Optimising nitrogen applications for wheat grown for the biofuels market

Abstract

Previous HGCA-funded work has shown that the alcohol yield from wheat is inversely related to its protein content. It seems likely therefore that growing wheat for bioethanol production might require lower nitrogen (N) rates to maximise alcohol yield per ha. This report presents a re-analysis of over 100 sets of wheat N response data, to investigate the extent to which the optimum N rate for bioethanol production differs from that for grain production for the feed market. In addition, for one typical N response, the implications of N fertiliser use on the greenhouse gas (GHG) and energy balance of the resulting biofuel is examined.

Response experiments were chosen where alcohol yield could be estimated from recorded grain protein data, using a previously established relationship showing alcohol yield to increase by 7.2 litres alcohol / tonne for every 1% reduction in grain protein. The response data sets were chosen to represent a range of varieties, soil types, fertiliser types and seasons, so that the results would be as broadly applicable as possible.

Assuming a scenario whereby the bioethanol processor is also growing the crop, the analysis indicated that the optimum N rate for alcohol production per ha was on average 12% lower than the economic optimum for grain production. There was no evidence that this difference in optimum varied significantly between varieties, soil types, fertiliser types or season. However, whilst a reduction in N rate of around 12% would be required to optimise alcohol yield per ha rather than grain yield overall, the adjustment to normal rates that should ultimately be adopted by a grower will depend entirely on the price structure being offered by the grain buyer. If no premium is being offered for low protein, high starch or high alcohol yield grain, then financially the grower will remain better off using the established optima for feed grain production. Where premiums are a possibility, it will be increasingly more important for growers to ensure that N rates above the optimum are avoided.

Analysis of the N rate needed to maximise the energy savings from bioethanol per ha of crop showed that the rate of N required was close to that needed to optimise the alcohol yield per ha. N fertiliser represents a greater proportion of the GHG costs of producing a wheat crop than it does the energy costs, due to the N2O emissions associated with its manufacture and use. To maximise the GHG savings per ha from bioethanol production therefore required a significantly lower N rate than that needed to optimise the alcohol production, about 100 kg N/ha compared to around 180 kg N/ha respectively. Whilst the best available information has been used to examine energy and GHG consequences, results should be regarded as provisional as considerable debate continues in the scientific community surrounding the calculation of such GHG and energy costs.