Estimate of the global warming potential of the Tasmanian pyrethrum industry in comparison to other crops, potato and onion

A desktop study was undertaken to assess the contribution of the pyrethrum industry in Tasmania, Australia to on-farm greenhouse gas (GHG) emissions in comparison to potato and onion. Pyrethrum used significantly less diesel in transport than potato or onion. This was mainly due to the lower weight of dried flowers harvested from pyrethrum crops in comparison to the weight of crop from potato and onion fields (60 t/ha). First harvest pyrethrum required a similar amount of diesel to potato and onion for tractor and harvester operations. However, older pyrethrum crops required only 41% of the diesel used in potato and onion crops due mainly to the perennial nature of pyrethrum and the absence of cultivation in years subsequent to planting. Pyrethrum required greater inputs (kg a.i./ha) of herbicides and lower inputs of fungicides than potato and onion. All three crops required little input of insecticides. Pyrethrum had substantially lower inputs of nitrogen (N) and phosphorus (P) than potato or onion. First-harvest pyrethrum required only 38 and 59% of the N required by potato and onion, respectively. Moreover, pyrethrum required lower potassium (K) than potato, but higher K than onion. The electricity consumption associated with irrigation of pyrethrum and onion were similar, and amounted to 39% that of potato, due to the higher irrigation requirement of potato (5.1 ML/ha) in comparison to pyrethrum and onion (2.0 ML/ha) in Tasmania. The global warming potential (GWP) per hectare associated with planting to harvest of first year pyrethrum (18 months) and subsequently from one harvest to another (12 months) was estimated at 4,128.8 and 2,184.7 kg CO₂-e/ha, respectively. By comparison the GWP resulting from planting to harvest of potato and onion in Tasmania (4 months) was estimated at 5,284.0 and 4,875.3 kg CO₂-e/ha. The GWP of first year pyrethrum was therefore 78% that of potato and 85% that of onion, while the annual GWP per hectare of older pyrethrum fields was 41% that of potato and 45% that of onion. This study was based on strict assumptions and on generic emission factors for greenhouse gases. Therefore caution is required with the absolute figure for GWP. However, this study indicated that the on-farm production of pyrethrum is a relatively low contributor to GHG production on a per hectare basis, in comparison to other annual crops often grown in rotation.