubble were to implement stubble incorporation then, in the absence of other changes to the supply chain, they will achieve a 30% reduction in emissions from 'cradle to farm-gate'. In absolute terms, when the soil carbon dioxide is included (even though soil carbon dioxide in this instance is not counted as a greenhouse gas in national and international greenhouse gas inventories), our measurements indicate that carbon dioxide and greenhouse gas emissions from farms that produce maize using stubble incorporation are 56% lower than emissions from farms that burn their stubble. The pre-farm and on-farm operations add $0.40 value per kg of CO₂-e greenhouse gas emitted. Post-farm processing added $2 value per kg of CO₂-e greenhouse gas emitted. Processing maize for corn chips emitted more greenhouse gases than processing the same amount of corn for starch or ethanol.