Emissions of nitrous oxide from grassland systems are attributable largely to the use of nitrogen
fertilisers and the excreta deposited by grazing animals. There is increasing interest in using gibberellins
as a naturally-occurring growth promotant of herbage that could be used to reduce the use of nitrogen
fertilisers while leading to similar or greater increases in dry matter. This may provide practical
opportunities to reduce nitrogen intake by ruminants and extend the seasonality of herbage growth in
spring and autumn while reducing nitrogen losses, resulting in lower rates of nitrogen excretion by
grazing animals and reduced nitrous oxide emissions. Our findings from a review of previous studies
con firm that gibberellins promote dry matter production, especially when applied in early spring or late
summer/early autumn. When gibberellins are applied alone without nitrogen fertiliser, the nitrogen
concentration of herbage is reduced and the impacts on forage quality are small and of ten not
significantly different from those for untreated controls. We calculated the consequences of enhanced
herbage production on nitrogen excreta returned to the soil as urine by a grazing dairy cow and estimated
that one application of gibberellins will result in a relative reduction in nitrous oxide emission per
urination event of 18 % when compared with emissions from using nitrogen fertiliser. We used the
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model and nitrous oxide emissions factors to estimate the impacts of changing herbage dry
matter production, foliage nitrogen concentration and timing of one application of gibberellins on annual
nitrous oxide emissions for a dairy farm. For one application of gibberellins in late summer and early
spring, we estimate reductions in nitrous oxide emissions of 1.6% and 1.3%, respectively, relative to the
response for an untreated control. Incorporating the effects of reduced use of nitrogen fertiliser by
substituting one split application of fertiliser in late summer or autumn with gibberellins, we estimate
reductions on nitrous oxide emissions of between 5 and 6% relative to the response for the untreated
control. We conclude that the use of gibberellins with reduced addition of nitrogen fertiliser has the
potential to reduce nitrous emissions from grazed grassland. However, acceptance of widespread use of
gibberellins will be dependent on cost benefit analysis for farmers.
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